(Slip Opinion)              OCTOBER TERM, 2005                                       1

                                       Syllabus

         NOTE: Where it is feasible, a syllabus (headnote) will be released, as is
       being done in connection with this case, at the time the opinion is issued.
       The syllabus constitutes no part of the opinion of the Court but has been
       prepared by the Reporter of Decisions for the convenience of the reader.
       See United States v. Detroit Timber & Lumber Co., 200 U. S. 321, 337.


SUPREME COURT OF THE UNITED STATES

                                       Syllabus

                            CLARK v. ARIZONA

      CERTIORARI TO THE COURT OF APPEALS OF ARIZONA

      No. 05–5966. Argued April 19, 2006—Decided June 29, 2006
Petitioner Clark was charged with first-degree murder under an Ari
  zona statute prohibiting “[i]nten[tionally] or knowing[ly]” killing a
  police officer in the line of duty. At his bench trial, Clark did not con
  test that he shot the officer or that the officer died, but relied on his
  own undisputed paranoid schizophrenia at the time of the incident to
  deny that he had the specific intent to shoot an officer or knowledge
  that he was doing so. Accordingly, the prosecutor offered circumstan
  tial evidence that Clark knew the victim was a police officer and tes
  timony indicating that Clark had previously stated he wanted to
  shoot police and had lured the victim to the scene to kill him. In pre
  senting the defense case, Clark claimed mental illness, which he
  sought to introduce for two purposes. First, he raised the affirmative
  defense of insanity, putting the burden on himself to prove by clear
  and convincing evidence that, in the words of another state statute,
  “at the time of the [crime, he] was afflicted with a mental disease or
  defect of such severity that [he] did not know the criminal act was
  wrong.” Second, he aimed to rebut the prosecution’s evidence of the
  requisite mens rea, that he had acted intentionally or knowingly to
  kill an officer.
     Ruling that Clark could not rely on evidence bearing on insanity to
  dispute the mens rea, the trial court cited the Arizona Supreme
  Court’s decision in State v. Mott, 187 Ariz. 536, 931 P. 2d 1046, which
  refused to allow psychiatric testimony to negate specific intent and
  held that Arizona does not allow evidence of a mental disorder short
  of insanity to negate the mens rea element of a crime. As to his in
  sanity, then, Clark presented lay testimony describing his increas
  ingly bizarre behavior over the year before the shooting. Other lay
  and expert testimony indicated, among other things, that Clark
  thought that “aliens” (some impersonating government agents) were
2                           CLARK v. ARIZONA

                                  Syllabus

    trying to kill him and that bullets were the only way to stop them. A
    psychiatrist testified that Clark was suffering from paranoid schizo
    phrenia with delusions about “aliens” when he killed the officer, and
    concluded that Clark was incapable of luring the officer or under
    standing right from wrong and was thus insane at the time of the
    killing. In rebuttal, the State’s psychiatrist gave his opinion that
    Clark’s paranoid schizophrenia did not keep him from appreciating
    the wrongfulness of his conduct before and after the shooting. The
    judge then issued a first-degree murder verdict, finding that in light
    of that the facts of the crime, the expert evaluations, Clark’s actions
    and behavior both before and after the shooting, and the observations
    of those who knew him, Clark had not established that his schizo
    phrenia distorted his perception of reality so severely that he did not
    know his actions were wrong.
       Clark moved to vacate the judgment and life sentence, arguing,
    among other things, that Arizona’s insanity test and its Mott rule
    each violate due process. He claimed that the Arizona Legislature
    had impermissibly narrowed its insanity standard in 1993 when it
    eliminated the first of the two parts of the traditional M’Naghten in
    sanity test. The trial court denied the motion. Affirming, the Ari
    zona Court of Appeals held, among other things, that the State’s in
    sanity scheme was consistent with due process. The court read Mott
    as barring the trial court’s consideration of evidence of Clark’s mental
    illness and capacity directly on the element of mens rea.
Held:
    1. Due process does not prohibit Arizona’s use of an insanity test
 stated solely in terms of the capacity to tell whether an act charged
 as a crime was right or wrong. Pp. 6–15.
       (a) The first part of the landmark English rule in M’Naghten’s
 Case asks about cognitive capacity: whether a mental defect leaves a
 defendant unable to understand what he was doing. The second part
 presents an ostensibly alternative basis for recognizing a defense of
 insanity understood as a lack of moral capacity: whether a mental
 disease or defect leaves a defendant unable to understand that his
 action was wrong. Although the Arizona Legislature at first adopted
 the full M’Naghten statement, it later dropped the cognitive incapac
 ity part. Under current Arizona law, a defendant will not be ad
 judged insane unless he demonstrates that at the time of the crime,
 he was afflicted with a mental disease or defect of such severity that
 he did not know the criminal act was wrong. Pp. 6–7.
       (b) Clark insists that the side-by-side M’Naghten test represents
 the minimum that a government must provide, and he argues that
 eliminating the first part “ ‘offends [a] principle of justice so rooted in
 the traditions and conscience of our people as to be ranked as funda
                   Cite as: 548 U. S. ____ (2006)                    3

                              Syllabus

mental,’ ” Patterson v. New York, 432 U. S. 197, 202. The claim entails
no light burden, and Clark does not carry it. History shows no defer
ence to M’Naghten that could elevate its formula to the level of fun
damental principle, so as to limit the traditional recognition of a
State’s capacity to define crimes and defenses. See, e.g., Patterson,
supra, at 210. Even a cursory examination of the traditional Anglo-
American approaches to insanity reveals significant differences a
mong them, with four traditional strains variously combined to yield
a diversity of American standards. Although 17 States and the Fed
eral Government have adopted recognizable versions of the
M’Naghten test with both its components, other States have adopted
a variety of standards based on all or part of one or more of four vari
ants. The alternatives are multiplied further by variations in the
prescribed insanity verdict. This varied background makes clear that
no particular formulation has evolved into a baseline for due process,
and that the insanity rule, like the conceptualization of criminal of
fenses, is substantially open to state choice. Pp. 7–12.
     (c) Nor does Arizona’s abbreviation of the M’Naghten statement
raise a proper claim that some constitutional minimum has been
shortchanged. Although Arizona’s former statement of the full
M’Naghten rule was constitutionally adequate, the abbreviated rule
is no less so, for cognitive incapacity is relevant under that state
ment, just as it was under the more extended formulation, and evi
dence going to cognitive incapacity has the same significance under
the short form as it had under the long. Though Clark is correct that
applying the moral incapacity test (telling right from wrong) does not
necessarily require evaluation of a defendant’s cognitive capacity to
appreciate the nature and quality of the acts charged against him,
his argument fails to recognize that cognitive incapacity is itself
enough to demonstrate moral incapacity, so that evidence bearing on
whether the defendant knew the nature and quality of his actions is
both relevant and admissible. In practical terms, if a defendant did
not know what he was doing when he acted, he could not have known
that he was performing the wrongful act charged as a crime. The
Arizona appeals court acknowledged as much in this case. Clark
adopted this very analysis in the trial court, which apparently agreed
when it admitted his cognitive incapacity evidence for consideration
under the State’s moral incapacity formulation. Clark can point to no
evidence bearing on insanity that was excluded. Pp. 12–15.
  2. The Arizona Supreme Court’s Mott rule does not violate due
process. Pp. 15–38.
     (a) Mott held that testimony of a professional psychologist or psy
chiatrist about a defendant’s mental incapacity owing to mental dis
ease or defect was admissible, and could be considered, only for its
4                           CLARK v. ARIZONA

                                   Syllabus

    bearing on an insanity defense, but could not be considered on the
    element of mens rea. Of the three categories of evidence that poten
    tially bear on mens rea—(1) everyday “observation evidence” either
    by lay or expert witnesses of what Clark did or said, which may sup
    port the professional diagnoses of disease and in any event is the
    kind of evidence that can be relevant to show what was on Clark’s
    mind when he fired his gun; (2) “mental-disease evidence,” typically
    from professional psychologists or psychiatrists based on factual re
    ports, professional observations, and tests about Clark’s mental dis
    ease, with features described by the witness; and (3) “capacity evi
    dence,” typically by the same experts, about Clark’s capacity for
    cognition and moral judgment (and ultimately also his capacity to
    form mens rea)—Mott imposed no restriction on considering evidence
    of the first sort, but applies to the latter two. Although the trial court
    seems to have applied the Mott restriction to all three categories of
    evidence Clark offered for the purpose of showing what he called his
    inability to form the required mens rea, his objection to Mott’s appli
    cation does not turn on the distinction between lay and expert wit
    nesses or the kinds of testimony they were competent to present.
    Rather, the issue here is Clark’s claim that the Mott rule violates due
    process. Pp. 15–25.
         (b) Clark’s Mott challenge turns on the application of the pre
    sumption of innocence in criminal cases, the presumption of sanity,
    and the principle that a criminal defendant is entitled to present
    relevant and favorable evidence on an element of the offense charged
    against him. Pp. 25–30.
           (i) The presumption of innocence is that a defendant is inno
    cent unless and until the government proves beyond a reasonable
    doubt each element of the offense changed, including the mental ele
    ment or mens rea. The modern tendency is to describe the mens rea
    required to prove particular offenses in specific terms, as shown in
    the Arizona statute requiring the State to prove that in acting to kill
    the victim, Clark intended to kill a law enforcement officer on duty or
    knew that the victim was such an officer on duty. As applied to mens
    rea (and every other element), the force of the presumption of inno
    cence is measured by the force of the showing needed to overcome it,
    which is proof beyond a reasonable doubt that a defendant’s state of
    mind was in fact what the charge states. See In re Winship, 397
    U. S. 358, 361–363. Pp. 25–26.
           (ii) The presumption of sanity dispenses with a requirement
    that the government include as an element of every criminal charge
    an allegation that the defendant had the capacity to form the mens
    rea necessary for conviction and criminal responsibility. Unlike the
    presumption of innocence, the presumption of sanity’s force varies
                    Cite as: 548 U. S. ____ (2006)                      5

                               Syllabus

across the many state and federal jurisdictions, and prior law has
recognized considerable leeway on the part of the legislative branch
in defining the presumption’s strength through the kind of evidence
and degree of persuasiveness necessary to overcome it, see Fisher v.
United States, 328 U. S. 463, 466–476. There are two points where the
sanity or capacity presumption may be placed in issue. First, a State
may allow a defendant to introduce (and a factfinder to consider) evi
dence of mental disease or incapacity for the bearing it can have on
the government’s burden to show mens rea. Second, the sanity pre
sumption’s force may be tested in the consideration of an insanity de
fense raised by a defendant. Insanity rules like M’Naghten and the
variants noted above are attempts to define or indicate the kinds of
mental differences that overcome the presumption of sanity or capac
ity and therefore excuse a defendant from customary criminal re
sponsibility, see, e.g., Jones v. United States, 463 U. S. 354, 373, n. 4,
even if the prosecution has otherwise overcome the presumption of
innocence by convincing the factfinder of all the elements charged be
yond a reasonable doubt. The burden a defendant raising the insan
ity issue must carry defines the strength of the sanity presumption.
A State may, for example, place the burden of persuasion on a defen
dant to prove insanity as the applicable law defines it, whether by a
preponderance of the evidence or to some more convincing degree.
See, e.g., Leland v. Oregon, 343 U. S. 790, 798. Pp. 26–29.
       (iii) A defendant has a due process right to present evidence
favorable to himself on an element that must be proven to convict
him. Evidence tending to show that a defendant suffers from mental
disease and lacks capacity to form mens rea is relevant to rebut evi
dence that he did in fact form the required mens rea at the time in
question. Thus, Clark claims a right to require the factfinder in this
case to consider testimony about his mental illness and his incapacity
directly, when weighing the persuasiveness of other evidence tending
to show mens rea, which the prosecution has the burden to prove.
However, the right to introduce relevant evidence can be curtailed if
there is a good reason for doing so. For example, trial judges may
“exclude evidence if its probative value is outweighed by certain other
factors such as unfair prejudice, confusion of the issues, or potential
to mislead the jury.” Holmes v. South Carolina, 547 U. S. ___, ___.
And if evidence may be kept out entirely, its consideration may be
subject to limitation, which Arizona claims the power to impose here.
Under state law, mental-disease and capacity evidence may be con
sidered only for its bearing on the insanity defense, and it will avail a
defendant only if it is persuasive enough to satisfy the defendant’s
burden as defined by the terms of that defense. Such evidence is thus
being channeled or restricted to one issue; it is not being excluded en
6                            CLARK v. ARIZONA

                                   Syllabus

    tirely, and the question is whether reasons for requiring it to be
    channeled and restricted satisfy due process’s fundamental fairness
    standard. Pp. 29–30.
          (c) The reasons supporting the Arizona rule satisfy due process.
    Pp. 30–38.
             (i) The first such reason is Arizona’s authority to define its pre
    sumption of sanity (or capacity or responsibility) by choosing an in
    sanity definition and placing the burden of persuasion on criminal
    defendants claiming incapacity as an excuse. Consistent with due
    process, a State can require defendants to bear that burden, see
    Leland, supra, at 797–799, and Clark does not object to Arizona’s deci
    sion to require persuasion to a clear and convincing degree before the
    presumption of sanity and normal responsibility is overcome. If a
    State is to have this authority in practice as well as in theory, it must
    be able to deny a defendant the opportunity to displace the sanity
    presumption more easily when addressing a different issue during
    the criminal trial. Yet just such an opportunity would be available if
    expert testimony of mental disease and incapacity could be consid
    ered for whatever a factfinder might think it was worth on the mens
    rea issue. The sanity presumption would then be only as strong as
    the evidence a factfinder would accept as enough to raise a reason
    able doubt about mens rea; once reasonable doubt was found, acquit
    tal would be required, and the standards established for the insanity
    defense would go by the boards. What counts for due process is sim
    ply that a State wishing to avoid a second avenue for exploring capac
    ity, less stringent for a defendant, has a good reason for confining the
    consideration of mental disease and incapacity evidence to the insan
    ity defense. Pp. 30–32.
             (ii) Arizona’s rule also serves to avoid confusion and misunder
    standing on the part of jurors. The controversial character of some
    categories of mental disease, the potential of mental-disease evidence
    to mislead, and the danger of according greater certainty to capacity
    evidence than experts claim for it give rise to risks that may rea
    sonably be hedged by channeling the consideration of such evidence
    to the insanity issue on which, in States like Arizona, a defendant
    has the burden of persuasion. First, the diagnosis may mask vigor
    ous debate within the psychiatric profession about the very contours
    of the mental disease itself. See, e.g., Jones, supra, at 364–365, n. 13.
    Though mental-disease evidence is certainly not condemned wholesale,
    the consequence of this professional ferment is a general caution in
    treating psychological classifications as predicates for excusing other
    wise criminal conduct. Next, there is the potential of mental-disease
    evidence to mislead jurors (when they are the factfinders) through
    the power of this kind of evidence to suggest that a defendant suffer
                      Cite as: 548 U. S. ____ (2006)                      7

                                 Syllabus

  ing from a recognized mental disease lacks cognitive, moral, voli
  tional, or other capacity, when that may not be a sound conclusion at
  all. Even when a category of mental disease is broadly accepted and
  the assignment of a defendant’s behavior to that category is uncon
  troversial, the classification may suggest something very significant
  about a defendant’s capacity, when in fact the classification tells little
  or nothing about the defendant’s ability to form mens rea or to exer
  cise the cognitive, moral, or volitional capacities that define legal san
  ity. The limits of the utility of a professional disease diagnosis are
  evident in the dispute between the two testifying experts in this case;
  they agree that Clark was schizophrenic, but they reach opposite con
  clusions on whether his mental disease left him bereft of cognitive or
  moral capacity. Finally, there are particular risks inherent in the
  opinions of the experts who supplement the mental-disease classifica
  tions with opinions on incapacity: on whether the mental disease
  rendered a particular defendant incapable of the cognition necessary
  for moral judgment or mens rea or otherwise incapable of under
  standing the wrongfulness of the conduct charged. Unlike observa
  tional evidence bearing on mens rea, capacity evidence consists of
  judgment, and judgment is fraught with multiple perils. Although
  such capacity judgments may be given in the utmost good faith, their
  potentially tenuous character is indicated by the candor of the de
  fense expert in this very case. He testified that Clark lacked the ca
  pacity to appreciate the circumstances realistically and to under
  stand the wrongfulness of what he was doing, but he admitted that
  no one knew exactly what was on Clark’s mind at the time of the
  shooting. Even when an expert is confident that his understanding of
  the mind is reliable, judgment addressing the basic categories of ca
  pacity requires a leap from the concepts of psychology, which are de
  vised for thinking about treatment, to the concepts of legal sanity,
  which are devised for thinking about criminal responsibility. Pp. 33–
  38.
       (d) For these reasons, there is also no cause to claim that chan
  neling evidence on mental disease and capacity offends any “ ‘princi
  ple of justice so rooted in the traditions and conscience of our people
  as to be ranked as fundamental,’ ” Patterson, supra, at 202. P. 38.
Affirmed.

   SOUTER, J., delivered the opinion of the Court, in which ROBERTS,
C. J., and SCALIA, THOMAS, and ALITO, JJ., joined, and in which BREYER,
J., joined except as to Parts III–B and III–C and the ultimate disposi
tion. BREYER, J., filed an opinion concurring in part and dissenting in
part. KENNEDY, J., filed a dissenting opinion, in which STEVENS and
GINSBURG, JJ., joined.
                        Cite as: 548 U. S. ____ (2006)                              1

                             Opinion of the Court

     NOTICE: This opinion is subject to formal revision before publication in the
     preliminary print of the United States Reports. Readers are requested to
     notify the Reporter of Decisions, Supreme Court of the United States, Wash
     ington, D. C. 20543, of any typographical or other formal errors, in order
     that corrections may be made before the preliminary print goes to press.


SUPREME COURT OF THE UNITED STATES
                                   _________________

                                   No. 05–5966
                                   _________________


  ERIC MICHAEL CLARK, PETITIONER v. ARIZONA
   ON WRIT OF CERTIORARI TO THE COURT OF APPEALS OF

                 ARIZONA, DIVISION ONE

                                 [June 29, 2006]

  JUSTICE SOUTER delivered the opinion of the Court.
  The case presents two questions: whether due process
prohibits Arizona’s use of an insanity test stated solely in
terms of the capacity to tell whether an act charged as a
crime was right or wrong; and whether Arizona violates
due process in restricting consideration of defense evi
dence of mental illness and incapacity to its bearing on a
claim of insanity, thus eliminating its significance directly
on the issue of the mental element of the crime charged
(known in legal shorthand as the mens rea, or guilty
mind). We hold that there is no violation of due process in
either instance.
                              I
  In the early hours of June 21, 2000, Officer Jeffrey
Moritz of the Flagstaff Police responded in uniform to
complaints that a pickup truck with loud music blaring
was circling a residential block. When he located the
truck, the officer turned on the emergency lights and siren
of his marked patrol car, which prompted petitioner Eric
Clark, the truck’s driver (then 17), to pull over. Officer
Moritz got out of the patrol car and told Clark to stay
where he was. Less than a minute later, Clark shot the
2                        CLARK v. ARIZONA

                         Opinion of the Court

officer, who died soon after but not before calling the police
dispatcher for help. Clark ran away on foot but was ar
rested later that day with gunpowder residue on his
hands; the gun that killed the officer was found nearby,
stuffed into a knit cap.
   Clark was charged with first-degree murder under Ariz.
Rev. Stat. Ann. §13–1105(A)(3) (West Supp. 2005) for
intentionally or knowingly killing a law enforcement
officer in the line of duty.1 In March 2001, Clark was
found incompetent to stand trial and was committed to a
state hospital for treatment, but two years later the same
trial court found his competence restored and ordered him
to be tried. Clark waived his right to a jury, and the case
was heard by the court.
   At trial, Clark did not contest the shooting and death,
but relied on his undisputed paranoid schizophrenia at the
time of the incident in denying that he had the specific
intent to shoot a law enforcement officer or knowledge
that he was doing so, as required by the statute. Accord
ingly, the prosecutor offered circumstantial evidence that
Clark knew Officer Moritz was a law enforcement officer.
The evidence showed that the officer was in uniform at the
time, that he caught up with Clark in a marked police car
with emergency lights and siren going, and that Clark
acknowledged the symbols of police authority and stopped.
The testimony for the prosecution indicated that Clark
had intentionally lured an officer to the scene to kill him,
having told some people a few weeks before the incident
that he wanted to shoot police officers. At the close of the
State’s evidence, the trial court denied Clark’s motion for
judgment of acquittal for failure to prove intent to kill a

——————
    1 Section
           13–1105(A)(3) provides that “[a] person commits first degree
murder if . . . [i]ntending or knowing that the person’s conduct will
cause death to a law enforcement officer, the person causes the death of
a law enforcement officer who is in the line of duty.”
                     Cite as: 548 U. S. ____ (2006)                   3

                         Opinion of the Court

law enforcement officer or knowledge that Officer Moritz
was a law enforcement officer.
   In presenting the defense case, Clark claimed mental
illness, which he sought to introduce for two purposes.
First, he raised the affirmative defense of insanity, putting
the burden on himself to prove by clear and convincing
evidence, §13–502(C) (West 2001), that “at the time of the
commission of the criminal act [he] was afflicted with a
mental disease or defect of such severity that [he] did not
know the criminal act was wrong,” §13–502(A).2 Second,
he aimed to rebut the prosecution’s evidence of the requi
site mens rea, that he had acted intentionally or know
ingly to kill a law enforcement officer. See, e.g., Record in
No. CR 2000–538 (Ariz. Super. Ct.), Doc. 374 (hereinafter
Record).
   The trial court ruled that Clark could not rely on evi
dence bearing on insanity to dispute the mens rea. The
court cited State v. Mott, 187 Ariz. 536, 931 P. 2d 1046 (en
banc), cert. denied, 520 U. S. 1234 (1997), which “refused
to allow psychiatric testimony to negate specific intent,”
187 Ariz., at 541, 931 P. 2d, at 1051, and held that “Ari
——————
  2 Section 13–502(A) provides in full that

“A person may be found guilty except insane if at the time of the
commission of the criminal act the person was afflicted with a mental
disease or defect of such severity that the person did not know the
criminal act was wrong. A mental disease or defect constituting legal
insanity is an affirmative defense. Mental disease or defect does not
include disorders that result from acute voluntary intoxication or
withdrawal from alcohol or drugs, character defects, psychosexual
disorders or impulse control disorders. Conditions that do not consti
tute legal insanity include but are not limited to momentary, temporary
conditions arising from the pressure of the circumstances, moral
decadence, depravity or passion growing out of anger, jealousy, revenge,
hatred or other motives in a person who does not suffer from a mental
disease or defect or an abnormality that is manifested only by criminal
conduct.”
  A defendant found “guilty except insane” is committed to a state
mental health facility for treatment. See §13–502(D).
4                        CLARK v. ARIZONA

                          Opinion of the Court

zona does not allow evidence of a defendant’s mental
disorder short of insanity . . . to negate the mens rea ele
ment of a crime,” ibid.3
   As to his insanity, then, Clark presented testimony from
classmates, school officials, and his family describing his
increasingly bizarre behavior over the year before the
shooting. Witnesses testified, for example, that paranoid
delusions led Clark to rig a fishing line with beads and
wind chimes at home to alert him to intrusion by invaders,
and to keep a bird in his automobile to warn of airborne
poison. There was lay and expert testimony that Clark
thought Flagstaff was populated with “aliens” (some im
personating government agents), the “aliens” were trying
to kill him, and bullets were the only way to stop them. A
psychiatrist testified that Clark was suffering from para
noid schizophrenia with delusions about “aliens” when he
killed Officer Moritz, and he concluded that Clark was
incapable of luring the officer or understanding right from
wrong and that he was thus insane at the time of the
killing. In rebuttal, a psychiatrist for the State gave his
opinion that Clark’s paranoid schizophrenia did not keep
him from appreciating the wrongfulness of his conduct, as
shown by his actions before and after the shooting (such as
circling the residential block with music blaring as if to
lure the police to intervene, evading the police after the
shooting, and hiding the gun).
   At the close of the defense case consisting of this evi
dence bearing on mental illness, the trial court denied
Clark’s renewed motion for a directed verdict grounded on
failure of the prosecution to show that Clark knew the

——————
  3 The trial court permitted Clark to introduce this evidence, whether

primarily going to insanity or lack of intent, “because it goes to the
insanity issue and because we’re not in front of a jury.” App. 9. It also
allowed him to make an offer of proof as to intent to preserve the issue
on appeal. Ibid.
                     Cite as: 548 U. S. ____ (2006)                   5

                         Opinion of the Court

victim was a police officer.4 The judge then issued a spe
cial verdict of first-degree murder, expressly finding that
Clark shot and caused the death of Officer Moritz beyond
a reasonable doubt and that Clark had not shown that he
was insane at the time. The judge noted that though
Clark was indisputably afflicted with paranoid schizo
phrenia at the time of the shooting, the mental illness “did
not . . . distort his perception of reality so severely that he
did not know his actions were wrong.” App. 334. For this
conclusion, the judge expressly relied on “the facts of the
crime, the evaluations of the experts, [Clark’s] actions and
behavior both before and after the shooting, and the ob
servations of those that knew [Clark].” Id., at 333. The
sentence was life imprisonment without the possibility of
release for 25 years.
   Clark moved to vacate the judgment and sentence,
arguing, among other things, that Arizona’s insanity test
and its Mott rule each violate due process. As to the in
sanity standard, Clark claimed (as he had argued earlier)
that the Arizona Legislature had impermissibly narrowed
its standard in 1993 when it eliminated the first part of
the two-part insanity test announced in M’Naghten’s Case,
10 Cl. & Fin. 200, 8 Eng. Rep. 718 (1843). The court de
nied the motion.
   The Court of Appeals of Arizona affirmed Clark’s convic
tion, treating the conclusion on sanity as supported by
enough evidence to withstand review for abuse of discre
tion, and holding the State’s insanity scheme consistent
with due process. App. 336. As to the latter, the Court of
Appeals reasoned that there is no constitutional require
ment to recognize an insanity defense at all, the bounds of

——————
  4 Clark did not at this time make an additional offer of proof, as con
templated by the trial court when it ruled that it would consider
evidence bearing on insanity as to insanity but not as to mens rea. See
n. 3, supra.
6                    CLARK v. ARIZONA

                     Opinion of the Court

which are left to the State’s discretion. Beyond that, the
appellate court followed Mott, reading it as barring the
trial court’s consideration of evidence of Clark’s mental
illness and capacity directly on the element of mens rea.
The Supreme Court of Arizona denied further review.
   We granted certiorari to decide whether due process
prohibits Arizona from thus narrowing its insanity test or
from excluding evidence of mental illness and incapacity
due to mental illness to rebut evidence of the requisite
criminal intent. 546 U. S. ___ (2005). We now affirm.
                             II
  Clark first says that Arizona’s definition of insanity,
being only a fragment of the Victorian standard from
which it derives, violates due process. The landmark
English rule in M’Naghten’s Case, 10 Cl. & Fin. 200, 8
Eng. Rep. 718 (1843), states that
    “the jurors ought to be told . . . that to establish a de
    fence on the ground of insanity, it must be clearly
    proved that, at the time of the committing of the act,
    the party accused was laboring under such a defect of
    reason, from disease of the mind, as not to know the
    nature and quality of the act he was doing; or, if he
    did know it, that he did not know he was doing what
    was wrong.” 10 Cl. & Fin., at 210, 8 Eng. Rep., at 722.
The first part asks about cognitive capacity: whether a
mental defect leaves a defendant unable to understand
what he is doing. The second part presents an ostensibly
alternative basis for recognizing a defense of insanity
understood as a lack of moral capacity: whether a mental
disease or defect leaves a defendant unable to understand
that his action is wrong.
  When the Arizona Legislature first codified an insanity
rule, it adopted the full M’Naghten statement (subject to
modifications in details that do not matter here):
                    Cite as: 548 U. S. ____ (2006)                  7

                        Opinion of the Court

     “A person is not responsible for criminal conduct if at
     the time of such conduct the person was suffering
     from such a mental disease or defect as not to know
     the nature and quality of the act or, if such person did
     know, that such person did not know that what he
     was doing was wrong.” Ariz. Rev. Stat. Ann. §13–502
     (West 1978).5
In 1993, the legislature dropped the cognitive incapacity
part, leaving only moral incapacity as the nub of the
stated definition. See 1993 Ariz. Sess. Laws ch. 256, §§2–
3.6 Under current Arizona law, a defendant will not be
adjudged insane unless he demonstrates that “at the time
of the commission of the criminal act [he] was afflicted
with a mental disease or defect of such severity that [he]
did not know the criminal act was wrong,” Ariz. Rev. Stat.
Ann. §13–502(A) (West 2001).
                              A
  Clark challenges the 1993 amendment excising the
express reference to the cognitive incapacity element. He
insists that the side-by-side M’Naghten test represents the
minimum that a government must provide in recognizing
an alternative to criminal responsibility on grounds of
mental illness or defect, and he argues that elimination of
——————
  5 This statutory standard followed the Arizona Supreme Court’s dec

laration that Arizona has “uniformly adhered” to the two-part
M’Naghten standard. State v. Schantz, 98 Ariz. 200, 206, 403 P. 2d
521, 525 (1965) (citing cases), cert. denied, 382 U. S. 1015 (1966).
  6 This change was accompanied by others, principally an enumeration

of mental states excluded from the category of “mental disease or
defect,” such as voluntary intoxication and other conditions, and a
change of the insanity verdict from “not responsible for criminal con
duct” by reason of insanity to “guilty except insane.” See 1993 Ariz.
Sess. Laws ch. 256, §§2–3. The 1993 amendments were prompted, at
least in part, by an acquittal by reason of insanity in a murder case.
See Note, Arizona’s Insane Response to Insanity, 40 Ariz. L. Rev. 287,
290 (1998).
8                        CLARK v. ARIZONA

                          Opinion of the Court

the M’Naghten reference to nature and quality “ ‘offends
[a] principle of justice so rooted in the traditions and
conscience of our people as to be ranked as fundamental,’ ”
Patterson v. New York, 432 U. S. 197, 202 (1977) (quoting
Speiser v. Randall, 357 U. S. 513, 523 (1958)); see also
Leland v. Oregon, 343 U. S. 790, 798 (1952).
  The claim entails no light burden, see Montana v. Egel
hoff, 518 U. S. 37, 43 (1996) (plurality opinion), and Clark
does not carry it.        History shows no deference to
M’Naghten that could elevate its formula to the level of
fundamental principle, so as to limit the traditional recog
nition of a State’s capacity to define crimes and defenses,
see Patterson, supra, at 210; see also Foucha v. Louisiana,
504 U. S. 71, 96 (1992) (KENNEDY, J., dissenting).
  Even a cursory examination of the traditional Anglo-
American approaches to insanity reveals significant dif
ferences among them, with four traditional strains vari
ously combined to yield a diversity of American standards.
The main variants are the cognitive incapacity, the moral
incapacity, the volitional incapacity, and the product-of
mental-illness tests.7 The first two emanate from the
alternatives stated in the M’Naghten rule. The volitional
incapacity or irresistible-impulse test, which surfaced over
two centuries ago (first in England,8 then in this country9),
asks whether a person was so lacking in volition due to a
——————
  7 “Capacity” is understood to mean the ability to form a certain state

of mind or motive, understand or evaluate one’s actions, or control
them.
  8 See Queen v. Oxford, 9 Car. & P. 525, 546, 173 Eng. Rep. 941, 950

(1840) (“If some controlling disease was, in truth, the acting power
within [the defendant] which he could not resist, then he will not be
responsible”); Hadfield’s Case, 27 How. St. Tr. 1281, 1314–1315, 1354–
1355 (K. B. 1800). But cf. Queen v. Burton, 3 F. & F. 772, 780, 176 Eng.
Rep. 354, 357 (1863) (rejecting the irresistible-impulse test as “a most
dangerous doctrine”).
  9 E.g., Parsons v. State, 81 Ala. 577, 2 So. 854 (1887); State v. Thomp

son, Wright’s Ohio Rep. 617 (1834).
                     Cite as: 548 U. S. ____ (2006)                    9

                          Opinion of the Court

mental defect or illness that he could not have controlled
his actions. And the product-of-mental-illness test was
used as early as 1870,10 and simply asks whether a per
son’s action was a product of a mental disease or defect.11
Seventeen States and the Federal Government have
adopted a recognizable version of the M’Naghten test with
both its cognitive incapacity and moral incapacity compo
nents.12 One State has adopted only M’Naghten’s cogni
——————
  10 State v. Jones, 50 N. H. 369 (1871); State v. Pike, 49 N. H. 399

(1870).
  11 This distillation of the Anglo-American insanity standards into

combinations of four building blocks should not be read to signify that
no other components contribute to these insanity standards or that
there are no material distinctions between jurisdictions testing insanity
with the same building blocks. For example, the jurisdictions limit, in
varying degrees, which sorts of mental illness or defect can give rise to
a successful insanity defense. Compare, e.g., Ariz. Rev. Stat. Ann. §13–
502(A) (West 2001) (excluding from definition of “mental disease or
defect” acute voluntary intoxication, withdrawal from alcohol or drugs,
character defects, psychosexual disorders, and impulse control disor
ders) with, e.g., Ind. Code §35–41–3–6(b) (West 2004) (excluding from
definition of “mental disease or defect” “abnormality manifested only by
repeated unlawful or antisocial conduct”). We need not compare the
standards under a finer lens because our coarser analysis shows that
the standards vary significantly.
  12 See 18 U. S. C. §17; Ala. Code §13A–3–1 (1994); Cal. Penal Code

Ann. §25 (West 1999); Colo. Rev. Stat. §16–8–101.5 (2005); Fla. Stat.
§775.027 (2003); Iowa Code §701.4 (2005); Minn. Stat. §611.026 (2004);
Stevens v. State, 806 So. 2d 1031, 1050–1051 (Miss. 2001); Mo. Rev.
Stat. §562.086 (2000); State v. Harms, 263 Neb. 814, 836–837, 643
N. W. 2d 359, 378–379 (2002); Nev. Rev. Stat. §194.010 (2003); Finger
v. State, 117 Nev. 548, 553–577, 27 P. 3d 66, 70–85 (2001); N. J. Stat.
Ann. §2C:4–1 (West 2005); N. Y. Penal Law Ann. §40.15 (West 2004);
State v. Thompson, 328 N. C. 477, 485–486, 402 S. E. 2d 386, 390
(1991); Burrows v. State, 640 P. 2d 533, 540–541 (Okla. Crim. App.
1982) (interpreting statutory language excusing from criminal respon
sibility mentally ill defendants when “at the time of committing the act
charged against them they were incapable of knowing its wrongful
ness,” Okla. Stat., Tit. 21, §152(4) (West 2001), to mean the two-part
M’Naghten test); 18 Pa. Cons. Stat. §315 (2002); Tenn. Code Ann. §39–
11–501 (2002); Wash. Rev. Code §9A.12.010 (2004). North Dakota has
10                        CLARK v. ARIZONA

                          Opinion of the Court

tive incapacity test,13 and 10 (including Arizona) have
adopted the moral incapacity test alone.14 Fourteen juris
dictions, inspired by the Model Penal Code,15 have in place
an amalgam of the volitional incapacity test and some
variant of the moral incapacity test, satisfaction of either
(generally by showing a defendant’s substantial lack of
capacity) being enough to excuse.16 Three States combine
a full M’Naghten test with a volitional incapacity for
mula.17 And New Hampshire alone stands by the product
——————
a unique test, which appears to be a modified version of M’Naghten,
asking whether a defendant “lacks substantial capacity to comprehend
the harmful nature or consequences of the conduct, or the conduct is
the result of a loss or serious distortion of the individual’s capacity to
recognize reality,” N. D. Cent. Code Ann. §12.1–04.1–01(1)(a) (Lexis
1997), when “[i]t is an essential element of the crime charged that the
individual act willfully,” §12.1–04.1–01(1)(b).
  13 Alaska Stat. §12.47.010 (2004).
  14 Ariz. Rev. Stat. Ann. §13–502 (West 2001); Del. Code Ann., Tit. 11,

§401 (1995); Ind. Code §35–41–3–6 (West 2004); Ill. Comp. Stat., ch.
720, §5/6-2 (West 2004); La. Stat. Ann. §14:14 (West 1997); Me. Rev.
Stat. Ann., Tit. 17–A, §39 (2006); Ohio Rev. Code Ann. §2901.01(A)(14)
(Lexis 2006); S. C. Code Ann. §17–24–10 (2003); S. D. Codified Laws
§22–1–2(20) (2005 Supp. Pamphlet); Tex. Penal Code Ann. §8.01 (West
2003).
  15 ALI, Model Penal Code §4.01(1) (Proposed Official Draft 1962) (“A

person is not responsible for criminal conduct if at the time of such
conduct as a result of mental disease or defect he lacks substantial
capacity either to appreciate the criminality [wrongfulness] of his
conduct or to conform his conduct to the requirements of the law”).
  16 Ark. Code Ann. §5–2–312 (2006); Conn. Gen. Stat. §53a–13 (2005);

Malede v. United States, 767 A. 2d 267, 269 (D. C. 2001); Ga. Code Ann.
§§16–3–2, 16–3–3 (2003); Haw. Rev. Stat. §704–400 (1993); Ky. Rev.
Stat. Ann. §504.020 (West 2003); Md. Crim. Proc. Code Ann. §3–109
(Lexis 2001); Commonwealth v. McLaughlin, 431 Mass. 506, 508, 729
N. E. 2d 252, 255 (2000); Ore. Rev. Stat. §161.295 (2005); State v.
Martinez, 651 A. 2d 1189, 1193 (R. I. 1994); Vt. Stat. Ann., Tit. 13,
§4801 (1998); State v. Lockhart, 208 W. Va. 622, 630, 542 S. E. 2d 443,
451 (2000); Wis. Stat. §971.15 (2003–2004); Wyo. Stat. Ann. §7–11–304
(2005).
  17 Mich. Comp. Laws Ann. §768.21a (West 2000); State v. Hartley, 90

N. M. 488, 490–491, 565 P. 2d 658, 660–661 (1977); Bennett v. Com
                     Cite as: 548 U. S. ____ (2006)                  11

                         Opinion of the Court

of-mental-illness test.18 The alternatives are multiplied
further by variations in the prescribed insanity verdict: a
significant number of these jurisdictions supplement the
traditional “not guilty by reason of insanity” verdict with
an alternative of “guilty but mentally ill.”19 Finally, four
States have no affirmative insanity defense,20 though one
provides for a “guilty and mentally ill” verdict.21 These
four, like a number of others that recognize an affirmative
insanity defense, allow consideration of evidence of mental
illness directly on the element of mens rea defining the
offense.22
   With this varied background, it is clear that no particu
lar formulation has evolved into a baseline for due process,
and that the insanity rule, like the conceptualization of
criminal offenses, is substantially open to state choice.
Indeed, the legitimacy of such choice is the more obvious
when one considers the interplay of legal concepts of men
tal illness or deficiency required for an insanity defense,

——————
monwealth, 29 Va. App. 261, 277, 511 S. E. 2d 439, 446–447 (1999).
  18 State v. Plante, 134 N. H. 456, 461, 594 A. 2d 1279, 1283 (1991).
  19 See, e.g., Alaska Stat. §§12.47.020(c), 12.47.030 (2004); Del. Code

Ann., Tit. 11, §401 (1995); Ga. Code Ann. §17–7–131 (2004); Ill. Comp.
Stat., ch. 720, §5/6–2 (West 2004); Ind. Code §§35–35–2–1, 35–36–1–1,
35–36–2–3 (West 2004); Ky. Rev. Stat. Ann. §504.130 (West 2003);
Mich. Comp. Laws Ann. §768.36 (West Supp. 2006); N. M. Stat. Ann.
§31–9–3 (2000); 18 Pa. Cons. Stat. §314 (2002); S. C. Code Ann. §17–
24–20 (2003); S. D. Codified Laws §23A–26–14 (2004). Usually, a
defendant found “guilty but mentally ill” will receive mental-health
treatment until his mental health has rebounded, at which point he
must serve the remainder of his imposed sentence. See, e.g., Alaska
Stat. §12.47.050 (2004).
  20 Idaho Code §18–207 (Lexis 2004); Kan. Stat. Ann. §22–3220 (1995);

Mont. Code Ann. §§46–14–102, 46–14–311 (2005); Utah Code Ann. §76–
2–305 (Lexis 2003). We have never held that the Constitution man
dates an insanity defense, nor have we held that the Constitution does
not so require. This case does not call upon us to decide the matter.
  21 §§77–16a–101, 77–16a–103, 77–16a–104 (Lexis 2003).
  22 See statutes cited in n. 20, supra.
12                   CLARK v. ARIZONA

                     Opinion of the Court

with the medical concepts of mental abnormality that
influence the expert opinion testimony by psychologists
and psychiatrists commonly introduced to support or
contest insanity claims. For medical definitions devised to
justify treatment, like legal ones devised to excuse from
conventional criminal responsibility, are subject to flux
and disagreement. See infra, at 31–33; cf. Leland, 343
U. S., at 800–801 (no due process violation for adopting the
M’Naghten standard rather than the irresistible-impulse
test because scientific knowledge does not require other
wise and choice of test is a matter of policy). There being
such fodder for reasonable debate about what the cognate
legal and medical tests should be, due process imposes no
single canonical formulation of legal insanity.
                             B
   Nor does Arizona’s abbreviation of the M’Naghten
statement raise a proper claim that some constitutional
minimum has been shortchanged. Clark’s argument of
course assumes that Arizona’s former statement of the
M’Naghten rule, with its express alternative of cognitive
incapacity, was constitutionally adequate (as we agree).
That being so, the abbreviated rule is no less so, for cogni
tive incapacity is relevant under that statement, just as it
was under the more extended formulation, and evidence
going to cognitive incapacity has the same significance
under the short form as it had under the long.
   Though Clark is correct that the application of the
moral incapacity test (telling right from wrong) does not
necessarily require evaluation of a defendant’s cognitive
capacity to appreciate the nature and quality of the acts
charged against him, see Brief for Petitioner 46–47, his
argument fails to recognize that cognitive incapacity is
itself enough to demonstrate moral incapacity. Cognitive
incapacity, in other words, is a sufficient condition for
establishing a defense of insanity, albeit not a necessary
                    Cite as: 548 U. S. ____ (2006)                  13

                         Opinion of the Court

one. As a defendant can therefore make out moral inca
pacity by demonstrating cognitive incapacity, evidence
bearing on whether the defendant knew the nature and
quality of his actions is both relevant and admissible. In
practical terms, if a defendant did not know what he was
doing when he acted, he could not have known that he was
performing the wrongful act charged as a crime.23 Indeed,
when the two-part rule was still in effect, the Supreme
Court of Arizona held that a jury instruction on insanity
containing the moral incapacity part but not a full recita
tion of the cognitive incapacity part was fine, as the cogni
tive incapacity part might be “ ‘treated as adding nothing
to the requirement that the accused know his act was
wrong.’ ” State v. Chavez, 143 Ariz. 238, 239, 693 P. 2d
893, 894 (1984) (quoting A. Goldstein, The Insanity De
fense 50 (1967)).
   The Court of Appeals of Arizona acknowledged as much
in this case, too, see App. 350 (“It is difficult to imagine
that a defendant who did not appreciate the ‘nature and
quality’ of the act he committed would reasonably be able
to perceive that the act was ‘wrong’ ”), and thus aligned
itself with the long-accepted understanding that the cogni
tively incapacitated are a subset of the morally incapaci
tated within the meaning of the standard M’Naghten rule,
see, e.g., Goldstein, supra, at 51 (“In those situations
where the accused does not know the nature and quality of
his act, in the broad sense, he will not know that it was
wrong, no matter what construction ‘wrong’ is given”); 1
——————
  23He  might, of course, have thought delusively he was doing some
thing just as wrongful as the act charged against him, but this is not
the test: he must have understood that he was committing the act
charged and that it was wrongful, see Ariz. Rev. Stat. Ann. §13–502(A)
(West 2001) (“A person may be found guilty except insane if at the time
of the commission of the criminal act the person was afflicted with a
mental disease or defect of such severity that the person did not know
the criminal act was wrong”).
14                         CLARK v. ARIZONA

                           Opinion of the Court

W. LaFave, Substantive Criminal Law §7.2(b)(3), p. 536
(2d ed. 2003) (“Many courts feel that knowledge of ‘the
nature and quality of the act’ is the mere equivalent of the
ability to know that the act was wrong” (citing cases)); id.,
§7.2(b)(4), at 537 (“If the defendant does not know the
nature and quality of his act, then quite obviously he does
not know that his act is ‘wrong,’ and this is true without
regard to the interpretation given to the word ‘wrong’ ”); cf.
1 R. Gerber, Criminal Law of Arizona 502–7, n. 1 (2d ed.
1993).24
  Clark, indeed, adopted this very analysis himself in the
——————
   24 We think this logic holds true in the face of the usual rule of statu

tory construction of “ ‘ “giv[ing] effect, if possible, to every clause and
word of a statute,” ’ ” Duncan v. Walker, 533 U. S. 167, 174 (2001) (quot
ing United States v. Menasche, 348 U. S. 528, 538–539 (1955)); see also 2
J. Sutherland, Statutes and Statutory Construction §4705 (3d ed. 1943).
Insanity standards are formulated to guide the factfinder to determine the
blameworthiness of a mentally ill defendant. See, e.g., Jones v. United
States, 463 U. S. 354, 373, n. 4 (1983) (Brennan, J., dissenting). The
M’Naghten test is a sequential test, first asking the factfinder to conduct
the easier enquiry whether a defendant knew the nature and quality of
his actions. If not, the defendant is to be considered insane and there is no
need to pass to the harder and broader enquiry whether the defendant
knew his actions were wrong. And, because, owing to this sequence, the
factfinder is to ask whether a defendant lacks moral capacity only when
he possesses cognitive capacity, the only defendants who will be found to
lack moral capacity are those possessing cognitive capacity. Cf. 2 C.
Torcia, Wharton’s Criminal Law §101 (15th ed. 1994). Though, before
1993, Arizona had in place the full M’Naghten test with this sequential
enquiry, see, e.g., Schantz, 98 Ariz., at 207, 403 P. 2d, at 525, it would
appear that the legislature eliminated the cognitive capacity part not to
change the meaning of the insanity standard but to implement its judg
ment that a streamlined standard with only the moral capacity part
would be easier for the jury to apply, see Arizona House of Representative
Judiciary Committee Notes 3 (Mar. 18, 1993); 1 R. Gerber, Criminal Law
of Arizona 502–6, 502–11 (2d ed. 1993 and Supp. 2000). This is corrobo
rated by the State’s choice for many years against revising the applicable
recommended jury instruction (enumerating the complete M’Naghten test)
in order to match the amended statutory standard. See 1 Gerber, supra,
at 502–6.
                 Cite as: 548 U. S. ____ (2006)           15

                     Opinion of the Court

trial court: “[I]f [Clark] did not know he was shooting at a
police officer, or believed he had to shoot or be shot, even
though his belief was not based in reality, this would
establish that he did not know what he was doing was
wrong.” Record, Doc. 374, at 1. The trial court apparently
agreed, for the judge admitted Clark’s evidence of cogni
tive incapacity for consideration under the State’s moral
incapacity formulation. And Clark can point to no evi
dence bearing on insanity that was excluded. His psychi
atric expert and a number of lay witnesses testified to his
delusions, and this evidence tended to support a descrip
tion of Clark as lacking the capacity to understand that
the police officer was a human being. There is no doubt
that the trial judge considered the evidence as going to an
issue of cognitive capacity, for in finding insanity not
proven he said that Clark’s mental illness “did not . . .
distort his perception of reality so severely that he did not
know his actions were wrong,” App. 334.
   We are satisfied that neither in theory nor in practice
did Arizona’s 1993 abridgment of the insanity formulation
deprive Clark of due process.
                             III
  Clark’s second claim of a due process violation chal
lenges the rule adopted by the Supreme Court of Arizona
in State v. Mott, 187 Ariz. 536, 931 P. 2d 1046 (en banc),
cert. denied, 520 U. S. 1234 (1997). This case ruled on the
admissibility of testimony from a psychologist offered to
show that the defendant suffered from battered women’s
syndrome and therefore lacked the capacity to form the
mens rea of the crime charged against her. The opinion
variously referred to the testimony in issue as “psychologi
cal testimony,” 187 Ariz., at 541, 931 P. 2d, at 1051, and
“expert testimony,” ibid., and implicitly equated it with
“expert psychiatric evidence,” id., at 540, 931 P. 2d, at
1050 (internal quotation marks omitted), and “psychiatric
16                       CLARK v. ARIZONA

                         Opinion of the Court

testimony,” id., at 541, 931 P. 2d, at 1051.25 The state
court held that testimony of a professional psychologist or
psychiatrist about a defendant’s mental incapacity owing
to mental disease or defect was admissible, and could be
considered, only for its bearing on an insanity defense;
such evidence could not be considered on the element of
mens rea, that is, what the State must show about a de
fendant’s mental state (such as intent or understanding)
when he performed the act charged against him. See id.,
at 541, 544, 931 P. 2d, at 1051, 1054.26
                             A
   Understanding Clark’s claim requires attention to the
categories of evidence with a potential bearing on mens
rea. First, there is “observation evidence” in the everyday
sense, testimony from those who observed what Clark did
and heard what he said; this category would also include
testimony that an expert witness might give about Clark’s
tendency to think in a certain way and his behavioral
characteristics. This evidence may support a professional
diagnosis of mental disease and in any event is the kind of
evidence that can be relevant to show what in fact was on
Clark’s mind when he fired the gun. Observation evidence
in the record covers Clark’s behavior at home and with
friends, his expressions of belief around the time of the
killing that “aliens” were inhabiting the bodies of local
people (including government agents),27 his driving around
——————
  25 We thus think the dissent reads Mott too broadly. See post, at 6–7

(opinion of KENNEDY, J.) (no distinction between observation and
mental-disease testimony, see infra, at 16–17, or lay and expert).
  26 The more natural reading of Mott suggests to us that this evidence

cannot be considered as to mens rea even if the defendant establishes
his insanity, though one might read Mott otherwise.
  27 Clark’s parents testified that, in the months before the shooting

and even days beforehand, Clark called them “aliens” and thought that
“aliens” were out to get him. See, e.g., Tr. of Bench Trial in No. CR
2000–538, pp. 110–112, 136, 226–228 (Aug. 20, 2003). One night before
                      Cite as: 548 U. S. ____ (2006)                    17

                          Opinion of the Court

the neighborhood before the police arrived, and so on.
Contrary to the dissent’s characterization, see post, at 2
(opinion of KENNEDY, J.), observation evidence can be
presented by either lay or expert witnesses.
  Second, there is “mental-disease evidence” in the form of
opinion testimony that Clark suffered from a mental
disease with features described by the witness. As was
true here, this evidence characteristically but not always28
comes from professional psychologists or psychiatrists who
testify as expert witnesses and base their opinions in part
on examination of a defendant, usually conducted after the
events in question. The thrust of this evidence was that,
based on factual reports, professional observations, and
tests, Clark was psychotic at the time in question, with a
condition that fell within the category of schizophrenia.
  Third, there is evidence we will refer to as “capacity
evidence” about a defendant’s capacity for cognition and
moral judgment (and ultimately also his capacity to form
mens rea). This, too, is opinion evidence. Here, as it
usually does,29 this testimony came from the same experts
and concentrated on those specific details of the mental
condition that make the difference between sanity and
insanity under the Arizona definition.30 In their respec
——————
the shooting, according to Clark’s mother, Clark repeatedly viewed a
popular film characterized by her as telling a story about “aliens”
masquerading as government agents, a story Clark insisted was real
despite his mother’s protestations to the contrary. See id., at 59–60
(Aug. 21, 2003). And two months after the shooting, Clark purportedly
told his parents that his hometown, Flagstaff, was inhabited principally
by “aliens,” who had to be stopped, and that the only way to stop them
was with bullets. See, e.g., id., at 131–132 (Aug. 20, 2003); id., at 24–25
(Aug. 21, 2003).
  28 This is contrary to the dissent’s understanding. See post, at 2–3

(opinion of KENNEDY, J.).
  29 In conflict with the dissent’s characterization, see post, at 2 (opin

ion of KENNEDY, J.), it does not always, however, come from experts.
  30Arizona permits capacity evidence, see, e.g., State v. Sanchez, 117
18                        CLARK v. ARIZONA

                          Opinion of the Court

tive testimony on these details the experts disagreed: the
defense expert gave his opinion that the symptoms or
effects of the disease in Clark’s case included inability to
appreciate the nature of his action and to tell that it was
wrong, whereas the State’s psychiatrist was of the view
that Clark was a schizophrenic who was still sufficiently
able to appreciate the reality of shooting the officer and to
know that it was wrong to do that.31
   A caveat about these categories is in order. They at
tempt to identify different kinds of testimony offered in
this case in terms of explicit and implicit distinctions
made in Mott. What we can say about these categories
goes to their cores, however, not their margins. Exact
——————
Ariz. 369, 373, 573 P. 2d 60, 64 (1977); see also Ariz. Rule Evid. 704
(2006) (allowing otherwise admissible evidence on testimony “em
brac[ing] an ultimate issue to be decided by the trier of fact”), though
not every jurisdiction permits such evidence on the ultimate issue of
insanity. See, e.g., Fed. Rule Evid. 704(b) (“No expert witness testifying
with respect to the mental state or condition of a defendant in a crimi
nal case may state an opinion or inference as to whether the defendant
did or did not have the mental state or condition constituting an ele
ment of the crime charged or a defense thereto. Such ultimate issues
are matters for the trier of fact alone”); United States v. Dixon, 185
F. 3d 393, 400 (CA5 1999) (in the face of mental-disease evidence, Rule
704(b) prohibits an expert “from testifying that [the mental-disease
evidence] does or does not prevent the defendant from appreciating the
wrongfulness of his actions”).
   31 Arizona permits evidence bearing on insanity to be presented by

either lay or expert witnesses. See State v. Bay, 150 Ariz. 112, 116, 722
P. 2d 280, 284 (1986). According to Bay, “[f]oundationally, a lay wit
ness must have had an opportunity to observe the past conduct and
history of a defendant; the fact that he is a lay witness goes not to the
admissibility of the testimony but rather to its weight.” Ibid. (citation
omitted); see also State v. Hughes, 193 Ariz. 72, 83, 969 P. 2d 1184,
1195 (1998). In fact, a defendant can theoretically establish insanity
solely via lay testimony. See Bay, 150 Ariz., at 116, 722 P. 2d, at 284.
But cf. State v. McMurtrey, 136 Ariz. 93, 100, 664 P. 2d 637, 644 (1983)
(“[I]t is difficult to imagine how a defendant could place his or her
sanity in issue . . . without expert testimony as to the defendant’s state
of mind at the time of the crime”).
                 Cite as: 548 U. S. ____ (2006)          19

                     Opinion of the Court

limits have thus not been worked out in any Arizona law
that has come to our attention, and in this case, neither
the courts in their rulings nor counsel in objections in
voked or required precision in applying the Mott rule’s
evidentiary treatment, as we explain below. Necessarily,
then, our own decision can address only core issues, leav
ing for other cases any due process claims that may be
raised about the treatment of evidence whose categoriza
tion is subject to dispute.
                             B
   It is clear that Mott itself imposed no restriction on
considering evidence of the first sort, the observation
evidence. We read the Mott restriction to apply, rather, to
evidence addressing the two issues in testimony that
characteristically comes only from psychologists or psy
chiatrists qualified to give opinions as expert witnesses:
mental-disease evidence (whether at the time of the crime
a defendant suffered from a mental disease or defect, such
as schizophrenia) and capacity evidence (whether the
disease or defect left him incapable of performing or ex
periencing a mental process defined as necessary for san
ity such as appreciating the nature and quality of his act
and knowing that it was wrong).
   Mott was careful to distinguish this kind of opinion
evidence from observation evidence generally and even
from observation evidence that an expert witness might
offer, such as descriptions of a defendant’s tendency to
think in a certain way or his behavioral characteristics;
the Arizona court made it clear that this sort of testimony
was perfectly admissible to rebut the prosecution’s evi
dence of mens rea, 187 Ariz., at 544, 931 P. 2d, at 1054.
Thus, only opinion testimony going to mental defect or
disease, and its effect on the cognitive or moral capaci
ties on which sanity depends under the Arizona rule, is
restricted.
20                       CLARK v. ARIZONA

                          Opinion of the Court

  In this case, the trial court seems to have applied the
Mott restriction to all evidence offered by Clark for the
purpose of showing what he called his inability to form the
required mens rea, see, e.g., Record, Doc. 406, pp. 7–10,
(that is, an intent to kill a police officer on duty, or an
understanding that he was engaging in the act of killing
such an officer, see Ariz. Rev. Stat. Ann. §13–1105(A)(3)
(West Supp. 2005)). Thus, the trial court’s restriction may
have covered not only mental-disease and capacity evi
dence as just defined, but also observation evidence of
fered by lay (and expert) witnesses who described Clark’s
unusual behavior. Clark’s objection to the application of
the Mott rule does not, however, turn on the distinction
between lay and expert witnesses or the kinds of testi
mony they were competent to present.32
                              C
   There is some, albeit limited, disagreement between the
dissent and ourselves about the scope of the claim of error
properly before us. To start with matters of agreement, all
Members of the Court agree that Clark’s general attack on
the Mott rule covers its application in confining considera
tion of capacity evidence to the insanity defense.
   In practical terms, our agreement on issues presented
extends to a second point. JUSTICE KENNEDY understands
that Clark raised an objection to confining mental-disease
evidence to the insanity issue. As he sees it, Clark in
effect claimed that in dealing with the issue of mens rea
the trial judge should have considered expert testimony on
what may characteristically go through the mind of a
——————
  32 With respect to “the limited factual issues the trial court held it

could consider under [Ariz. Rev. Stat. Ann. §]13–502 and Mott, defense
counsel made no additional ‘offer of proof’ at the conclusion of the case
but preserved [Clark’s] legal contentions by asking the court to consider
all of the evidence presented in determining whether the state had
proved its case.” Brief for Petitioner 10, n. 20 (citations omitted).
                 Cite as: 548 U. S. ____ (2006)           21

                     Opinion of the Court

schizophrenic, when the judge considered what in fact was
in Clark’s mind at the time of the shooting. See post, at 3
(dissenting opinion) (“[T]he opinion that Clark had para
noid schizophrenia—an opinion shared by experts for both
the prosecution and defense—bears on efforts to deter
mine, as a factual matter, whether he knew he was killing
a police officer”). He thus understands that defense coun
sel claimed a right to rebut the State’s mens rea demon
stration with testimony about how schizophrenics may
hallucinate voices and other sounds, about their character
istic failure to distinguish the content of their imagination
from what most people perceive as exterior reality, and so
on. It is important to be clear that this supposed objection
was not about dealing with testimony based on observa
tion of Clark showing that he had auditory hallucinations
when he was driving around, or failed in fact to appreciate
objective reality when he shot; this objection went to use of
testimony about schizophrenics, not about Clark in par
ticular. While we might dispute how clearly Clark raised
this objection, we have no doubt that the objection falls
within a general challenge to the Mott rule; we understand
that Mott is meant to confine to the insanity defense any
consideration of characteristic behavior associated with
mental disease, see 187 Ariz., at 544, 931 P. 2d, at 1054
(contrasting State v. Christensen, 129 Ariz. 32, 628 P. 2d
580 (1991), and State v. Gonzales, 140 Ariz. 349, 681 P. 2d
1368 (1984)). We will therefore assume for argument that
Clark raised this claim, as we consider the due process
challenge to the Mott rule.
   The point on which we disagree with the dissent, how
ever, is this: did Clark apprise the Arizona courts that he
believed the trial judge had erroneously limited the con
sideration of observation evidence, whether from lay wit
nesses like Clark’s mother or (possibly) the expert wit
nesses who observed him? This sort of evidence was not
covered by the Mott restriction, and confining it to the
22                        CLARK v. ARIZONA

                           Opinion of the Court

insanity issue would have been an erroneous application
of Mott as a matter of Arizona law. For the following
reasons we think no such objection was made in a way the
Arizona courts could have understood it, and that no such
issue is before us now. We think the only issue properly
before us is the challenge to Mott on due process grounds,
comprising objections to limits on the use of mental-
disease and capacity evidence.
  It is clear that the trial judge intended to apply Mott:
     “[R]ecognizing that much of the evidence that [the de
     fense is] going to be submitting, in fact all of it, as far
     as I know . . . that has to do with the insanity could
     also arguably be made along the lines of the Mott is
     sue as to form and intent and his capacity for the in
     tent. I’m going to let you go ahead and get all that
     stuff in because it goes to the insanity issue and be
     cause we’re not in front of a jury. At the end, I’ll let
     you make an offer of proof as to the intent, the Mott
     issues, but I still think the supreme court decision is
     the law of the land in this state.” App. 9.
  At no point did the trial judge specify any particular
evidence that he refused to consider on the mens rea issue.
Nor did defense counsel specify any observation or other
particular evidence that he claimed was admissible but
wrongly excluded on the issue of mens rea, so as to pro
duce a clearer ruling on what evidence was being re
stricted on the authority of Mott and what was not. He
made no “offer of proof” in the trial court;33 and although
——————
  33 We do not agree with the State’s argument that the failure to make

an offer of proof, see n. 4, supra, is a bar to pressing Clark’s claim about
the admissibility of mental-illness or capacity evidence as to mens rea,
see Brief for Respondent 27–29, especially when the Arizona Court of
Appeals rejected Clark’s argument on the merits rather than clearly on
this ground, see App. 351–353; see also Michigan v. Long, 463 U. S.
1032, 1042 (1983) (“[I]t is not clear from the opinion itself that the state
                     Cite as: 548 U. S. ____ (2006)                  23

                         Opinion of the Court

his brief in the Arizona Court of Appeals stated at one
point that it was not inconsistent with Mott to consider
nonexpert evidence indicating mental illness on the issue
of mens rea, and argued that the trial judge had failed to
do so, Appellant’s Opening Brief in No. 1CA–CR–03–0851
etc. (Ariz. Ct. App.), pp. 48–49 (hereinafter Appellant’s
Opening Brief), he was no more specific than that, see,
e.g., id., at 52 (“The Court’s ruling in Mott and the trial
court’s refusal to consider whether as a result of suffering
from paranoid schizophrenia [Clark] could not formulate
the mens rea necessary for first degree murder violated his
right to due process”). Similarly, we read the Arizona
Court of Appeals to have done nothing more than rely on
Mott to reject the claim that due process forbids restricting
evidence bearing on “[a]bility to [f]orm [m]ens [r]ea,” App.
351 (emphasis in original), (i.e., mental-disease and capac
ity evidence) to the insanity determination. See id., at
351–353.
   This failure in the state courts to raise any clear claim
about observation evidence, see Appellant’s Opening Brief
46–52, is reflected in the material addressed to us, see
Brief for Petitioner 13–32. In this Court both the question
presented and the following statement of his position were
couched in similarly worded general terms:
     “I. ERIC WAS DENIED DUE PROCESS WHEN THE
     TRIAL    COURT    REFUSED  TO   CONSIDER
     EVIDENCE OF HIS SEVERE MENTAL ILLNESS IN
     DETERMINING FACTUALLY WHETHER THE
     PROSECUTION      PROVED    THE   MENTAL
     ELEMENTS OF THE CRIME CHARGED.” Id., at 13.
But as his counsel made certain beyond doubt in his reply
brief, 

—————— 

court relied upon an adequate and independent state ground and . . . it 

fairly appears that the state court rested its decision primarily on
federal law”).
24                        CLARK v. ARIZONA

                          Opinion of the Court

     “Eric’s Point I is and always has been an attack on the
     rule of State v. Mott, which both courts below held ap
     plicable and binding. Mott announced a categorical
     ‘rejection of the use of psychological testimony to chal
     lenge the mens rea element of a crime,’ and upheld
     this rule against federal due process challenge.” Re
     ply Brief for Petitioner 2 (citations omitted).
This explanation is supported by other statements in
Clark’s briefs in both the State Court of Appeals and this
Court, replete with the consistently maintained claim that
it was error to limit evidence of mental illness and inca
pacity to its bearing on the insanity defense, excluding it
from consideration on the element of mens rea. See, e.g.,
Appellant’s Opening Brief 46, 47, 51; Brief for Petitioner
11, 13, 16, 20–23.
   In sum, the trial court’s ruling, with its uncertain edges,
may have restricted observation evidence admissible on
mens rea to the insanity defense alone, but we cannot be
sure.34 But because a due process challenge to such a
restriction of observation evidence was, by our measure,
neither pressed nor passed upon in the Arizona Court of
Appeals, we do not consider it. See, e.g., Kentucky v. Stin
cer, 482 U. S. 730, 747, n. 22 (1987); Illinois v. Gates, 462
U. S. 213, 217–224 (1983). What we do know, and now
——————
  34We therefore have no reason to believe that the courts of Arizona

would have failed to restrict their application of Mott to the professional
testimony the Mott opinion was stated to cover, if Clark’s counsel had
specified any observation evidence he claimed to be generally admissi
ble and relevant to mens rea. Nothing that we hold here is authority
for restricting a factfinder’s consideration of observation evidence
indicating state of mind at the time of a criminal offense (conventional
mens rea evidence) as distinct from professional mental-disease or
capacity evidence going to ability to form a certain state of mind during
a period that includes the time of the offense charged. And, of course,
nothing held here prevents Clark from raising this discrete claim when
the case returns to the courts of Arizona, if consistent with the State’s
procedural rules.
                 Cite as: 548 U. S. ____ (2006)           25

                     Opinion of the Court

consider, is Clark’s claim that Mott denied due process
because it “preclude[d] Eric from contending that . . .
factual inferences” of the “mental states which were neces
sary elements of the crime charged” “should not be drawn
because the behavior was explainable, instead, as a mani
festation of his chronic paranoid schizophrenia.” Brief for
Petitioner 13 (emphasis in original). We consider the
claim, as Clark otherwise puts it, that “Arizona’s prohibi
tion of ‘diminished capacity’ evidence by criminal defen
dants violates” due process, ibid.
                            D
  Clark’s argument that the Mott rule violates the Four
teenth Amendment guarantee of due process turns on the
application of the presumption of innocence in criminal
cases, the presumption of sanity, and the principle that a
criminal defendant is entitled to present relevant and
favorable evidence on an element of the offense charged
against him.
                             1
  The first presumption is that a defendant is innocent
unless and until the government proves beyond a reason
able doubt each element of the offense charged, see Patter
son, 432 U. S., at 210–211; In re Winship, 397 U. S. 358,
361–364 (1970), including the mental element or mens rea.
Before the last century, the mens rea required to be proven
for particular offenses was often described in general
terms like “malice,” see, e.g., In re Eckart, 166 U. S. 481
(1897); 4 W. Blackstone, Commentaries *21 (“[A]n unwar
rantable act without a vicious will is no crime at all”), but
the modern tendency has been toward more specific de
scriptions, as shown in the Arizona statute defining the
murder charged against Clark: the State had to prove that
in acting to kill the victim, Clark intended to kill a law
enforcement officer on duty or knew that the victim was
26                        CLARK v. ARIZONA

                          Opinion of the Court

such an officer on duty. See generally Gardner, The Mens
Rea Enigma: Observations on the Role of Motive in the
Criminal Law Past and Present, 1993 Utah L. Rev. 635.
As applied to mens rea (and every other element), the force
of the presumption of innocence is measured by the force
of the showing needed to overcome it, which is proof be
yond a reasonable doubt that a defendant’s state of mind
was in fact what the charge states. See Winship, supra, at
361–363.
                              2
   The presumption of sanity is equally universal in some
variety or other, being (at least) a presumption that a
defendant has the capacity to form the mens rea necessary
for a verdict of guilt and the consequent criminal respon
sibility. See Leland, 343 U. S., at 799; Davis v. United
States, 160 U. S. 469, 486–487 (1895); M’Naghten’s Case, 10
Cl. & Fin., at 210, 8 Eng. Rep., at 722; see generally 1
LaFave, Substantive Criminal Law §8.3(a), at 598–599,
and n. 1. This presumption dispenses with a requirement
on the government’s part to include as an element of every
criminal charge an allegation that the defendant had such
a capacity.35 The force of this presumption, like the pre
sumption of innocence, is measured by the quantum of
evidence necessary to overcome it; unlike the presumption
of innocence, however, the force of the presumption of
sanity varies across the many state and federal jurisdic
tions, and prior law has recognized considerable leeway on
the part of the legislative branch in defining the presump
tion’s strength through the kind of evidence and degree of
persuasiveness necessary to overcome it, see Fisher v.

United States, 328 U. S. 463, 466–476 (1946).36

—————— 

  35 A  legislature is nonetheless free to require affirmative proof of san
ity by the way it describes a criminal offense, see Dixon v. United
States, ante, at ___ (slip op., at 7–9).
   36 Although a desired evidentiary use is restricted, that is not equiva
                      Cite as: 548 U. S. ____ (2006)                     27

                           Opinion of the Court

  There are two points where the sanity or capacity pre
sumption may be placed in issue. First, a State may allow
a defendant to introduce (and a factfinder to consider)
evidence of mental disease or incapacity for the bearing it
can have on the government’s burden to show mens rea.
See, e.g., State v. Perez, 882 A. 2d 574, 584 (R. I. 2005).37
In such States the evidence showing incapacity to form the
guilty state of mind, for example, qualifies the probative
force of other evidence, which considered alone indicates
that the defendant actually formed the guilty state of
mind. If it is shown that a defendant with mental disease
thinks all blond people are robots, he could not have in
tended to kill a person when he shot a man with blond
hair, even though he seemed to act like a man shooting
another man.38 In jurisdictions that allow mental-disease
and capacity evidence to be considered on par with any
——————
lent to a Sandstrom presumption. See Sandstrom v. Montana, 442 U. S.
510, 514–524 (1979) (due process forbids use of presumption that relieves
the prosecution of burden of proving mental state by inference of intent
from an act).
   37 In fact, Oregon had this scheme in place when we decided Leland v.

Oregon, 343 U. S. 790, 794–796 (1952). We do not, however, read any
part of Leland to require as a matter of due process that evidence of
incapacity be considered to rebut the mens rea element of a crime.
   38We reject the State’s argument that mens rea and insanity, as cur

rently understood, are entirely distinguishable, so that mental-disease
and capacity evidence relevant to insanity is simply irrelevant to mens
rea. Not only does evidence accepted as showing insanity trump mens
rea, but evidence of behavior close to the time of the act charged may
indicate both the actual state of mind at that time and also an enduring
incapacity to form the criminal state of mind necessary to the offense
charged. See Brief for American Psychiatric Association et al. as Amici
Curiae 12–13; Arenella, The Diminished Capacity and Diminished
Responsibility Defenses: Two Children of a Doomed Marriage, 77
Colum. L. Rev. 827, 834–835 (1977); cf. Powell v. Texas, 392 U. S. 514,
535–536 (1968) (plurality opinion) (the “doctrines of actus reus, mens rea,
insanity, mistake, justification, and duress” are a “collection of interlock
ing and overlapping concepts which the common law has utilized to assess
the moral accountability of an individual for his antisocial deeds”).
28                   CLARK v. ARIZONA

                     Opinion of the Court

other relevant evidence when deciding whether the prose
cution has proven mens rea beyond a reasonable doubt,
the evidence of mental disease or incapacity need only
support what the factfinder regards as a reasonable doubt
about the capacity to form (or the actual formation of) the
mens rea, in order to require acquittal of the charge.
Thus, in these States the strength of the presumption of
sanity is no greater than the strength of the evidence of
abnormal mental state that the factfinder thinks is
enough to raise a reasonable doubt.
   The second point where the force of the presumption of
sanity may be tested is in the consideration of a defense of
insanity raised by a defendant.          Insanity rules like
M’Naghten and the variants discussed in Part II, supra,
are attempts to define, or at least to indicate, the kinds of
mental differences that overcome the presumption of
sanity or capacity and therefore excuse a defendant from
customary criminal responsibility, see Jones, 463 U. S., at
373, n. 4 (Brennan, J., dissenting); D. Hermann, The Insan
ity Defense: Philosophical, Historical and Legal Perspec
tives 4 (1983) (“A central significance of the insanity de
fense . . . is the separation of nonblameworthy from
blameworthy offenders”), even if the prosecution has
otherwise overcome the presumption of innocence by
convincing the factfinder of all the elements charged be
yond a reasonable doubt. The burden that must be carried
by a defendant who raises the insanity issue, again, de
fines the strength of the sanity presumption. A State may
provide, for example, that whenever the defendant raises
a claim of insanity by some quantum of credible evidence,
the presumption disappears and the government must
prove sanity to a specified degree of certainty (whether
beyond reasonable doubt or something less). See, e.g.,
Commonwealth v. Keita, 429 Mass. 843, 846, 712 N. E. 2d
65, 68 (1999). Or a jurisdiction may place the burden of
persuasion on a defendant to prove insanity as the appli
                     Cite as: 548 U. S. ____ (2006)                    29

                          Opinion of the Court

cable law defines it, whether by a preponderance of the
evidence or to some more convincing degree, see Ariz. Rev.
Stat. Ann. §13–502(C) (West 2001); Leland, 343 U. S., at
798. In any case, the defendant’s burden defines the
presumption of sanity, whether that burden be to burst a
bubble or to show something more.
                             3
  The third principle implicated by Clark’s argument is a
defendant’s right as a matter of simple due process to
present evidence favorable to himself on an element that
must be proven to convict him.39 As already noted, evi
dence tending to show that a defendant suffers from men
tal disease and lacks capacity to form mens rea is relevant
to rebut evidence that he did in fact form the required
mens rea at the time in question; this is the reason that
Clark claims a right to require the factfinder in this case
to consider testimony about his mental illness and his
incapacity directly, when weighing the persuasiveness of
other evidence tending to show mens rea, which the prose
cution has the burden to prove.
  As Clark recognizes, however, the right to introduce
relevant evidence can be curtailed if there is a good reason
for doing that. “While the Constitution . . . prohibits the
exclusion of defense evidence under rules that serve no
legitimate purpose or that are disproportionate to the ends
that they are asserted to promote, well-established rules of
evidence permit trial judges to exclude evidence if its
probative value is outweighed by certain other factors
such as unfair prejudice, confusion of the issues, or poten

——————
  39 Clark’s argument assumes that Arizona’s rule is a rule of evidence,

rather than a redefinition of mens rea, see Montana v. Egelhoff, 518
U. S. 37, 58–59 (1996) (GINSBURG, J., concurring in judgment); id., at 71
(O’Connor, J., dissenting). We have no reason to view the rule otherwise,
and on this assumption, it does not violate due process, see infra, at 31–
39.
30                   CLARK v. ARIZONA

                      Opinion of the Court

tial to mislead the jury.” Holmes v. South Carolina, 547
U. S. ___, ___ (2006) (slip op., at 6); see Crane v. Kentucky,
476 U. S. 683, 689–690 (1986) (permitting exclusion of
evidence that “poses an undue risk of ‘harassment, preju
dice, [or] confusion of the issues’” (quoting Delaware v. Van
Arsdall, 475 U. S. 673, 679 (1986))); see also Egelhoff, 518
U. S. 37; Chambers v. Mississippi, 410 U. S. 284, 302 (1973).
And if evidence may be kept out entirely, its consideration
may be subject to limitation, which Arizona claims the
power to impose here. State law says that evidence of
mental disease and incapacity may be introduced and
considered, and if sufficiently forceful to satisfy the defen
dant’s burden of proof under the insanity rule it will dis
place the presumption of sanity and excuse from criminal
responsibility. But mental-disease and capacity evidence
may be considered only for its bearing on the insanity
defense, and it will avail a defendant only if it is persua
sive enough to satisfy the defendant’s burden as defined
by the terms of that defense. The mental-disease and
capacity evidence is thus being channeled or restricted to
one issue and given effect only if the defendant carries the
burden to convince the factfinder of insanity; the evidence
is not being excluded entirely, and the question is whether
reasons for requiring it to be channeled and restricted are
good enough to satisfy the standard of fundamental fair
ness that due process requires. We think they are.
                             E
                             1
  The first reason supporting the Mott rule is Arizona’s
authority to define its presumption of sanity (or capacity
or responsibility) by choosing an insanity definition, as
discussed in Part II, supra, and by placing the burden of
persuasion on defendants who claim incapacity as an
excuse from customary criminal responsibility. No one,
certainly not Clark here, denies that a State may place a
                      Cite as: 548 U. S. ____ (2006)                    31

                          Opinion of the Court

burden of persuasion on a defendant claiming insanity, see
Leland, supra, at 797–799 (permitting a State, consistent
with due process, to require the defendant to bear this
burden). And Clark presses no objection to Arizona’s
decision to require persuasion to a clear and convincing
degree before the presumption of sanity and normal re
sponsibility is overcome. See Brief for Petitioner 18, n. 25.
   But if a State is to have this authority in practice as
well as in theory, it must be able to deny a defendant the
opportunity to displace the presumption of sanity more
easily when addressing a different issue in the course of
the criminal trial. Yet, as we have explained, just such an
opportunity would be available if expert testimony of
mental disease and incapacity could be considered for
whatever a factfinder might think it was worth on the
issue of mens rea.40 As we mentioned, the presumption of
sanity would then be only as strong as the evidence a
factfinder would accept as enough to raise a reasonable
doubt about mens rea for the crime charged; once reason
able doubt was found, acquittal would be required, and the
standards established for the defense of insanity would go
by the boards.
   Now, a State is of course free to accept such a possibility
in its law. After all, it is free to define the insanity de
fense by treating the presumption of sanity as a bursting
bubble, whose disappearance shifts the burden to the
prosecution to prove sanity whenever a defendant pre
sents any credible evidence of mental disease or incapac
ity. In States with this kind of insanity rule, the legisla
ture may well be willing to allow such evidence to be
considered on the mens rea element for whatever the
——————
   40 Cf. post, at 3 (KENNEDY, J., dissenting) (“The psychiatrist’s explana

tion of Clark’s condition was essential to understanding how he proc
esses sensory data and therefore to deciding what information was in
his mind at the time of the shooting. Simply put, knowledge relies on
cognition, and cognition can be affected by schizophrenia”).
32                        CLARK v. ARIZONA

                          Opinion of the Court

factfinder thinks it is worth. What counts for due process,
however, is simply that a State that wishes to avoid a
second avenue for exploring capacity, less stringent for a
defendant, has a good reason for confining the considera
tion of evidence of mental disease and incapacity to the
insanity defense.
   It is obvious that Arizona’s Mott rule reflects such a
choice. The State Supreme Court pointed out that the
State had declined to adopt a defense of diminished capac
ity (allowing a jury to decide when to excuse a defendant
because of greater than normal difficulty in conforming to
the law).41 The court reasoned that the State’s choice
would be undercut if evidence of incapacity could be con
sidered for whatever a jury might think sufficient to raise
a reasonable doubt about mens rea, even if it did not show
insanity. 187 Ariz., at 541, 931 P. 2d, at 1051. In other
words, if a jury were free to decide how much evidence of
mental disease and incapacity was enough to counter
evidence of mens rea to the point of creating a reasonable
doubt, that would in functional terms be analogous to
allowing jurors to decide upon some degree of diminished
capacity to obey the law, a degree set by them, that would

——————
   41 Though the term “diminished capacity” has been given different

meanings, see, e.g., Morse, Undiminished Confusion in Diminished
Capacity, 75 J. Crim. L. & C. 1 (1984) (“The diminished capacity
doctrine allows a criminal defendant to introduce evidence of mental
abnormality at trial either to negate a mental element of the crime
charged, thereby exonerating the defendant of that charge, or to reduce
the degree of crime for which the defendant may be convicted, even if
the defendant’s conduct satisfied all the formal elements of a higher
offense”), California, a jurisdiction with which the concept has tradi
tionally been associated, understood it to be simply a “ ‘showing that the
defendant’s mental capacity was reduced by mental illness, mental
defect or intoxication,’ ” People v. Berry, 18 Cal. 3d 509, 517, 556 P. 2d
777, 781 (1976) (in banc) (quoting People v. Castillo, 70 Cal. 2d 264,
270, 449 P. 2d 449, 452 (1969); emphasis deleted), abrogated by Cal.
Penal Code Ann. §§25(a), 28(a)–(b), 29 (West 1999 and Supp. 2006).
                      Cite as: 548 U. S. ____ (2006)                    33

                          Opinion of the Court

prevail as a stand-alone defense.42
                              2
   A State’s insistence on preserving its chosen standard of
legal insanity cannot be the sole reason for a rule like
Mott, however, for it fails to answer an objection the dis
sent makes in this case. See post, at 10–18 (opinion of
KENNEDY, J.). An insanity rule gives a defendant already
found guilty the opportunity to excuse his conduct by
showing he was insane when he acted, that is, that he did
not have the mental capacity for conventional guilt and
criminal responsibility. But, as the dissent argues, if the
same evidence that affirmatively shows he was not guilty
by reason of insanity (or “guilty except insane” under
Arizona law, Ariz. Rev. Stat. Ann. §13–502(A) (West
2001)) also shows it was at least doubtful that he could
form mens rea, then he should not be found guilty in the
first place; it thus violates due process when the State
impedes him from using mental-disease and capacity
evidence directly to rebut the prosecution’s evidence that
he did form mens rea.
   Are there, then, characteristics of mental-disease and
capacity evidence giving rise to risks that may reasonably
be hedged by channeling the consideration of such evi
dence to the insanity issue on which, in States like Ari
——————
   42 It is beyond question that Arizona may preclude such a defense, see

Fisher v. United States, 328 U. S. 463, 466–476 (1946), and there is no
doubt that the Arizona Legislature meant to do so, see Ariz. Rev. Stat.
Ann. §13–502(A) (West 2001) (“Mental disease or defect does not include
disorders that result from acute voluntary intoxication or withdrawal
from alcohol or drugs, character defects, psychosexual disorders or im
pulse control disorders. Conditions that do not constitute legal insanity
include but are not limited to momentary, temporary conditions arising
from the pressure of the circumstances, moral decadence, depravity or
passion growing out of anger, jealousy, revenge, hatred or other motives in
a person who does not suffer from a mental disease or defect or an abnor
mality that is manifested only by criminal conduct”).
34                    CLARK v. ARIZONA

                      Opinion of the Court

zona, a defendant has the burden of persuasion? We think
there are: in the controversial character of some categories
of mental disease, in the potential of mental-disease evi
dence to mislead, and in the danger of according greater
certainty to capacity evidence than experts claim for it.
   To begin with, the diagnosis may mask vigorous debate
within the profession about the very contours of the men
tal disease itself. See, e.g., American Psychiatric Associa
tion, Diagnostic and Statistical Manual of Mental Disor
ders xxxiii (4th ed. text rev. 2000) (hereinafter DSM–IV–
TR) (“DSM–IV reflects a consensus about the classification
and diagnosis of mental disorders derived at the time of its
initial publication. New knowledge generated by research
or clinical experience will undoubtedly lead to an in
creased understanding of the disorders included in DSM–
IV, to the identification of new disorders, and to the re
moval of some disorders in future classifications. The text
and criteria sets included in DSM–IV will require recon
sideration in light of evolving new information”); P.
Caplan, They Say You’re Crazy: How the World’s Most
Powerful Psychiatrists Decide Who’s Normal (1995) (criti
cism by former consultant to the DSM against some of the
DSM’s categories). And Members of this Court have pre
viously recognized that the end of such debate is not im
minent. See Jones, 463 U. S., at 364–365, n. 13 (“‘The only
certain thing that can be said about the present state of
knowledge and therapy regarding mental disease is that
science has not reached finality of judgment’ ” (quoting
Greenwood v. United States, 350 U. S. 366, 375 (1956)));
Powell v. Texas, 392 U. S. 514, 537 (1968) (plurality opinion)
(“It is simply not yet the time to write into the Constitution
formulas cast in terms whose meaning, let alone relevance,
is not yet clear . . . to doctors”). Though we certainly do not
“condem[n mental-disease evidence] wholesale”, Brief for
American Psychiatric Association et al. as Amici Curiae
15, the consequence of this professional ferment is a general
                      Cite as: 548 U. S. ____ (2006)                      35

                           Opinion of the Court

caution in treating psychological classifications as predi
cates for excusing otherwise criminal conduct.
   Next, there is the potential of mental-disease evidence to
mislead jurors (when they are the factfinders) through the
power of this kind of evidence to suggest that a defendant
suffering from a recognized mental disease lacks cognitive,
moral, volitional, or other capacity, when that may not be
a sound conclusion at all. Even when a category of mental
disease is broadly accepted and the assignment of a defen
dant’s behavior to that category is uncontroversial, the
classification may suggest something very significant
about a defendant’s capacity, when in fact the classifica
tion tells us little or nothing about the ability of the defen
dant to form mens rea or to exercise the cognitive, moral,
or volitional capacities that define legal sanity.43 See
DSM–IV–TR xxxii–xxxiii (“When the DSM–IV categories,
criteria, and textual descriptions are employed for forensic
purposes, there are significant risks that diagnostic infor
mation will be misused or misunderstood. These dangers
arise because of the imperfect fit between the questions of
ultimate concern to the law and the information contained
in a clinical diagnosis. In most situations, the clinical
diagnosis of a DSM–IV mental disorder is not sufficient to
establish the existence for legal purposes of . . . ‘mental
diseas[e]’ or ‘mental defect.’ In determining whether an
individual meets a specified legal standard (e.g., for . . .
criminal responsibility . . .), additional information is
usually required beyond that contained in the DSM–IV
diagnosis”). The limits of the utility of a professional
——————
   43 Our observation about the impact of mental-disease evidence on

understandings of capacity in no way undermines the assertion by the
American Psychiatric Association, the American Psychological Associa
tion, and the American Academy of Psychiatry in this case that
“[e]xpert evidence of mental disorders . . . is . . . relevant to the mental-
state issues raised by mens rea requirements,” Brief for American
Psychiatric Association et al. as Amici Curiae 15.
36                   CLARK v. ARIZONA

                     Opinion of the Court

disease diagnosis are evident in the dispute between the
two testifying experts in this case; they agree that Clark
was schizophrenic, but they come to opposite conclusions
on whether the mental disease in his particular case left
him bereft of cognitive or moral capacity. Evidence of
mental disease, then, can easily mislead; it is very easy to
slide from evidence that an individual with a profession
ally recognized mental disease is very different, into
doubting that he has the capacity to form mens rea,
whereas that doubt may not be justified. And of course, in
the cases mentioned before, in which the categorization is
doubtful or the category of mental disease is itself subject
to controversy, the risks are even greater that opinions
about mental disease may confuse a jury into thinking the
opinions show more than they do. Because allowing men
tal-disease evidence on mens rea can thus easily mislead,
it is not unreasonable to address that tendency by con-
fining consideration of this kind of evidence to insanity,
on which a defendant may be assigned the burden of
persuasion.
   There are, finally, particular risks inherent in the opin
ions of the experts who supplement the mental-disease
classifications with opinions on incapacity: on whether the
mental disease rendered a particular defendant incapable
of the cognition necessary for moral judgment or mens rea
or otherwise incapable of understanding the wrongfulness
of the conduct charged. Unlike observational evidence
bearing on mens rea, capacity evidence consists of judg
ment, and judgment fraught with multiple perils: a defen
dant’s state of mind at the crucial moment can be elusive
no matter how conscientious the enquiry, and the law’s
categories that set the terms of the capacity judgment are
not the categories of psychology that govern the expert’s
professional thinking. Although such capacity judgments
may be given in the utmost good faith, their potentially
tenuous character is indicated by the candor of the defense
                  Cite as: 548 U. S. ____ (2006)             37

                      Opinion of the Court

expert in this very case. Contrary to the State’s expert, he
testified that Clark lacked the capacity to appreciate the
circumstances realistically and to understand the wrong
fulness of what he was doing, App. 48–49, but he said that
“no one knows exactly what was on [his] mind” at the time
of the shooting, id., at 48. And even when an expert is
confident that his understanding of the mind is reliable,
judgment addressing the basic categories of capacity
requires a leap from the concepts of psychology, which are
devised for thinking about treatment, to the concepts of
legal sanity, which are devised for thinking about criminal
responsibility. See Insanity Defense Work Group, Ameri
can Psychiatric Association Statement on the Insanity
Defense, 140 Am. J. Psychiatry 681, 686 (1983), reprinted
in 2 The Role of Mental Illness in Criminal Trials 117, 122
(J. Moriarty ed. 2001) (“The American Psychiatric Associa
tion is not opposed to legislatures restricting psychiatric
testimony about the . . . ultimate legal issues concerning
the insanity defense. . . . When . . . ‘ultimate issue’ ques
tions are formulated by the law and put to the expert
witness who must then say ‘yea’ or ‘nay,’ then the expert
witness is required to make a leap in logic. He no longer
addresses himself to medical concepts but instead must
infer or intuit what is in fact unspeakable, namely, the
probable relationship between medical concepts and legal
or moral constructs such as free will. These impermissible
leaps in logic made by expert witnesses confuse the
jury. . . . This state of affairs does considerable injustice to
psychiatry and, we believe, possibly to criminal defen
dants. These psychiatric disagreements . . . cause less
than fully understanding juries or the public to conclude
that psychiatrists cannot agree. In fact, in many criminal
insanity trials both prosecution and defense psychiatrists
do agree about the nature and even the extent of mental
disorder exhibited by the defendant at the time of the act”
(emphasis in original; footnote omitted)); DSM–IV–TR
38                       CLARK v. ARIZONA

                         Opinion of the Court

xxxii–xxxiii; P. Giannelli & E. Imwinkelried, Scientific
Evidence §9–3(B), p. 286 (1986) (“[N]o matter how the test
for insanity is phrased, a psychiatrist or psychologist is no
more qualified than any other person to give an opinion
about whether a particular defendant’s mental condition
satisfies the legal test for insanity”); cf. R. Slovenko, Psy
chiatry and Criminal Culpability 55 (1995) (“The scope of
the DSM is wide-ranging and includes ‘conduct disorders’
but ‘evil’ is not mentioned”). In sum, these empirical and
conceptual problems add up to a real risk that an expert’s
judgment in giving capacity evidence will come with an
apparent authority that psychologists and psychiatrists do
not claim to have. We think that this risk, like the diffi
culty in assessing the significance of mental-disease evi
dence, supports the State’s decision to channel such expert
testimony to consideration on the insanity defense, on
which the party seeking the benefit of this evidence has
the burden of persuasion.
   It bears repeating that not every State will find it
worthwhile to make the judgment Arizona has made, and
the choices the States do make about dealing with the
risks posed by mental-disease and capacity evidence will
reflect their varying assessments about the presumption
of sanity as expressed in choices of insanity rules.44 The
point here simply is that Arizona has sensible reasons
to assign the risks as it has done by channeling the
evidence.45
——————
   44 A State in which the burden of persuasion as to a defendant’s san

ity lies with the prosecution might also be justified in restricting
mental-disease and capacity evidence to insanity determinations owing
to the potential of mental-disease evidence to mislead and the risk of
misjudgment inherent in capacity evidence. We need not, in the
context of this case, address that issue.
   45 Arizona’s rule is supported by a further practical reason, though

not as weighty as those just considered. As mentioned before, if sub
stantial mental-disease and capacity evidence is accepted as rebutting
mens rea in a given case, the affirmative defense of insanity will proba
                      Cite as: 548 U. S. ____ (2006) 
                  39

                          Opinion of the Court


                                 F

  Arizona’s rule serves to preserve the State’s chosen
standard for recognizing insanity as a defense and to avoid
confusion and misunderstanding on the part of jurors.46
For these reasons, there is no violation of due process
under Chambers and its progeny, and no cause to claim
that channeling evidence on mental disease and capacity
offends any “ ‘principle of justice so rooted in the traditions
and conscience of our people as to be ranked as fundamen
tal,’ ” Patterson, 432 U. S., at 202 (quoting Speiser, 357 U. S.,
at 523).
                       * *    *
  The judgment of the Court of Appeals of Arizona is,
accordingly, affirmed.
                                      It is so ordered.




——————
bly not be reached or ruled upon; the defendant will simply be acquitted
(or perhaps convicted of a lesser included offense). If an acquitted
defendant suffers from a mental disease or defect that makes him
dangerous, he will neither be confined nor treated psychiatrically
unless a judge so orders after some independent commitment proceed
ing. But if a defendant succeeds in showing himself insane, Arizona
law (and presumably that of every other State with an insanity rule)
will require commitment and treatment as a consequence of that
finding without more. It makes sense, then, to channel capacity evi
dence to the issue structured to deal with mental incapacity when such
a claim is raised successfully. See, e.g., Jones, 463 U. S., at 368 (“The
purpose of commitment following an insanity acquittal . . . is to treat the
individual’s mental illness and protect him and society from his potential
dangerousness”).
   46 The rule also deals in a practical way with those whose insanity

has been shown to make them dangerous to others. See n. 45, supra.
                 Cite as: 548 U. S. ____ (2006)            1

                     Opinion of BREYER, J.

SUPREME COURT OF THE UNITED STATES
                         _________________

                         No. 05–5966
                         _________________


  ERIC MICHAEL CLARK, PETITIONER v. ARIZONA
   ON WRIT OF CERTIORARI TO THE COURT OF APPEALS OF

                 ARIZONA, DIVISION ONE

                        [June 29, 2006]

   JUSTICE BREYER, concurring in part and dissenting in
part.
   As I understand the Court’s opinion, it distinguishes
among three categories of evidence related to insanity: (1)
fact-related evidence as to the defendant’s specific state of
mind at the time of the crime, e.g., evidence that shows he
thought the policeman was not a human being; (2) expert
opinion evidence that the defendant suffered from a men
tal disease that would have affected his capacity to form
an intent to kill a policeman, e.g., that he suffers from a
disease of a kind where powerful voices command the
sufferer to kill; and (3) expert opinion evidence that the
defendant was legally insane, e.g., evidence that he did not
know right from wrong. Ante, at 16–18.
   I agree with the Court’s basic categorization. I also
agree that the Constitution permits a State to provide for
consideration of the second and third types of evidence
solely in conjunction with the insanity defense. A State
might reasonably fear that, without such a rule, the types
of evidence as to intent would become confused in the
jury’s mind, indeed that in some cases the insanity ques
tion would displace the intent question as the parties
litigate both simultaneously.
   Nonetheless, I believe the distinction among these
kinds of evidence will be unclear in some cases. And
though I accept the majority’s reading of the record, I
2                    CLARK v. ARIZONA

                     Opinion of BREYER, J.

remain concerned as to whether the lower courts, in set
ting forth and applying State v. Mott, 187 Ariz. 536, 931
P. 2d 1046 (en banc), cert. denied, 520 U. S. 1234 (1997),
focused with sufficient directness and precision upon the
distinction.
   Consequently, I would remand this case so that Ari
zona’s courts can determine whether Arizona law, as set
forth in Mott and other cases, is consistent with the dis
tinction the Court draws and whether the trial court so
applied Arizona law here. I would also reserve the ques
tion (as I believe the Court has done) as to the burden of
persuasion in a case where the defendant produces suffi
cient evidence of the second kind as to raise a reasonable
doubt suggesting that he suffered from a mental illness so
severe as to prevent him from forming any relevant intent
at all.
   For this reason, I dissent only from Parts III–B and III–
C of the Court’s opinion and the ultimate disposition of
this case, and I join the remainder.
                 Cite as: 548 U. S. ____ (2006)          1

                   KENNEDY, J., dissenting

SUPREME COURT OF THE UNITED STATES
                         _________________

                         No. 05–5966
                         _________________


  ERIC MICHAEL CLARK, PETITIONER v. ARIZONA
  ON WRIT OF CERTIORARI TO THE COURT OF APPEALS OF

                ARIZONA, DIVISION ONE

                        [June 29, 2006]

  JUSTICE KENNEDY, with whom JUSTICE STEVENS and
JUSTICE GINSBURG join, dissenting.
  In my submission the Court is incorrect in holding that
Arizona may convict petitioner Eric Clark of first-degree
murder for the intentional or knowing killing of a police
officer when Clark was not permitted to introduce critical
and reliable evidence showing he did not have that intent
or knowledge. The Court is wrong, too, when it concludes
the issue cannot be reached because of an error by Clark’s
counsel. Its reasons and conclusions lead me to file this
respectful dissent.
  Since I would reverse the judgment of the Arizona Court
of Appeals on this ground, and the Arizona courts might
well alter their interpretation of the State’s criminal re
sponsibility statute were my rationale to prevail, it is
unnecessary for me to address the argument that Ari
zona’s definition of insanity violates due process.
                             I
  Clark claims that the trial court erred in refusing to
consider evidence of his chronic paranoid schizophrenia in
deciding whether he possessed the knowledge or intent
required for first-degree murder. Seizing upon a theory
invented here by the Court itself, the Court narrows
Clark’s claim so he cannot raise the point everyone else
thought was involved in the case. The Court says the only
2                    CLARK v. ARIZONA

                    KENNEDY, J., dissenting

issue before us is whether there is a right to introduce
mental-disease evidence or capacity evidence, not a right
to introduce observation evidence. See ante, at 15–25.
This restructured evidentiary universe, with no convincing
authority to support it, is unworkable on its own terms.
Even were that not so, however, the Court’s tripartite
structure is something not addressed by the state trial
court, the state appellate court, counsel on either side in
those proceedings, or the briefs the parties filed with us.
The Court refuses to consider the key part of Clark’s claim
because his counsel did not predict the Court’s own inven
tion. It is unrealistic, and most unfair, to hold that Clark’s
counsel erred in failing to anticipate so novel an approach.
If the Court is to insist on its approach, at a minimum the
case should be remanded to determine whether Clark is
bound by his counsel’s purported waiver.
   The Court’s error, of course, has significance beyond this
case. It adopts an evidentiary framework that, in my
view, will be unworkable in many cases. The Court classi
fies Clark’s behavior and expressed beliefs as observation
evidence but insists that its description by experts must be
mental-disease evidence or capacity evidence. See ante, at
16–18. These categories break down quickly when it is
understood how the testimony would apply to the question
of intent and knowledge at issue here. The most common
type of schizophrenia, and the one Clark suffered from, is
paranoid schizophrenia. See P. Berner et al., Diagnostic
Criteria for Functional Psychoses 37 (2d ed. 1992). The
existence of this functional psychosis is beyond dispute,
but that does not mean the lay witness understands it or
that a disputed issue of fact concerning its effect in a
particular instance is not something for the expert to
address. Common symptoms of the condition are delu
sions accompanied by hallucinations, often of the auditory
type, which can cause disturbances of perception. Ibid.
Clark’s expert testified that people with schizophrenia
                 Cite as: 548 U. S. ____ (2006)           3

                    KENNEDY, J., dissenting

often play radios loudly to drown out the voices in their
heads. See App. 32. Clark’s attorney argued to the trial
court that this, rather than a desire to lure a policeman to
the scene, explained Clark’s behavior just before the kill
ing. Id., at 294–295. The observation that schizophrenics
play radios loudly is a fact regarding behavior, but it is
only a relevant fact if Clark has schizophrenia.
   Even if this evidence were, to use the Court’s term,
mental-disease evidence, because it relies on an expert
opinion, what would happen if the expert simply were to
testify, without mentioning schizophrenia, that people
with Clark’s symptoms often play the radio loudly? This
seems to be factual evidence, as the term is defined by the
Court, yet it differs from mental-disease evidence only in
forcing the witness to pretend that no one has yet come up
with a way to classify the set of symptoms being described.
More generally, the opinion that Clark had paranoid
schizophrenia—an opinion shared by experts for both the
prosecution and defense—bears on efforts to determine, as
a factual matter, whether he knew he was killing a police
officer. The psychiatrist’s explanation of Clark’s condition
was essential to understanding how he processes sensory
data and therefore to deciding what information was in his
mind at the time of the shooting. Simply put, knowledge
relies on cognition, and cognition can be affected by
schizophrenia. See American Psychiatric Association,
Diagnostic and Statistical Manual of Mental Disorders
299 (4th ed. text rev. 2000) (“The characteristic symptoms
of Schizophrenia involve a range of cognitive and emo
tional dysfunctions that include perception”); ibid. (Symp
toms include delusions, which are “erroneous beliefs that
usually involve a misinterpretation of perceptions or
experiences”). The mental-disease evidence at trial was
also intertwined with the observation evidence because it
lent needed credibility. Clark’s parents and friends testi
fied Clark thought the people in his town were aliens
4                    CLARK v. ARIZONA

                    KENNEDY, J., dissenting

trying to kill him. These claims might not be believable
without a psychiatrist confirming the story based on his
experience with people who have exhibited similar behav
iors. It makes little sense to divorce the observation evi
dence from the explanation that makes it comprehensible.
   Assuming the Court’s tripartite structure were feasible,
the Court is incorrect when it narrows Clark’s claim to
exclude any concern about observation evidence. In decid
ing Clark’s counsel failed to raise this issue, the Court
relies on a series of perceived ambiguities regarding how
the claim fits within the Court’s own categories. See ante,
at 15–25. The Court cites no precedent for construing
these ambiguities against the claimant and no prudential
reason for ignoring the breadth of Clark’s claim. It is
particularly surprising that the Court does so to the det
riment of a criminal defendant asserting the fundamental
challenge that the trier of fact refused to consider critical
evidence showing he is innocent of the crime charged.
   The alleged ambiguities are, in any event, illusory. The
evidence at trial addressed more than the question of
general incapacity or opinions regarding mental illness; it
went further, as it included so-called observation evidence
relevant to Clark’s mental state at the moment he shot the
officer. There was testimony, for example, that Clark
thought the people in his town, particularly government
officials, were not human beings but aliens who were
trying to kill him. See App. 119–121, 131–132, 192–197,
249–256; Tr. of Bench Trial in No. CR–2000–538, pp. 110–
112, 131–132, 136, 226–228 (Aug. 20, 2003); id., at 24–25,
59–60 (Aug. 21, 2003). The Court recognizes the existence
of this essential observation evidence. See ante, at 16–17.
   The Court holds, nonetheless, that “we cannot be sure”
whether the trial court failed to consider this evidence.
Ante, at 24. It is true the trial court ruling was not per
fectly clear. Its language does strongly suggest, though,
that it did not consider any of this testimony in deciding
                  Cite as: 548 U. S. ____ (2006)            5

                    KENNEDY, J., dissenting

whether Clark had the knowledge or intent required for
first-degree murder. After recognizing that “much of the
evidence that [the defense is] going to be submitting, in
fact all of it, as far as I know . . . that has to do with the
insanity could also arguably be made . . . as to form and
intent and his capacity for the intent,” the court concluded
“we will be focusing, as far as I’m concerned, strictly on
the insanity defense.” App. 9. In announcing its verdict,
the trial court did not mention any of the mental-illness
evidence, observation or otherwise, in deciding Clark’s
guilt. Id., at 331–335. The most reasonable assumption,
then, would seem to be that the trial court did not consider
it, and the Court does not hold otherwise. See ante, at 20.
   Clark’s objection to this refusal by the trier of fact to
consider the evidence as it bore on his key defense was
made at all stages of the proceeding. In his post-trial
motion to vacate the judgment, Clark argued that “prohib
iting consideration of any evidence reflecting upon a men
tally ill criminal defendant’s ability to form the necessary
mens rea violates due process.” Record, Doc. 406, p. 8.
Clark pressed the same argument in the Arizona Court of
Appeals. See Appellant’s Opening Brief in No. 1CA–CR–
03–0851 etc., pp. 46–52 (hereinafter Appellant’s Opening
Brief). He also noted that the trial judge had erred in
refusing to consider non-expert testimony—presumably
what the Court would call observation evidence—on
Clark’s mental illness. Id., at 47–48. (“The trial court
therefore violated [Clark’s] right to present a defense
because [the] court refused to consider any evidence, in
cluding the multiple testimonials of lay witnesses . . . in
deciding whether he could form the requisite mens rea”).
The appeals court decided the issue on the merits, holding
that the trial court was correct not to consider the evi
dence of mental illness in determining whether Clark had
the mens rea for first-degree murder. See App. 351–353.
It offered no distinction at all between observation or
6                     CLARK v. ARIZONA

                     KENNEDY, J., dissenting

mental-disease evidence.
    Notwithstanding the appeals court’s decision, the Court
states that the issue was not clearly presented to the state
courts. See ante, at 21–24. According to the Court, Clark
only raised an objection based on State v. Mott, 187 Ariz.
536, 931 P. 2d 1046 (1997) (en banc), see ante, at 21–24,
and Mott’s holding was limited to the exclusion of mental-
disease and capacity evidence, see ante, at 19. The Court
is incorrect, and on both counts.
    First, Clark’s claim goes well beyond an objection to
Mott. In fact, he specifically attempted to distinguish Mott
by noting that the trial court in this case refused to con
sider all evidence of mental illness. See Record, Doc. 406,
at 8; see also Appellant’s Opening Brief 48. The Court
notices these arguments but criticizes Clark’s counsel for
not being specific about the observation evidence he
wanted the trial court to consider. See ante, at 22. There
was no reason, though, for Clark’s counsel to believe addi
tional specificity was required, since there was no evident
distinction in Arizona law between observation evidence
and mental-disease testimony.
    Second, Mott’s holding was not restricted to mental-
disease evidence. The Arizona Supreme Court did not
refer to any distinction between observation and mental-
disease evidence, or lay and expert testimony. Its holding
was stated in broad terms: “Arizona does not allow evi
dence of a defendant’s mental disorder short of insanity
either as an affirmative defense or to negate the mens rea
element of a crime.” 187 Ariz., at 541, 931 P. 2d, at 1051;
see id., at 540, 931 P. 2d, at 1050 (“The legislature’s decision
. . . evidences its rejection of the use of psychological testi
mony to challenge the mens rea element of a crime”). The
Court attempts to divine a fact/opinion distinction in Mott
based on Mott’s distinguishing a case, State v. Christensen,
129 Ariz. 32, 628 P. 2d 580 (1981) (in banc), where evidence
about behavioral tendencies was deemed admissible. See
                  Cite as: 548 U. S. ____ (2006)            7

                    KENNEDY, J., dissenting

ante, at 19. Christensen, though, also addressed an expert
opinion; the difference was that the evidence there con
cerned a “character trait of acting reflexively in response to
stress,” not a mental illness. Mott, supra, at 544, 931 P. 2d,
at 1054. Since the Court recognizes the Arizona Court of
Appeals relied on Mott, the expansive rule of exclusion in
Mott—without any suggestion of a limitation depending on
the kind of evidence—should suffice for us to reach the so-
called observation-evidence issue. Even if, as the Court
contends, see ante, at 15, Mott is limited to expert testi
mony, the Court’s categories still do not properly interpret
Mott, because the Court’s own definition of observation
evidence includes some expert testimony, see ante, at 17.
   It makes no difference that in the appeals court Clark
referred to the issue as inability to form knowledge or
intent. See Appellant’s Opening Brief 46–52. He did not
insist on some vague, general incapacity. He stated,
instead, that he “suffered from a major mental illness and
was psychotic at the time of the offense.” Id., at 48. Even
if Clark’s arguments were insufficient to apprise the state
court of the argument, “[o]ur traditional rule is that ‘[o]nce
a federal claim is properly presented, a party can make
any argument in support of that claim; parties are not
limited to the precise arguments they made below.’ ”
Lebron v. National Railroad Passenger Corporation, 513
U. S. 374, 379 (1995) (quoting Yee v. Escondido, 503 U. S.
519, 534 (1992)). The claim is clear. Though it seems to be
obscure to this Court, it was understood by the Arizona
Court of Appeals, which stated: “Clark argues that the trial
court erred in refusing to consider evidence of his mental
disease or defect in determining whether he had the req
uisite mens rea to commit first-degree murder.” App. 351.
When the question is what the state court held, it is not
instructive for this Court to recast the words the state
court used.
   The razor-thin distinction the Court draws between
8                    CLARK v. ARIZONA

                    KENNEDY, J., dissenting

evidence being used to show incapacity and evidence being
used to show lack of mens rea directly does not identify
two different claims. Clark’s single claim, however char
acterized, involves the use of the same mental-illness
evidence to decide whether he had the requisite knowledge
or intent. The various ways in which the evidence is
relevant in disproving mens rea hardly qualify as separate
claims. The new arguments allowed in Lebron and Yee, by
comparison, were far more disconnected from the initial
bases for the alleged violations. See Lebron, supra, at 378,
379 (for purposes of showing state action, petitioner could
argue that Amtrak was a Government entity even though
he argued below only that it was a private entity with
close connections to Government entities, because the
claim was simply “that Amtrak did not accord him the
rights it was obliged to provide by the First Amendment”);
Yee, supra, at 534, 535 (petitioners could argue that an
ordinance constituted a regulatory taking, even though they
arguably asserted in the Court of Appeals only a physical
taking, because the claim was simply “that the ordinance
effects an unconstitutional taking”). If we give this latitude
to litigants in civil cases, surely we must do so here. Fur
thermore, to the extent any ambiguity remains on whether
the claim was raised, the proper course is to remand. See
Bradshaw v. Richey, 546 U. S. ___, ___ (2005) (slip op., at 6)
(per curiam). Unless the state court clearly decides an
issue on state-law grounds, which the Court does not
contend occurred here, there is no bar to our review of the
federal question. See Harris v. Reed, 489 U. S. 255, 261–
262 (1989).
   Before this Court Clark framed the issue in broad terms
that encompass the question whether the evidence of his
mental illness should have been considered to show he did
not at the time of the offense have the knowledge or intent
to shoot a police officer. See Brief for Petitioner i (“Ques
tions Presented for Review: (1) Whether Arizona’s blanket
                 Cite as: 548 U. S. ____ (2006)            9

                    KENNEDY, J., dissenting

exclusion of evidence and refusal to consider mental dis
ease or defect to rebut the state’s evidence on the element
of mens rea violated Petitioner’s right to due process under
the United States Constitution, Fourteenth Amend
ment?”), 22 (“Here, the trial court held that under the
Mott rule it was obliged to find as a fact that [Clark] knew
he was shooting a police officer to death—a necessary
factual element of the only form of first degree murder
charged against [Clark]—while simultaneously refusing to
consider [Clark’s] evidence that an acute episode of his
chronic paranoid schizophrenic illness prevented him from
actually having that knowledge” (emphasis omitted)), 31–
32 (the Arizona courts erred in holding Clark “could be
punished as though he had this knowledge and intent
although he may not in fact have had either”); Reply Brief
for Petitioner 3 (challenging the trial judge’s refusal “to
give any consideration to the mental-illness evidence in
making his factual findings as to whether [Clark] did or
did not act with the state of mind required for a first-
degree murder conviction”). An entire section of Clark’s
opening brief argues that the evidence of mental illness
should have been considered to rebut the prosecution’s
inference of knowledge or intent from the factual circum
stances of the crime. See Brief for Petitioner 13–21. This
line of argument concerns facts of behavior and amounts
to more than a claim of general incapacity.
   Clark seeks resolution of issues that can be complex and
somewhat overlapping. In the end, however, we must
decide whether he had the right to introduce evidence
showing he lacked the intent or knowledge the statute
itself sets forth in describing a basic element of the crime.
Clark has preserved this issue at all stages, including in
this Court.
                            II
  Clark was charged with first-degree murder for the shoot
10                   CLARK v. ARIZONA

                    KENNEDY, J., dissenting

ing of Officer Jeffrey Moritz. “A person commits first-
degree murder if,” as relevant here, “[i]ntending or know
ing that the person’s conduct will cause death to a law
enforcement officer, the person causes the death of a law
enforcement officer who is in the line of duty.” Ariz. Rev.
Stat. Ann. §13–1105(A)(3) (West Supp. 2005). Clark chal
lenges the trial court’s refusal to consider any evidence of
mental illness, from lay or expert testimony, in determining
whether he acted with the knowledge or intent element of
the crime. See App. 9; see also Mott, 187 Ariz., at 541, 931
P. 2d, at 1051.
   States have substantial latitude under the Constitution
to define rules for the exclusion of evidence and to apply
those rules to criminal defendants. See United States v.
Scheffer, 523 U. S. 303, 308 (1998). This authority, how
ever, has constitutional limits. “ ‘Whether rooted directly
in the Due Process Clause of the Fourteenth Amendment or
in the Compulsory Process or Confrontation Clauses of the
Sixth Amendment, the Constitution guarantees criminal
defendants “a meaningful opportunity to present a com
plete defense.” ’ ” Holmes v. South Carolina, 547 U. S. ___,
___ (2006) (slip op., at 4) (quoting Crane v. Kentucky, 476
U. S. 683, 690 (1986), in turn quoting California v. Trom
betta, 467 U. S. 479, 485 (1984)). “This right is abridged
by evidence rules that ‘infring[e] upon a weighty interest
of the accused’ and are ‘ “arbitrary” or “disproportionate to
the purposes they are designed to serve.” ’ ” Holmes, su
pra, at ___ (slip op., at 4) (quoting Scheffer, supra, at 308,
in turn quoting Rock v. Arkansas, 483 U. S. 44, 58, 56
(1987)).
   The central theory of Clark’s defense was that his
schizophrenia made him delusional. He lived in a uni
verse where the delusions were so dominant, the theory
was, that he had no intent to shoot a police officer or
knowledge he was doing so. It is one thing to say he acted
with intent or knowledge to pull the trigger. It is quite
                 Cite as: 548 U. S. ____ (2006)           11

                    KENNEDY, J., dissenting

another to say he pulled the trigger to kill someone he
knew to be a human being and a police officer. If the trier
of fact were to find Clark’s evidence sufficient to discount
the case made by the State, which has the burden to prove
knowledge or intent as an element of the offense, Clark
would not be guilty of first-degree murder under Arizona
law.
   The Court attempts to diminish Clark’s interest by treat
ing mental-illness evidence as concerning only “judgment,”
rather than fact. Ante, at 36. This view appears to derive
from the Court’s characterization of Clark’s claim as rais
ing only general incapacity. See ibid. This is wrong for
the reasons already discussed. It fails to recognize, more
over, the meaning of the offense element in question here.
The mens rea element of intent or knowledge may, at some
level, comprise certain moral choices, but it rests in the
first instance on a factual determination. That is the fact
Clark sought to put in issue. Either Clark knew he was
killing a police officer or he did not.
   The issue is not, as the Court insists, whether Clark’s
mental illness acts as an “excuse from customary criminal
responsibility,” ante, at 30, but whether his mental illness,
as a factual matter, made him unaware that he was shoot
ing a police officer. If it did, Clark needs no excuse, as
then he did not commit the crime as Arizona defines it.
For the elements of first-degree murder, where the ques
tion is knowledge of particular facts—that one is killing a
police officer—the determination depends not on moral
responsibility but on empirical fact. Clark’s evidence of
mental illness had a direct and substantial bearing upon
what he knew, or thought he knew, to be the facts when
he pulled the trigger; this lay at the heart of the matter.
   The trial court’s exclusion was all the more severe be
cause it barred from consideration on the issue of mens rea
all this evidence, from any source, thus preventing Clark
from showing he did not commit the crime as defined by
12                   CLARK v. ARIZONA

                    KENNEDY, J., dissenting

Arizona law. Quite apart from due process principles, we
have held that a bar of this sort can be inconsistent with
the Confrontation Clause. See Delaware v. Van Arsdall,
475 U. S. 673 (1986). In Van Arsdall the Court held a state
court erred in making a ruling that “prohibited all inquiry
into” an event. Id., at 679. At issue was a line of defense
questioning designed to show the bias of a prosecution
witness. In the instant case the ruling in question bars from
consideration all testimony from all witnesses necessary to
present the argument that was central to the whole case for
the defense: a challenge to the State’s own proof on an
element of the crime. The Due Process and Compulsory
Process Clauses, and not the Confrontation Clause, may be
the controlling standard; but the disability imposed on the
accused is every bit as substantial and pervasive here as it
was in Van Arsdall.
   Arizona’s rule is problematic because it excludes evi
dence no matter how credible and material it may be in
disproving an element of the offense. The Court’s cases
have noted the potential arbitrariness of per se exclusions
and, on this rationale, have invalidated various state
prohibitions. See Holmes, supra, at ___ (slip op., at 9)
(rule excluding, in certain cases, evidence that a third
party may have committed the crime “even if that evi
dence, if viewed independently, would have great proba
tive value and even if it would not pose an undue risk of
harassment, prejudice, or confusion of the issues”); Rock,
supra, at 56 (rule excluding all hypnotically refreshed
testimony “operates to the detriment of any defendant who
undergoes hypnosis, without regard to the reasons for it,
the circumstances under which it took place, or any inde
pendent verification of the information it produced”);
Washington v. Texas, 388 U. S. 14, 22 (1967) (rule excluding
accomplice testimony “prevent[s] whole categories of de
fense witnesses from testifying on the basis of a priori
categories that presume them unworthy of belief”).
                 Cite as: 548 U. S. ____ (2006)           13

                    KENNEDY, J., dissenting

   This is not to suggest all general rules on the exclusion
of certain types of evidence are invalid. If the rule does
not substantially burden the defense, then it is likely
permissible. See Scheffer, 523 U. S., at 316–317 (uphold
ing exclusion of polygraph evidence in part because this
rule “does not implicate any significant interest of the
accused”); id., at 318 (KENNEDY, J., concurring in part and
concurring in judgment) (“[S]ome later case might present
a more compelling case for introduction of the testimony
than this one does”). Where, however, the burden is sub
stantial, the State must present a valid reason for its per
se evidentiary rule.
   In the instant case Arizona’s proposed reasons are insuf
ficient to support its categorical exclusion. While the
State contends that testimony regarding mental illness
may be too incredible or speculative for the jury to con
sider, this does not explain why the exclusion applies in all
cases to all evidence of mental illness. “A State’s legiti
mate interest in barring unreliable evidence does not
extend to per se exclusions that may be reliable in an
individual case.” Rock, 483 U. S., at 61. States have
certain discretion to bar unreliable or speculative testi
mony and to adopt rules to ensure the reliability of expert
testimony. Arizona has done so, and there is no reason to
believe its rules are insufficient to avoid speculative evi
dence of mental illness. See Ariz. Rules of Evid. 403, 702
(West 2005). This is particularly true because Arizona
applies its usual case-by-case approach to permit admis
sion of evidence of mental illness for a variety of other
purposes. See, e.g., State v. Lindsey, 149 Ariz. 472, 474–
475, 720 P. 2d 73, 74–75 (1986) (en banc) (psychological
characteristics of molestation victims); State v. Hamilton,
177 Ariz. 403, 408–410, 868 P. 2d 986, 991–993 (App.
1993) (psychological evidence of child abuse accommoda
tion syndrome); Horan v. Indus. Comm’n, 167 Ariz. 322,
325–326, 806 P. 2d 911, 914–915 (App. 1991) (psychiatric
14                    CLARK v. ARIZONA

                     KENNEDY, J., dissenting

testimony regarding neurological deficits).
   The risk of jury confusion also fails to justify the rule.
The State defends its rule as a means to avoid the com
plexities of determining how and to what degree a mental
illness affects a person’s mental state. The difficulty of
resolving a factual issue, though, does not present a suffi
cient reason to take evidence away from the jury even
when it is crucial for the defense. “We have always
trusted juries to sort through complex facts in various
areas of law.” United States v. Booker, 543 U. S. 220, 289
(2005) (STEVENS, J., dissenting in part). Even were the risk
of jury confusion real enough to justify excluding evidence in
most cases, this would provide little basis for prohibiting all
evidence of mental illness without any inquiry into its likely
effect on the jury or its role in deciding the linchpin issue of
knowledge and intent. Indeed, Arizona has a rule in place
to serve this very purpose. See Ariz. Rule of Evid. 403.
   Even assuming the reliability and jury-confusion justifi
cations were persuasive in some cases, they would not
suffice here. It does not overcome the constitutional objec
tion to say that an evidentiary rule that is reasonable on
its face can be applied as well to bar significant defense
evidence without any rational basis for doing so. In Van
Arsdall, for example, the Court rejected the application of
Delaware Rule of Evidence 403, which allows relevant
evidence to be excluded where its probative value is sub
stantially outweighed by the risk of unfair prejudice or other
harms to the trial process. 475 U. S., at 676, and n. 2.
While the Rule is well established and designed for a legiti
mate function, the Constitution prevented an application
that deprived the defendant of all inquiry into an important
issue. Id., at 679. Other cases have applied this same case-
specific analysis in deciding the legitimacy of an exclusion.
See, e.g., Rock, supra, at 62 (the “circumstances present an
argument for admissibility of petitioner’s testimony in this
particular case, an argument that must be considered by the
                 Cite as: 548 U. S. ____ (2006)           15

                    KENNEDY, J., dissenting

trial court”); Chambers v. Mississippi, 410 U. S. 284, 302
(1973) (“In these circumstances, where constitutional rights
directly affecting the ascertainment of guilt are implicated,
the hearsay rule may not be applied mechanistically
to defeat the ends of justice”); cf. Scheffer, 523 U. S., at
318 (KENNEDY, J., concurring in part and concurring in
judgment).
   The Court undertakes little analysis of the interests
particular to this case. By proceeding in this way it de
values Clark’s constitutional rights. The reliability ra
tionale has minimal applicability here. The Court is
correct that many mental diseases are difficult to define
and the subject of great debate. See ante, at 33–34.
Schizophrenia, however, is a well-documented mental
illness, and no one seriously disputes either its definition
or its most prominent clinical manifestations. The State’s
own expert conceded that Clark had paranoid schizophre
nia and was actively psychotic at the time of the killing.
See App. 254–257. The jury-confusion rationale, if it is at
all applicable here, is the result of the Court’s own insis
tence on conflating the insanity defense and the question
of intent. Considered on its own terms, the issue of intent
and knowledge is a straightforward factual question. A
trier of fact is quite capable of weighing defense testimony
and then determining whether the accused did or did not
intend to kill or knowingly kill a human being who was a
police officer. True, the issue can be difficult to decide in
particular instances, but no more so than many matters
juries must confront.
   The Court says mental-illness evidence “can easily
mislead,” ante, at 36, and may “tel[l] us little or nothing
about the ability of the defendant to form mens rea,” ante,
at 35. These generalities do not, however, show how
relevant or misleading the evidence in this case would be
(or explain why Arizona Rule of Evidence 403 is insuffi
cient for weighing these factors). As explained above, the
16                   CLARK v. ARIZONA

                    KENNEDY, J., dissenting

evidence of Clark’s mental illness bears directly on mens
rea, for it suggests Clark may not have known he was
killing a human being. It is striking that while the Court
discusses at length the likelihood of misjudgment from
placing too much emphasis on evidence of mental illness,
see ante, at 33–38, it ignores the risk of misjudging an
innocent man guilty from refusing to consider this highly
relevant evidence at all. Clark’s expert, it is true, said no
one could know exactly what was on Clark’s mind at the
time of the shooting. See ante, at 37. The expert testified
extensively, however, about the effect of Clark’s delusions
on his perceptions of the world around him, and about
whether Clark’s behavior around the time of the shooting
was consistent with delusional thinking. This testimony
was relevant to determining whether Clark knew he was
killing a human being. It also bolstered the testimony of
lay witnesses, none of which was deemed unreliable or
misleading by the state courts.
   For the same reasons, the Court errs in seeking support
from the American Psychiatric Association’s statement
that a psychiatrist may be justifiably reluctant to reach
legal conclusions regarding the defendant’s mental state.
See ante, at 37. In this very case, the American Psychiat
ric Association made clear that psychiatric evidence plays
a crucial role regardless of whether the psychiatrist testi
fies on the ultimate issue: “Expert evidence of mental
disorders, presented by qualified professionals and subject
to adversarial testing, is both relevant to the mental-state
issues raised by mens rea requirements and reliable. . . .
Such evidence could not be condemned wholesale without
unsettling the legal system’s central reliance on such
evidence.” Brief for American Psychiatric Association
et al. as Amici Curiae 15.
   Contrary to the Court’s suggestion, see ante, at 35–36,
the fact that the state and defense experts drew different
conclusions about the effect of Clark’s mental illness on
                  Cite as: 548 U. S. ____ (2006)           17

                    KENNEDY, J., dissenting

his mental state only made Clark’s evidence contested; it
did not make the evidence irrelevant or misleading. The
trial court was capable of evaluating the competing con
clusions, as factfinders do in countless cases where there
is a dispute among witnesses. In fact, the potential to
mislead will be far greater under the Court’s new eviden
tiary system, where jurors will receive observation evi
dence without the necessary explanation from experts.
   The fact that mental-illness evidence may be considered
in deciding criminal responsibility does not compensate for
its exclusion from consideration on the mens rea elements
of the crime. Cf. ante, at 33. The evidence addresses
different issues in the two instances. Criminal responsi
bility involves an inquiry into whether the defendant
knew right from wrong, not whether he had the mens rea
elements of the offense. While there may be overlap be
tween the two issues, “the existence or nonexistence of
legal insanity bears no necessary relationship to the exis
tence or nonexistence of the required mental elements of
the crime.” Mullaney v. Wilbur, 421 U. S. 684, 706 (1975)
(Rehnquist, J., concurring).
   Even if the analyses were equivalent, there is a different
burden of proof for insanity than there is for mens rea.
Arizona requires the defendant to prove his insanity by
clear and convincing evidence. See Ariz. Rev. Stat. Ann.
§13–502(C) (West 2001). The prosecution, however, must
prove all elements of the offense beyond a reasonable
doubt. See Mullaney, supra, at 703–704; In re Winship,
397 U. S. 358, 364 (1970). The shift in the burden on the
criminal responsibility issue, while permissible under our
precedent, see Leland v. Oregon, 343 U. S. 790 (1952),
cannot be applied to the question of intent or knowledge
without relieving the State of its responsibility to establish
this element of the offense. See Sandstrom v. Montana, 442
U. S. 510, 524 (1979) (jury instruction that had the effect of
placing the burden on the defendant to disprove that he had
18                    CLARK v. ARIZONA

                     KENNEDY, J., dissenting

the requisite mental state violates due process). While
evidentiary rules do not generally shift the burden
impermissibly, where there is a right to have evidence
considered on an element of the offense, the right is not
respected by allowing the evidence to come in only on an
issue for which the defendant bears the burden of proof.
See Cool v. United States, 409 U. S. 100, 103 (1972) (per
curiam) (jury instruction that allowed jury to consider
accomplice’s testimony only if it was true beyond a rea
sonable doubt “places an improper burden on the defense
and allows the jury to convict despite its failure to find
guilt beyond a reasonable doubt”); Martin v. Ohio, 480
U. S. 228, 233–234 (1987) (State can shift the burden on a
claim of self-defense, but if the jury were disallowed from
considering self-defense evidence for purposes of deciding
the elements of the offense, it “would relieve the State of its
burden and plainly run afoul of Winship’s mandate”). By
viewing the Arizona rule as creating merely a “presump
tion of sanity (or capacity or responsibility),” ante, at 30,
rather than a presumption that the mens rea elements
were not affected by mental illness, the Court fails to
appreciate the implications for Winship.
   The State attempts to sidestep the evidentiary issue
entirely by claiming that its mental-illness exclusion
simply alters one element of the crime. The evidentiary
rule at issue here, however, cannot be considered a valid
redefinition of the offense. Under the State’s logic, a
person would be guilty of first-degree murder if he know
ingly or intentionally killed a police officer or committed
the killing under circumstances that would show knowl
edge or intent but for the defendant’s mental illness. To
begin with, Arizona law does not say this. And if it did, it
would be impermissible. States have substantial discretion
in defining criminal offenses. In some instances they may
provide that the accused has the burden of persuasion
with respect to affirmative defenses. See Patterson v. New
                 Cite as: 548 U. S. ____ (2006)          19

                   KENNEDY, J., dissenting

York, 432 U. S. 197, 210 (1977). “But there are obviously
constitutional limits beyond which the States may not go
in this regard.” Ibid. If it were otherwise, States could
label all evidentiary exclusions as redefinitions and so
evade constitutional requirements. There is no rational
basis, furthermore, for criminally punishing a person who
commits a killing without knowledge or intent only if that
person has a mental illness. Cf. Robinson v. California,
370 U. S. 660, 666 (1962). The State attempts to bring the
instant case within the ambit of Montana v. Egelhoff, 518
U. S. 37 (1996); but in Egelhoff the excluded evidence con
cerned voluntary intoxication, for which a person can be
held responsible. Viewed either as an evidentiary rule or a
redefinition of the offense, it was upheld because it “com
ports with and implements society’s moral perception that
one who has voluntarily impaired his own faculties should
be responsible for the consequences.” Id., at 50 (plurality
opinion). An involuntary mental illness does not implicate
this justification.
   Future dangerousness is not, as the Court appears to
conclude, see ante, at 38–39, n. 45, a rational basis for
convicting mentally ill individuals of crimes they did not
commit. Civil commitment proceedings can ensure that
individuals who present a danger to themselves or others
receive proper treatment without unfairly treating them
as criminals. The State presents no evidence to the con
trary, and the Court ought not to imply otherwise.
   The State gains little support from Fisher v. United
States, 328 U. S. 463 (1946). There the defendant re
quested an instruction from the trial court that the jury
consider his mental deficiencies in determining his capac
ity for premeditation and deliberation. Id., at 470. The
Court noted that “[i]n view of the status of the defense of
partial responsibility in the District and the nation no
contention is or could be made of the denial of due proc
ess.” Id., at 466. This dictum may be attributable to the
20                    CLARK v. ARIZONA

                     KENNEDY, J., dissenting

fact that the cases recognizing a defendant’s evidentiary
rights and the prosecution’s duty to prove all elements
beyond a reasonable doubt were still decades away. It
may also reflect the fact that the jury instructions as given
did seem to allow the jury to consider evidence of mental
deficiency if it disproved the elements of the offense. See
id., at 467, n. 3 (The jury instructions stated, “ ‘It is further
contended that even if sane and responsible, there was no
deliberate intent to kill, nor in fact any actual intent to
kill. Therefore if not guilty by reason of insanity, the
defendant at most is guilty only of second degree murder
or manslaughter’ ”). Even further ambiguity comes from
the fact that the defense in Fisher concerned a claim that
the petitioner was “mentally somewhat below the average”
with a “psychopathic personality” of aggression. Id., at
467. This general claim of mental deficiencies was rele
vant to the “theory of partial responsibility,” id., at 470, he
wanted the jury to consider. Unlike the mental illness
here, though, which concerns inadequacy of perception
and information processing, the petitioner’s claim may not
have been relevant to mens rea unless mens rea were
redefined to include an element of responsibility. Fisher’s
language, then, does not control this case.
   While Arizona’s rule is not unique, either historically or
in contemporary practice, this fact does not dispose of
Clark’s constitutional argument. To the extent Fisher
may have suggested the contrary, subsequent cases make
clear that while the existence of the rule in some jurisdic
tions is a significant factor to consider, see Egelhoff, supra,
at 43 (plurality opinion), it is not dispositive for evaluation
of a claim that the accused was foreclosed from introduc
ing evidence crucial to the defense. The evidentiary exclu
sion of accomplice testimony the Court invalidated in
Washington was, in fact, well established. See 388 U. S.,
at 21–22. The exclusion of hypnotically refreshed testi
mony likewise had some support when the Court held it
                  Cite as: 548 U. S. ____ (2006)           21

                    KENNEDY, J., dissenting

unconstitutional as applied to a defendant’s own testi
mony. Rock, 483 U. S., at 57. While 13 States still impose
significant restrictions on the use of mental-illness evi
dence to negate mens rea, a substantial majority of the
States currently allow it. Brief for United States as
Amicus Curiae 22–23, and n. 13. The fact that a reason
able number of States restrict this evidence weighs into
the analysis, but applying the rule as a per se bar, as
Arizona does, is so plainly unreasonable that it cannot be
sustained.
   Putting aside the lack of any legitimate state interest
for application of the rule in this case, its irrationality is
apparent when considering the evidence that is allowed.
See Washington, supra, at 22 (“The absurdity of the rule is
amply demonstrated by the exceptions that have been
made to it”). Arizona permits the defendant to introduce,
for example, evidence of “behavioral tendencies” to show
he did not have the required mental state. See Mott, 187
Ariz., at 544, 931 P. 2d, at 1054; Christensen, 129 Ariz., at
35–36, 628 P. 2d, at 583–584. While defining mental
illness is a difficult matter, the State seems to exclude the
evidence one would think most reliable by allowing unex
plained and uncategorized tendencies to be introduced
while excluding relatively well-understood psychiatric
testimony regarding well-documented mental illnesses. It
is unclear, moreover, what would have happened in this
case had the defendant wanted to testify that he thought
Officer Moritz was an alien. If disallowed, it would be
tantamount to barring Clark from testifying on his behalf
to explain his own actions. If allowed, then Arizona’s rule
would simply prohibit the corroboration necessary to make
sense of Clark’s explanation. In sum, the rule forces the
jury to decide guilt in a fictional world with undefined and
unexplained behaviors but without mental illness. This
rule has no rational justification and imposes a significant
burden upon a straightforward defense: He did not commit
22                  CLARK v. ARIZONA

                   KENNEDY, J., dissenting

the crime with which he was charged.
  These are the reasons for my respectful dissent.
