               Corrected Reprint 1/19/2007

                 FOR PUBLICATION
 UNITED STATES COURT OF APPEALS
      FOR THE NINTH CIRCUIT

UNITED STATES OF AMERICA,                 No. 04-10118
                Plaintiff-Appellee,          D.C. No.
               v.                         CR-01-40201-
EDUARDO SANDOVAL-MENDOZA,                     SBA-1
             Defendant-Appellant.
                                           OPINION

       Appeal from the United States District Court
          for the Northern District of California
      Saundra B. Armstrong, District Judge, Presiding
                  Argued April 12, 2005
                 Submitted August 3, 2005
                 San Francisco, California
                 Filed December 27, 2006
 Before: Alfred T. Goodwin, Diarmuid F. O’Scannlain, and
            Andrew J. Kleinfeld, Circuit Judges.
                Opinion by Judge Kleinfeld




                           19943
             UNITED STATES v. SANDOVAL-MENDOZA             19947


                          COUNSEL

Marc J. Zilversmit, San Francisco, California, for appellant
Eduardo Sandoval-Mendoza.

Erika R. Frick, Assistant U.S. Attorney, San Francisco, Cali-
fornia, for the appellee.


                          OPINION

KLEINFELD, Circuit Judge:

   This drug conspiracy case presents two principal issues.
The first is whether the district court erred in ordering defense
counsel not to talk to his client during an overnight recess.
The second is whether the district court abused its discretion
in excluding expert testimony about the defendant’s subnor-
mal intelligence. We reverse.

                            FACTS

   Twin brothers, Eduardo and Ricardo Sandoval-Mendoza,
were convicted of conspiring to sell methamphetamine.
Eduardo Sandoval-Mendoza sold the drugs. He argues that
the government entrapped him as a matter of law. He also
argues that the district court erroneously excluded medical
evidence of an enormous brain tumor that made him espe-
19948        UNITED STATES v. SANDOVAL-MENDOZA
cially vulnerable to entrapment. Ricardo Sandoval-Mendoza
argues that the government presented insufficient evidence to
convict him. We treat Ricardo Sandoval-Mendoza’s appeal
separately in an unpublished disposition.

   A family friend named “Marcos” introduced Eduardo
Sandoval-Mendoza to “Tony” in February of 2000. Marcos
and Tony were government informants. Sandoval-Mendoza
sold them about 12 pounds of methamphetamine in three sep-
arate deals.

   Sandoval-Mendoza testified. He admitted selling the drugs,
but claimed the government entrapped him. He claimed the
government informants knew a large brain tumor rendered
him especially susceptible to suggestion and preyed upon his
weakness. The tumor was diagnosed in 1992, eight years
before the methamphetamine sales. At first, Sandoval-
Mendoza took medication to shrink the tumor, but quit
because of the side effects. He resumed only when his doctor
told him the tumor would kill him without the medicine.

   Sandoval-Mendoza was depressed. He was worried about
dying and about providing for his wife and five children. And
he was worried about the impotence his tumor caused. He
talked about his problems with Marcos, his sister’s boyfriend.
Sandoval Mendoza testified that Marcos told him he made
$5,000 to $10,000 a week selling drugs. Marcos and Tony
suggested that Sandoval-Mendoza sell drugs to make some
money to support his family after he died.

   Sandoval-Mendoza testified that he refused to sell drugs for
several months, lacking both the experience and the inclina-
tion. But eventually he caved in, making three sales to Marcos
and Tony. He testified that he sold them drugs only because
they used his depression and fear to persuade him, and that he
never sold drugs to anyone else.

  Sandoval-Mendoza’s account is not entirely credible. On
wiretap recordings he sounds suspiciously like an experienced
              UNITED STATES v. SANDOVAL-MENDOZA            19949
drug dealer, not a neophyte. He testified that a relative, a fugi-
tive drug dealer in Mexico, told him what to say and how to
portray himself. This relative also connected him with suppli-
ers in Los Angeles.

   Neither of the government informants took the stand to
contradict Sandoval-Mendoza’s entrapment defense or offer
an alternative explanation. Tapes of their conversations with
Sandoval-Mendoza came into evidence, but the government
did not put them on the stand. And the informants had an
incentive to entrap Sandoval-Mendoza. The government paid
them money for their assistance as well as offering benefits in
their own criminal cases.

  To bolster his entrapment defense, Sandoval-Mendoza
sought to introduce expert testimony explaining that his large
brain tumor damaged his intelligence, memory, and judgment,
making him especially susceptible to suggestion. Sandoval-
Mendoza’s lawyer’s theme for the jury was “thou shalt not
put a stumbling block before the blind.”1 His defense theory
was that the government improperly induced a sick and sug-
gestible man to sell drugs.

   The district court admitted some evidence in support of
Sandoval-Mendoza’s defense, permitting his sister and ex-
wife to testify that his tumor made him forgetful. But it
excluded all the defense expert testimony. The defense had a
neuropsychologist and a neurologist ready to testify that the
brain tumor did indeed impair Eduardo’s intellect and judg-
ment. After an in camera Daubert2 hearing, the district court
excluded the expert testimony, partly because it did not dem-
onstrate the tumor caused suggestibility and partly because it
would be long and confusing. Sandoval-Mendoza argues that
this error requires reversal.
  1
   Leviticus 19:14.
  2
   See Daubert v. Merrell Dow Pharm., Inc., 509 U.S. 579, 592-95
(1993).
19950           UNITED STATES v. SANDOVAL-MENDOZA
   The government cross examined Sandoval-Mendoza over
the course of two days, spanning a morning recess, a lunch
recess, an overnight recess, and another recess on the second
day. The district court ordered Sandoval-Mendoza and his
lawyer not to communicate with each other during the
recesses regarding Sandoval-Mendoza’s testimony, including
the overnight recess. The district court allowed communica-
tion on other matters,“just not concerning his testimony.”
Once cross examination was over, the prohibition was lifted.

   Sandoval-Mendoza was convicted and sentenced to 235
months. He appeals, claiming entrapment as a matter of law,
error in excluding his expert witnesses’ testimony, error in
limiting his consultation with counsel, and error on other
grounds.

                              ANALYSIS

I.       Entrapment.

   The jury instruction required the government to prove
beyond a reasonable doubt that Sandoval-Mendoza was not
entrapped. The jury decided that Sandoval-Mendoza was not
entrapped. But Sandoval-Mendoza argues that he was entrap-
ped as a matter of law. We review de novo.3 We “will not dis-
turb the jury’s finding unless, viewing the evidence in the
government’s favor, no reasonable jury could have concluded
that the government disproved the elements of the entrapment
defense.”4

  [1] Entrapment has two elements: “government inducement
of the crime and the absence of predisposition on the part of
     3
     United States v. Si, 343 F.3d 1116, 1125 (9th Cir. 2003) (citation omit-
ted).
   4
     United States v. Mendoza-Prado, 314 F.3d 1099, 1102 (9th Cir. 2002)
(citation omitted).
                UNITED STATES v. SANDOVAL-MENDOZA                   19951
the defendant.”5 Inducement is “any government conduct cre-
ating a substantial risk that an otherwise law-abiding citizen
would commit an offense.”6 We assume Sandoval-Mendoza
proved inducement because the government did not dispute
that its informants proposed the drug sales.

   [2] Even so, Sandoval-Mendoza does not establish absence
of predisposition as a matter of law. “Where the Government
has induced an individual to break the law and the defense of
entrapment is at issue, as it was in this case, the prosecution
must prove beyond reasonable doubt that the defendant was
disposed to commit the criminal act prior to first being
approached by Government agents.”7 The government pres-
ented evidence Sandoval-Mendoza was predisposed to sell
drugs, including wiretap recordings of him talking as though
he were an experienced drug dealer. Offering to buy drugs
from a drug dealer is not entrapment, even if the government
“sets the dealer up” by providing an informant pretending to
be a customer, because the dealer is already predisposed to sell.8

   In order to prove he was entrapped as a matter of law,
Sandoval-Mendoza must “point to undisputed evidence mak-
ing it patently clear that an otherwise innocent person was
induced to commit the illegal act by trickery, persuasion, or
fraud of a government agent.”9 He argues that his testimony
that he resisted the pressure to sell drugs to the government
informants and never sold drugs to anyone else was undis-
  5
     United States v. Skarie, 971 F.2d 317, 320 (9th Cir. 1992).
  6
     United States v. Poehlman, 217 F.3d 692, 698 (9th Cir. 2000) (citation
omitted).
   7
     Jacobson v. United States, 503 U.S. 540, 548-549 (1992) (citation
omitted).
   8
     United States v. Poehlman, 217 F.3d 692, 701 (9th Cir. 2000) (“Where
government agents merely make themselves available to participate in a
criminal transaction, such as standing ready to buy or sell illegal drugs,
they do not induce commission of the crime.”).
   9
     United States v. Mendoza-Prado, 314 F.3d 1099, 1102 (9th Cir. 2002).
19952            UNITED STATES v. SANDOVAL-MENDOZA
puted because he testified and the informants did not. But the
jury did not have to believe Sandoval-Mendoza. Uncontra-
dicted testimony is not necessarily undisputed evidence.10
Jurors may reject uncontradicted testimony when cross exam-
ination, other evidence, or their own common sense and ordi-
nary experience convince them the testimony is probably
false. “Even perfectly plausible allegations can be disbelieved
if they occur during the course of a generally implausible
account.”11 The wiretap recordings in which Sandoval-
Mendoza pretended to be or really was an experienced drug
dealer belie his testimony. The jury could have believed
Sandoval-Mendoza’s drug dealer relative coached him and he
was just pretending. But it didn’t have to.

   [3] The jury could have found that Sandoval-Mendoza was
entrapped. But its conclusion to the contrary was supported by
enough evidence to meet the Jackson12 standard. This is not
a case like Jacobson,13 where the government failed to present
any evidence of predisposition at all. Entrapment was prop-
erly left to the jury.

  10
    Wilbur-Ellis Co. v. The M/V Captayannis “S”, 451 F.2d 973, 974 (9th
Cir. 1971) (per curiam) (holding “the court is not bound to accept uncon-
troverted testimony at face value if it is improbable, unreasonable or other-
wise questionable.”) (citing Quock Ting v. United States, 140 U.S. 417,
420-21 (1891)).
  11
     Grotemeyer v. Hickman, 393 F.3d 871, 879 (9th Cir. 2004) (“When
a witness’s account is as unlikely as the events portrayed in a Marc Cha-
gall painting, the jury is entitled to reject the testimony in its entirety, dis-
believing both the reasonable and the unreasonable aspects.”).
  12
     Jackson v. Virginia, 443 U.S. 307, 319 (1979) (holding “the relevant
question is whether, after viewing the evidence in the light most favorable
to the prosecution, any rational trier of fact could have found the essential
elements of the crime beyond a reasonable doubt.”) (emphasis in original)
(citation omitted).
  13
    Jacobson v. United States, 503 U.S. 540 (1992).
                UNITED STATES v. SANDOVAL-MENDOZA                   19953
II.    The order limiting attorney-client discussion.

   Sandoval-Mendoza testified over the course of three days.
The government’s cross examination spanned a morning
recess, a lunch recess, an overnight recess, and another morn-
ing recess the following day. The district court instructed
Sandoval-Mendoza and his lawyer not to discuss his testi-
mony during any of the recesses, but permitted them to dis-
cuss anything else. Sandoval-Mendoza’s lawyer objected and
was overruled. When the cross examination ended, the district
court permitted Sandoval-Mendoza and his lawyer to discuss
his testimony before redirect.

   Sandoval-Mendoza argues that the district court’s order
prohibiting him from discussing his testimony with his lawyer
during the recesses amounted to a structural error under
Geders v. United States14 and Perry v. Leeke.15 Perry and
Geders reach opposite conclusions based on different facts. In
Geders, the trial court prohibited all communication between
the defendant and his lawyer during an overnight recess
between direct and cross examination. The Supreme Court
held that this prohibition required reversal because it deprived
the defendant of his Sixth Amendment right to counsel.16 In
Perry, the trial court prohibited all communication between
the defendant and his lawyer during a fifteen minute recess
between direct and cross examination. The Supreme Court
held that this prohibition did not violate the Sixth Amendment.17
Perry distinguished Geders, on the ground that “the normal
consultation between attorney and client that occurs during an
overnight recess would encompass matters that go beyond the
content of the defendant’s own testimony — matters that the
  14
     Geders v. United States, 425 U.S. 80 (1976).
  15
     Perry v. Leeke, 488 U.S. 272 (1989). See also United States v. Santos,
201 F.3d 953 (7th Cir. 2000); Mudd v. United States, 798 F.2d 1509 (D.C.
Cir. 1986).
  16
     Geders v. United States, 425 U.S. 80, 91-92 (1976).
  17
     Perry v. Leeke, 488 U.S. 272 (1989).
19954          UNITED STATES v. SANDOVAL-MENDOZA
defendant does have a constitutional right to discuss with his
lawyer, such as the availability of other witnesses, trial tactics,
or even the possibility of negotiating a plea bargain.”18

   The facts of this case fall in the middle. The district court
instructed Sandoval-Mendoza’s lawyer, “You can communi-
cate. Just not concerning cross, his testimony, now that he’s
on cross-examination, unless that’s concluded. That doesn’t
mean you can’t talk with your client at all, just not concerning
his testimony.” On the one hand, the district court prohibited
communication between Sandoval-Mendoza and his lawyer
during an overnight recess, suggesting Geders controls. On
the other hand, the district court only prohibited Sandoval-
Mendoza and his lawyer from discussing his testimony, rather
than anything at all, suggesting Perry controls.

   The core issue is whether prohibiting a defendant and his
lawyer from discussing his testimony during an overnight
recess violates the Sixth Amendment. Two other circuits have
addressed this question. In Mudd v. United States, the District
of Columbia Circuit held a similar prohibition unconstitu-
tional under Geders.19 And in United States v. Santos, the
Seventh Circuit held a similar prohibition unconstitutional
under Perry.20

   This is a difficult question. Cross examination best exposes
the truth when a witness must answer questions unaided.
Coaching may vitiate its value. But it is hard to see how a
defendant and his lawyer can communicate without implicit
coaching. The Seventh Circuit suggests that “the judge may
instruct the lawyer not to coach his client” but may not pro-
hibit discussion of the client’s testimony.21 But that is not a
workable rule, because coaching is implicit in any discussion
  18
     Perry v. Leeke, 488 U.S. 272, 284 (1989).
  19
     Mudd v. United States, 798 F.2d 1509 (D.C. Cir. 1986).
  20
     United States v. Santos, 201 F.3d 953 (7th Cir. 2000).
  21
     United States v. Santos, 201 F.3d 953, 965 (7th Cir. 2000).
                UNITED STATES v. SANDOVAL-MENDOZA                     19955
of a defendant’s testimony, even if the defendant’s lawyer
tries his best to avoid coaching.22

   [4] We conclude that any overnight ban on communication
falls on the Geders side of the line and violates the Sixth
Amendment. That seems the fairer reading of Perry, which
only permitted prohibitions on communication between a
defendant and his lawyer during a “brief recess.”23 Perry rec-
ognized a defendant has a “constitutional right” to discuss
matters other than his own testimony with his lawyer, “such
as the availability of other witnesses, trial tactics, or even the
possibility of negotiating a plea bargain,” during an overnight
recess.24 And it conceded that “such discussions will inevita-
bly include some consideration of defendant’s ongoing testimo-
ny.”25 Indeed, it is hard to see how a defendant’s lawyer could
ask him for the name of a witness who could corroborate his
testimony or advise him to change his plea after disastrous
testimony, subjects Perry expressly says a defendant has a
right to discuss with his lawyer during an overnight recess,
without discussing the testimony itself.

   [5] Thus, we conclude that trial courts may prohibit all
communication between a defendant and his lawyer during a
brief recess before or during cross-examination, but may not
restrict communications during an overnight recess.26 This
  22
      As has been recognized for millennia, even neutral judges find it hard
to avoid teaching a witness what would be useful to him, when judges
question the witness. “Be thorough in the interrogation of witnesses, and
be careful in thy words, lest from them they learn to utter falsehood.”
Aboth 1:9 (J. Israelstam trans.), in 4 The Babylonian Talmud 7 (I. Epstein
ed. 1935).
   23
      Perry v. Leeke, 488 U.S. 272, 283-84 (1989).
   24
      Perry v. Leeke, 488 U.S. 272, 284 (1989).
   25
      Perry v. Leeke, 488 U.S. 272, 284 (1989).
   26
      “We merely hold that the Federal Constitution does not compel every
trial judge to allow the defendant to consult with his lawyer while his testi-
mony is in progress if the judge decides that there is a good reason to
interrupt the trial for a few minutes.” Perry v. Leeke, 488 U.S. 272, 284-
285 (1989).
19956          UNITED STATES v. SANDOVAL-MENDOZA
simple rule is consistent with the reasoning of Geders and
Perry. And it has several other advantages. First, it is easy to
understand and apply. Second, it dispenses with pretense.
Jurors can believe that a defendant did not communicate with
his lawyer during a bathroom break. But only a lawyer more
wedded to words than common sense can believe that a defen-
dant communicated with his lawyer during an overnight
recess without at least implicitly discussing his testimony.

   Third, as Geders explains, prosecutors and judges can
address the coaching problem without prohibiting communi-
cation during an overnight recess. The trial court may “exer-
cise reasonable control” over the order and timing of direct
and cross examination in order to “make the interrogation and
presentation effective for the ascertainment of the truth.”27 For
instance, it “may direct that the examination of the witness
continue without interruption until completed” or “arrange the
sequence of testimony so that direct- and cross-examination
of a witness will be completed without interruption.”28 If a
defense lawyer strings out direct examination until the usual
time for the evening recess, the court can delay the recess and
give the prosecutor a few minutes to get in some cross exami-
nation.

   Thus, we conclude that the district court erred in prohibit-
ing Sandoval-Mendoza and his lawyer from discussing his
testimony during an overnight recess. But does the error
require reversal? While Geders implies it does,29 Geders, a
1976 decision, preceded many recent Supreme Court deci-
sions requiring prejudice as well as constitutional error for rever-
sal.30 Still, under the recent Supreme Court decision in United
States v. Gonzales-Lopez,31 if an error is structural, prejudice
  27
     Fed. R. Evid. 611(a).
  28
     Geders v. United States, 425 U.S. 80, 90 (1976).
  29
     Geders v. United States, 425 U.S. 80, 91 (1976).
  30
     E.g. Neder v. United States, 527 U.S. 1, 8 (1999).
  31
     United States v. Gonzales-Lopez, 548 U.S. ___ (2006).
              UNITED STATES v. SANDOVAL-MENDOZA              19957
is irrelevant. We need not decide whether or not an overnight
prohibition of communications regarding the defendant’s tes-
timony is structural error, because another error, described
below, independently requires reversal.

III.   The Excluded Expert Witness Testimony.

  “We review the district court’s decision to exclude expert
witness testimony for abuse of discretion.”32

   Sandoval-Mendoza wanted to present expert testimony
concerning his mental condition and susceptibility to sugges-
tion. He offered two expert witnesses: Dr. Michael Shore, a
neuropsychologist, and Dr. J. Richard Mendius, a neurologist.
The prosecutors also proposed expert witnesses: Dr. Ronald
H. Roberts, a neuropsychologist, and Dr. Richard Cuneo, a
neurologist. The district court held a Daubert33 hearing on
whether any of these expert witnesses would be permitted to
testify.

   Defense witness Michael Shore, Ph.D., is a psychologist
with extensive clinical and teaching experience in neuropsy-
chology, focusing on the rehabilitation of patients suffering
from brain damage caused by strokes, tumors, and other
causes. He testified that Sandoval-Mendoza suffered from an
unusually large pituitary tumor measuring 2 x 2 x 3 centime-
ters, about the size of an apricot, when diagnosed. The tumor
compressed Sandoval-Mendoza’s frontal lobe, temporal lobe,
and thalamus, probably causing damage. Medication eventu-
ally shrank Sandoval-Mendoza’s tumor to some degree, but
could not reverse any brain damage.

  The relationship between brain damage and cognitive
impairment is well-documented. Tumors like Sandoval-
  32
     United States v. Bahena-Cardenas, 411 F.3d 1067, 1078 (9th Cir.
2005) (citation omitted).
  33
     Daubert v. Merrell Dow Pharm., 509 U.S. 579 (1993).
19958        UNITED STATES v. SANDOVAL-MENDOZA
Mendoza’s may affect mental condition in two ways. First,
damage to the pituitary gland may affect thyroid production,
causing mood disorders, including depression. Second, direct
damage to the frontal lobe, temporal lobe, and thalamus may
affect memory, decision-making, judgment, mental flexibility,
and overall intellectual capacity. In particular, damage to the
frontal lobe often affects concentration, focus, learning, mem-
ory, decision-making, reasoning, judgment, and problem-
solving, according to Dr. Shore.

   A battery of routine and widely-accepted tests showed
Eduardo suffered from brain damage affecting his cognitive
condition. Although an performance IQ test showed border-
line mental retardation and a mental age of eleven, Dr. Shore
concluded Sandoval-Mendoza’s school and work history were
not consistent with retardation, and attributed the test results
to his tumor. A classic nineteenth-century study showed fron-
tal lobe damage causes a person to have “the passions of a
man but the mind of a child,” increasing suggestibility. In
Sandoval-Mendoza’s case, the brain damage apparently
affected the “passions of a man” as well, because the tumor
made him impotent.

   Defense witness Dr. J. Richard Mendius, M.D., is a board-
certified neurologist with additional expertise in clinical
neurophysiology. He testified that a magnetic resonance
imaging test showed Sandoval-Mendoza suffers from an
unusually large pituitary tumor. When the tumor shrank after
treatment, the frontal lobe herniated into the empty space. The
tumor also caused atrophy of the inside of the left temporal
lobe and penetrated a bone separating the pituitary gland from
the brain stem. Brain damage of this kind tends to affect judg-
ment, memory, and emotions connected to memory. A perfor-
mance IQ test suggested a very low level of intellectual
function. Both Shore and Mendius testified that they knew of
no studies specifically linking brain damage of this kind with
susceptibility to inducement to commit crimes. But they noted
that it commonly causes disinhibition.
             UNITED STATES v. SANDOVAL-MENDOZA             19959
   Prosecution witness Ronald H. Roberts, Ph.D., is a neurop-
sychologist who mainly testifies as an expert witness. Roberts
agreed Sandoval-Mendoza suffered from an unusually large
tumor. However, he believed Sandoval-Mendoza was deliber-
ately underperforming on memory texts. He also believed the
tumor did not significantly affect Sandoval-Mendoza’s perfor-
mance IQ test result. Though he did not contradict the deter-
minations from the MRI films that Sandoval-Mendoza had an
extremely large tumor, he gave the opinion that on the mem-
ory tests, Sandoval-Mendoza was faking a worse memory
than he really had.

   Prosecution witness Dr. Richard Cuneo, M.D., is a neurolo-
gist. Cuneo agreed Sandoval-Mendoza suffered from an
unusually large pituitary tumor near areas of the brain control-
ling behavior and cognition. But medical understanding of
behavior and cognition is preliminary and inconclusive. Dr.
Cuneo thought the studies presented were inadequate to show
that Sandoval-Mendoza’s tumor and brain damage affected
his behavior and cognition because the studies were retrospec-
tive and involve small samples. A patient of Dr. Cuneo’s had
a similar tumor that neither caused brain damage nor affected
behavior and cognition. Sandoval-Mendoza’s magnetic reso-
nance imaging test did not show any brain damage, in Dr.
Cuneo’s opinion. While some brain tumors may cause disin-
hibition or greater susceptibility to influence, pituitary tumors
do not, unless they are even larger than Sandoval-Mendoza’s.
Dr. Cuneo conceded that Sandoval-Mendoza’s performance
IQ test was borderline “retarded” and mentally retarded peo-
ple are “known to be susceptible to the influence of others.”

   After the Daubert hearing, the district court excluded the
expert testimony as “not relevant to the entrapment defense”
because it “does not tend to show either inducement or a lack
of predisposition attributable to the tumor.” The court based
its ruling on the expert testimony’s “lack of scientific validi-
ty” and “absence of ability to make a causal connection”
between the tumor and inducement or predisposition. Alterna-
19960           UNITED STATES v. SANDOVAL-MENDOZA
tively, it concluded that the “probative value” of the expert
testimony was “outweighed by the dangers of confusing the
issues, misleading the jury, and creating undue delay,” and
“would be extremely confusing to both the court and the jury”
especially “given the fact that the defense witnesses will then
be rebutted by government witnesses.”

   As a consequence, only Sandoval-Mendoza himself, his ex-
wife, and his sister could testify that his brain tumor made
him forgetful and suggestible. The defense had another wit-
ness prepared to testify that Sandoval-Mendoza once drank
his own urine sample, having forgotten what it was, but the
witness disappeared after the prosecutor advised her that as an
illegal alien she could be putting herself at risk of deportation.
But jurors might well disregard the lay evidence that came in
as biased and lacking scientific foundation, since they were
deprived of medical evidence.

   Although the abuse of discretion standard of review is lib-
eral, the district court’s decision to exclude the expert testi-
mony creates a “definite and firm conviction that the district
court committed a clear error of judgment.”34 Daubert makes
the district court a gatekeeper, not a fact finder. When credi-
ble, qualified experts disagree, a criminal defendant is entitled
to have the jury, not the judge, decide whether the govern-
ment has proved its case.35

  [6] Federal Rule of Evidence 702 governs the admission of
expert opinion testimony.36 Under Daubert37 and Kumho Tire,38
  34
      Clausen v. M/V New Carissa, 339 F.3d 1049, 1055 (9th Cir. 2003).
  35
      Dorn v. Burlington N. Santa Fe R.R. Co., 397 F.3d 1183, 1196 (9th
Cir. 2005).
   36
      “If scientific, technical, or other specialized knowledge will assist the
trier of fact to understand the evidence or to determine a fact in issue, a
witness qualified as an expert by knowledge, skill, experience, training, or
education, may testify thereto in the form of an opinion or otherwise.”
Fed. R. Evid. 702.
   37
      Daubert v. Merrell Dow Pharms, 509 U.S. 579 (1993).
   38
      Kumho Tire Co. v. Carmichael, 526 U.S. 137 (1999).
                UNITED STATES v. SANDOVAL-MENDOZA                    19961
only relevant and reliable expert opinion testimony is admissi-
ble. Expert opinion testimony is relevant if the knowledge
underlying it has a “valid . . . connection to the pertinent inqui-
ry.”39 And it is reliable if the knowledge underlying it “has a
reliable basis in the knowledge and experience of [the rele-
vant] discipline.”40

   [7] Trial courts must exercise reasonable discretion in eval-
uating and in determining how to evaluate the relevance and
reliability of expert opinion testimony.41 Daubert and Kumho
Tire suggest factors trial courts may consider when evaluating
the relevance and reliability of expert opinion testimony. For
example, in evaluating the reliability of scientific expert opin-
ion testimony, trial courts may consider: “(1) whether the sci-
entific theory or technique can be (and has been) tested, (2)
whether the theory or technique has been subjected to peer
review and publication, (3) whether there is a known or
potential error rate, and (4) whether the theory or technique
is generally accepted in the relevant scientific community.”42
Of course, “there are many different kinds of experts, and
many different kinds of expertise,” so these factors “may or
may not be pertinent in assessing reliability, depending on the
nature of the issue, the expert’s particular expertise, and the
subject of his testimony.”43
   39
      Kumho Tire Co. v. Carmichael, 526 U.S. 137, 149 (1999) (quoting
Daubert v. Merrell Dow Pharms, 509 U.S. 579, 592 (1993)).
   40
      Kumho Tire Co. v. Carmichael, 526 U.S. 137, 149 (1999) (quoting
Daubert v. Merrell Dow Pharms, 509 U.S. 579, 592 (1993)).
   41
      Kumho Tire Co. v. Carmichael, 526 U.S. 137, 152 (1999); Mukhtar
v. Cal. State Univ., 299 F.3d 1053, 1064 (9th Cir. 2002) (“A trial court not
only has broad latitude in determining whether an expert’s testimony is
reliable, but also in deciding how to determine the testimony’s reliabili-
ty.”) (citation omitted).
   42
      Mukhtar v. Cal. State Univ., 299 F.3d 1053, 1064 (9th Cir. 2002)
(summarizing Daubert v. Merrell Dow Pharms., 509 U.S. 579, 592-94
(1993)).
   43
      Kumho Tire Co. v. Carmichael, 526 U.S. 137, 150 (1999).
19962           UNITED STATES v. SANDOVAL-MENDOZA
   [8] When evaluating specialized or technical expert opinion
testimony, “the relevant reliability concerns may focus upon
personal knowledge or experience.”44 Because medical expert
opinion testimony “is based on specialized as distinguished
from scientific knowledge, the Daubert factors are not
intended to be exhaustive or unduly restrictive.”45 Under our
decision in Sullivan v. United States Dep’t of the Navy, the
district court “applied an inappropriately rigid Daubert stan-
dard to medical expert testimony” by not accepting what “a
good [physician] would in determining what is reliable
knowledge in the [medical] profession.”46 A trial court should
admit medical expert testimony if physicians would accept it
as useful and reliable. Utility to the jury of medical expert tes-
timony should be determined by what physicians would
accept as useful.

   [9] The district court concluded that the proposed medical
expert opinion testimony was unreliable because it did not
conclusively prove Sandoval-Mendoza’s brain tumor caused
susceptibility to inducement or a lack of predisposition. But
medical knowledge is often uncertain. The human body is
complex, etiology is often uncertain, and ethical concerns
often prevent double-blind studies calculated to establish sta-
tistical proof. This does not preclude the introduction of medi-
cal expert opinion testimony when medical knowledge
“permits the assertion of a reasonable opinion.”47

  [10] Predisposition or its absence is the focus of an entrap-
ment defense.48 Therefore, medical expert opinion testimony
showing that a medical condition renders a person unusually
  44
     Kumho Tire Co. v. Carmichael, 526 U.S. 137, 150 (1999).
  45
     Sullivan v. U.S. Dep’t of the Navy, 365 F.3d 827, 834 (9th Cir. 2004).
  46
     Sullivan v. U.S. Dep’t of the Navy, 365 F.3d 827, 833-34 (9th Cir.
2004).
  47
     United States v. Finley, 301 F.3d 1000, 1007 (9th Cir. 2002) (citation
omitted).
  48
     United States v. Slaughter, 891 F.2d 691, 697 (9th Cir. 1989)
               UNITED STATES v. SANDOVAL-MENDOZA                    19963
vulnerable to inducement is highly relevant to an entrapment
defense. If it is adequately supported by medical expert opin-
ion, it is admissible. Sandoval-Mendoza’s experts were well
qualified and had sufficient expertise in the neurology of
brain tumors and his particular case to be useful to the jury.
The district court’s exclusion of medical expert opinion testi-
mony prevented Sandoval-Mendoza from showing lack of
predisposition, “and thereby deprived him of a fair opportu-
nity to defend himself.”49 In this case, the foundation was suf-
ficient. After hearing Drs. Mendius and Shore, the jury could
have decided to disbelieve them. But Sandoval-Mendoza was
entitled to have the jury decide upon their credibility, rather
than the judge. As it was, the jury was left with nothing but
unpersuasive lay evidence on a medical matter beyond what
laymen could usefully testify about.

   [11] The district court excluded the medical expert opinion
testimony alternatively in order to avoid “unfair prejudice,
confusion of the issues, or misleading the jury, or by consider-
ations of undue delay, waste of time, or needless presentation
of cumulative evidence.”50 But there could be no “confusion
of the issues” because predisposition was the issue. Sandoval-
Mendoza’s only defense was entrapment, and entrapment
came down to predisposition. And there was no risk of “mis-
leading the jury.” The experts agreed Sandoval-Mendoza has
an unusually large brain tumor. Their only disagreement was
whether it caused susceptibility to inducement. The jury was
capable of weighing the conflicting medical expert opinion
testimony against the rest of the evidence presented and deter-
mining whether or not predisposition existed. As for “undue
delay,” testimony would likely consume no more time than
the Daubert hearing, and probably much less.

   [12] Without the medical expert opinion testimony, the real
issue in dispute was hidden from the jury. It could not deter-
  49
    United States v. Slaughter, 891 F.2d 691, 698 (9th Cir. 1989)
  50
    Fed. R. Evid. 403.
19964        UNITED STATES v. SANDOVAL-MENDOZA
mine whether the government’s informants induced a vulnera-
ble and suggestible man to break the law. The informants did
not testify, so the jury could not evaluate the pressure they put
on Sandoval-Mendoza. It could not evaluate the merits of
Sandoval-Mendoza’s suggestibility, because the medical
expert opinion testimony concerning the possibility his tumor
or limited mental capacity made him susceptible to induce-
ment was excluded. All the jury had was proof that Sandoval-
Mendoza sold drugs, wiretap recordings in which he sounded
like an experienced drug dealer, and a couple of lay witnesses
testifying that he was addled by a brain tumor. Sandoval-
Mendoza is entitled to present his case to the jury. For that,
he deserves a new trial.

  Because this error requires reversal, we need not reach
Sandoval-Mendoza’s other claims.

  REVERSED.
