                               In the

    United States Court of Appeals
                 For the Seventh Circuit
                     ____________________
No. 16-1603
UNITED STATES OF AMERICA,
                                                   Plaintiff-Appellee.

                                 v.

ALDO BROWN
                                               Defendant-Appellant.
                     ____________________

         Appeal from the United States District Court for the
           Northern District of Illinois, Eastern Division.
           No. 14 CR 674 — Virginia M. Kendall, Judge.
                     ____________________

  ARGUED SEPTEMBER 8, 2016 — DECIDED SEPTEMBER 8, 2017
                     ____________________

   Before FLAUM, ROVNER, and SYKES, Circuit Judges.
    SYKES, Circuit Judge. While investigating a tip that illegal
drugs were being sold from a south-side convenience store,
Chicago Police Officer Aldo Brown sucker-punched a store
employee for no apparent reason. As the dazed employee
attempted to stagger away, Brown continued to beat and
kick him for about two minutes. The beating was caught on
the store’s surveillance camera. A federal grand jury indicted
2                                                 No. 16-1603

Brown for willfully depriving the employee of his Fourth
Amendment right to be free from excessive force inflicted by
a law-enforcement officer.
    At trial Brown sought to introduce expert testimony from
a former Chicago police officer who would testify that
Brown’s actions were consistent with departmental stand-
ards. Ruling on the government’s motion in limine, the
district judge excluded the expert witness, reasoning that
departmental policy was immaterial to the Fourth Amend-
ment inquiry and that the expert’s proposed testimony
might include an improper opinion about Brown’s state of
mind.
    The jury found Brown guilty. He challenges his convic-
tion, arguing that the judge wrongly excluded his expert
witness. We reject this argument and affirm. Expert testimo-
ny about police standards may appropriately assist the jury
in resolving some excessive-force questions, but sometimes
evidence of this type is unhelpful and thus irrelevant, par-
ticularly when no specialized knowledge is needed to de-
termine whether the officer’s conduct was objectively unrea-
sonable. The misconduct alleged here was easily within the
grasp of a lay jury, so the judge did not abuse her discretion
in excluding the expert.
                       I. Background
   On September 27, 2012, Chicago Police Officers Aldo
Brown and George Stacker entered a convenience store in
Chicago’s South Shore neighborhood to investigate a tip that
drugs were being sold there. The officers handcuffed several
people near the entrance, including a store employee named
Jecque Howard. The officers then searched the store. After
No. 16-1603                                                  3

completing his search, Stacker returned to the front of the
store and spoke to Howard for a few minutes, removing his
handcuffs. Brown approached and ordered Howard to show
his waistband. Howard lifted his shirt in compliance with
the officer’s order.
    Then, while Howard stood motionless, Brown punched
him in the face. As Howard reeled from the blow, Brown
grabbed him by the neck and held him against a large
refrigerator. At Brown’s direction Howard retrieved a small
bag of marijuana from his back pocket and turned it over to
Brown. Without provocation, Brown punched Howard in
the ribs and pulled him down an aisle toward the back of the
store where he forced him to lie on the floor on his back.
When Howard attempted to sit up, Brown hit him in the face
again and forced him back to the ground on his stomach.
Brown then handcuffed Howard, searched his back pockets,
and found a handgun. Brown confiscated the gun and
walked toward the front of the store to show it to his partner,
then returned to kick Howard in the ribs before placing him
under arrest. Surveillance cameras captured the episode on
silent video.
    In his arrest report, Officer Brown described the incident
as an emergency takedown and explained that it was neces-
sary because Howard reached for the firearm. In his tactical-
response report, the officer stated that Howard “fled” and
“pulled away” after the takedown. The tactical-response
report has a space for the officer to record whether he used a
“close hand strike/punch” or “kicks” during the incident in
question. Brown left those boxes unchecked.
   A federal grand jury returned an indictment charging
Brown with three crimes: two counts of falsifying a police
4                                                 No. 16-1603

record, see 18 U.S.C. § 1519 (one count for each report), and
one count of willfully depriving another of a federal right
under color of law, see id. § 242. The § 242 count alleged that
Brown used excessive force against Howard, depriving him
of his Fourth Amendment right to be free from unreasonable
seizure.
    At trial Brown testified that he delivered the first punch
because he noticed the gun in Howard’s back pocket. He
testified that he extended the confrontation only because
Howard threatened him, incited onlookers to rough him up,
failed to comply with his orders, tried to grab his gun, and
attempted to flee.
    To support his theory that his actions were reasonable
given the circumstances, Brown planned to call John Farrell,
a former Chicago police officer, as an expert witness. Ac-
cording to Farrell’s expert report, his proposed testimony
would take the jury through a frame-by-frame narration of
the surveillance video and describe how the Chicago Police
Department’s “Use of Force Model” applied to Brown’s
confrontation with Howard. Specifically, Farrell planned to
testify that Howard was an “active resister” and an “assail-
ant,” to use the parlance of the Use of Force Model. Farrell
based this opinion primarily on his review of the video and
an interview he conducted with Brown. Farrell also planned
to offer his conclusions that Brown’s actions were consistent
with departmental policy and that his response was appro-
priate under the circumstances.
   The government moved in limine to exclude Farrell’s tes-
timony on multiple grounds. The judge granted the motion.
Applying Rules 403 and 702 of the Federal Rules of Evi-
dence, the judge reasoned that Farrell’s testimony was
No. 16-1603                                                  5

largely immaterial and would not assist the jury. The judge
noted that the question for the jury was whether Brown
violated Howard’s constitutional rights, not whether he
violated the Chicago Police Department’s internal rules, and
the expert’s specialized knowledge of police procedures was
unhelpful because the alleged misconduct—punching and
kicking—was well within the average juror’s comprehen-
sion. She concluded as well that Farrell’s testimony would be
unfairly prejudicial because the jurors might defer to his
conclusions about the reasonableness of Brown’s actions
rather than reaching their own independent judgment.
    The judge also worried that Farrell’s proposed expert tes-
timony was likely to implicate Rule 704(b), which prohibits
expert opinion about a criminal defendant’s state of mind.
Farrell apparently intended to tell the jury what Brown was
likely thinking at each stage of the confrontation and explain
why his state of mind justified his actions. Finally, the judge
was concerned that Farrell’s testimony, which was based in
part on his interview with Brown, would introduce Brown’s
version of events through the expert.
    Though she excluded Brown’s expert witness, the judge
permitted the government to call two instructors from the
Chicago Police Department’s Education and Training Divi-
sion to lay a factual foundation for the report-falsification
charges. The judge allowed the instructors to testify for this
limited purpose, but she barred them from discussing the
substance of the training that officers receive on the Fourth
Amendment in general or the use of force in particular. She
also precluded them from offering opinions about whether
Brown used excessive force or had improperly filed a report.
6                                                   No. 16-1603

    More specifically, Andrea Hyfantis, the first departmental
witness, testified that in 2002 and 2003, she was an instructor
for Brown’s class of recruits. She told the jury that she taught
the recruits about the importance of truthfulness when filing
a police report. She also testified that she instructed them in
the basics of Fourth Amendment law, though she adhered to
the judge’s limitation and did not describe the substance of
this instruction. The second instructor, Yolanda Hatch,
explained the Chicago Police Department’s training on filing
postincident reports.
   The jury acquitted Brown of falsifying his reports but
convicted him of willfully violating Howard’s Fourth
Amendment right to be free from excessive force. This
appeal followed.
                        II. Discussion
    Brown limits his appeal to the exclusion of his expert
witness. We review de novo whether the district court
applied the proper legal framework for admitting or exclud-
ing expert testimony. Lees v. Carthage Coll., 714 F.3d 516, 520
(7th Cir. 2013). The judge applied the proper framework
here, so we review her evidentiary ruling for abuse of discre-
tion. See United States v. Trudeau, 812 F.3d 578, 590 (7th Cir.
2016). District judges have wide discretion over decisions to
admit or exclude evidence; we will reverse only if no rea-
sonable person could take the judge’s view of the matter.
United States v. Molton, 743 F.3d 479, 483 (7th Cir. 2014). Even
if we find an abuse of discretion, a new trial is warranted
only if the judge’s error affected the defendant’s substantial
rights. Trudeau, 812 F.3d at 590 (citing FED. R. CRIM. P. 52(a)).
That is, a new trial is appropriate only if the average juror
would have found the government’s case significantly less
No. 16-1603                                                    7

persuasive had the wrongly excluded evidence been admit-
ted. Id.
    The judge based her decision to exclude Farrell’s testi-
mony primarily on Rule 403, which permits the judge to
exclude evidence if its probative value is substantially
outweighed by the danger of unfair prejudice. She also
invoked Rule 704(b), which generally prohibits opinion
testimony about a criminal defendant’s state of mind.
A. Rule 403 and Expert Testimony on Departmental Policy
    The Fourth Amendment prohibits law-enforcement offic-
ers from using excessive force during an arrest as a neces-
sary corollary of the Amendment’s prohibition of unreason-
able seizures. Graham v. Connor, 490 U.S. 386, 395 (1989).
When an officer is accused of using excessive force, the
decisive question is whether the officer’s conduct meets the
Fourth Amendment’s objective standard of reasonableness.
Williams v. Indiana State Police Dep’t, 797 F.3d 468, 472–73 (7th
Cir. 2015). Objective reasonableness is “not capable of precise
definition or mechanical application.” Abdullahi v. City of
Madison, 423 F.3d 763, 768 (7th Cir. 2005) (quotation marks
omitted). Rather it “turns on the facts and circumstances of
each particular case.” Kingsley v. Hendrickson, 135 S. Ct. 2466,
2473 (2015) (internal quotation marks omitted). Ultimately
the officer flunks the test if, in light of the circumstances, he
“used greater force than was reasonably necessary to effec-
tuate the seizure.” Williams, 797 F.3d at 473.
   The excessive-force inquiry is governed by constitutional
principles, not police-department regulations. Scott v.
Edinburg, 346 F.3d 752, 760–61 (7th Cir. 2003). An officer’s
compliance with or deviation from departmental policy
8                                                   No. 16-1603

doesn’t determine whether he used excessive force. Put
another way, a police officer’s compliance with the rules of
his department is neither sufficient nor necessary to satisfy
the Fourth Amendment’s reasonableness requirement. Police
policies are not nationally uniform; nor are they static. If
compliance with departmental policy were decisive, the
Fourth Amendment’s reasonableness standard would “vary
from place to place and from time to time.” Whren v. United
States, 517 U.S. 806, 815 (1996). Worse, if compliance with
departmental policy were the applicable legal standard, the
police department itself would become the arbiter of Fourth
Amendment reasonableness—a prospect that would have
horrified those responsible for the Amendment’s ratification.
See Illinois v. Rodriguez, 497 U.S. 177, 191 (1990) (Fourth
Amendment rights were “deemed too precious to entrust to
the discretion of those whose job is the detection of crime
and the arrest of criminals.”) (quotation marks omitted);
1 ANNALS OF CONG. 439 (1789) (Joseph Gales ed., 1834)
(James Madison expressing the expectation that the coun-
try’s “independent tribunals of justice will consider them-
selves in a peculiar manner the guardians” of the individual
liberties secured in the Bill of Rights and that the courts “will
be an impenetrable bulwark against every assumption of
power in the Legislative or Executive”).
    With these background principles in mind, we reasoned
in Thompson v. City of Chicago that a police officer’s violation
of departmental policy is “completely immaterial [on] the
question … whether a violation of the federal constitution
has been established.” 472 F.3d 444, 454 (7th Cir. 2006).
Thompson involved an excessive-force claim under 42 U.S.C.
§ 1983, the civil analogue of § 242. We affirmed the district
court’s exclusion of the Chicago Police Department’s use-of-
No. 16-1603                                                    9

force orders. 472 F.3d at 453. We also affirmed the court’s
exclusion of expert testimony from a police sergeant who
would have offered an opinion about the reasonableness of
the officer’s conduct based in part on the use-of-force orders.
Id. at 457.
    Despite its strong language, Thompson should not be un-
derstood as establishing a rule that evidence of police policy
or procedure will never be relevant to the objective-
reasonableness inquiry. We recently clarified that expert
testimony concerning police policy is not categorically
barred. See Florek v. Village of Mundelein, 649 F.3d 594, 602–03
(7th Cir. 2011). Even though jurors can understand the
concept of reasonableness, in some cases they may not fully
grasp particular techniques or equipment used by police
officers in the field. In those instances an expert’s specialized
knowledge can “help the trier of fact to understand the
evidence or to determine a fact in issue,” as Rule 702 re-
quires. FED. R. EVID. 702(a).
    Expert testimony of this type may be relevant in cases
where specialized knowledge of law-enforcement custom or
training would assist the jury in understanding the facts or
resolving the contested issue. For example, if it’s standard
practice across the country to train officers to handle a given
situation in a particular way, expert testimony about that
training might aid a jury tasked with evaluating the conduct
of an officer in that specific situation. The legal standard
contemplates a reasonable officer, not a reasonable person, so
it may be useful in a particular case to know how officers
typically act in like cases. Florek, 649 F.3d at 602.
    Evidence of purely localized police procedure is less like-
ly to be helpful than nationally or widely used policy. The
10                                                 No. 16-1603

jury’s task is to determine how a reasonable officer would act
in the circumstances, not how an officer in a particular local
police department would act.
    The level of factual complexity in the case may also bear
on the relevance of expert testimony about police practices
or protocols. In many cases evaluating an officer’s conduct
will draw primarily on the jury’s collective common sense.
The everyday experience of lay jurors fully equips them to
answer the reasonableness question when a case involves
“facts that people of common understanding can easily
comprehend.” United States v. Lundy, 809 F.2d 392, 395 (7th
Cir. 1987). The jury’s common experience will suffice, for
example, when “police use[] their bare hands in making an
arrest, the most primitive form of force.” Florek, 649 F.3d at
602 (internal quotation marks omitted). But when “some-
thing peculiar about law enforcement (e.g., the tools they use
or the circumstances they face) informs the issues to be
decided by the finder of fact,” a juror’s everyday experience
may not be enough to effectively assess reasonableness. Id. If
a case involves “a gun, a slapjack, mace, or some other
tool, … the jury may start to ask itself: what is mace? what is
an officer’s training on using a gun? how much damage can
a slapjack do?” Kopf v. Skyrm, 993 F.2d 374, 379 (4th Cir.
1993).
   Importantly, a per se rule against expert testimony about
police policy or procedure is particularly inappropriate in
criminal cases. Brown stood accused of violating § 242,
which penalizes the willful deprivation of another’s federal
right under color of law. The statute codifies a specific-intent
crime; though the officer need not “have been thinking in
constitutional terms,” he can be convicted under § 242 only
No. 16-1603                                                  11

if he “is aware that what he does is precisely that which the
statute forbids.” Screws v. United States, 325 U.S. 91, 104, 106
(1945); see also United States v. Brown, 250 F.3d 580, 584–85
(7th Cir. 2001). It might be less likely that an officer knew
that his actions would deprive another of a federal right if
those actions fell entirely within widely used standardized
training or practice.
    Furthermore, in a criminal case, the defendant has a con-
stitutional right to a “meaningful opportunity to present a
complete defense.” Holmes v. South Carolina, 547 U.S. 319, 331
(2006) (quotation marks omitted). When evidence might
support a theory of innocence, the trial judge must be free to
“focus on the probative value or the potential adverse effects
of admitting the defense evidence.” Id. at 329. Categorically
excluding this type of evidence without a case-specific
inquiry under Rules 403 and 702 would raise serious consti-
tutional concerns. See Rock v. Arkansas, 483 U.S. 44, 61 (1987)
(A “legitimate interest in barring unreliable evidence does
not extend to per se exclusions that may be reliable in an
individual case.”).
   Though it’s not correct to read Thompson as establishing a
per se rule of exclusion, the judge appropriately exercised
her discretion in excluding the expert’s testimony here. To
repeat, the judge receives “special deference” in making
these determinations, and her decision will be upheld unless
“no reasonable person could take [her] view” of the matter.
United States v. LeShore, 543 F.3d 935, 939 (7th Cir. 2008)
(quotation marks omitted). That deference is more than
enough to carry the day.
   This case provides a textbook example of easily compre-
hensible facts. Brown was indicted for punching and kicking
12                                                 No. 16-1603

Howard. He didn’t use a sophisticated tool or technique; he
hit a motionless man in the face with his fist and continued
to beat and kick him before placing him under arrest. An
expert’s explanation of the Chicago Police Department’s Use
of Force Model would have added nothing that the jurors
could not ascertain on their own by viewing the surveillance
videotape and applying their everyday experience and
common sense. And as the district judge concluded, the
admission of Farrell’s testimony may have induced the jurors
to defer to his conclusion rather than drawing their own. See
Thompson, 472 F.3d at 458. Accordingly, the judge did not
abuse her discretion in excluding Farrell’s expert testimony
about departmental use-of-force standards.
B. Rule 704(b)
    Farrell also planned to offer his opinion that Brown acted
reasonably under the circumstances—an opinion based in
part on Brown’s version of events as recounted in exculpato-
ry statements he made to Farrell in an interview. As the
judge saw it, this expert testimony came too close to the line
drawn in Rule 704(b), which prohibits expert opinion about
a criminal defendant’s state of mind.
   Brown insists that Farrell relied on his own experience
and understanding of the facts to arrive at an admissible
opinion on the ultimate question of objective reasonableness.
Expert opinions on ultimate issues are not categorically
impermissible. See FED. R. EVID. 704(a). But Rules 403, 702,
and 704(b) operate (if in different ways) to prohibit expert
opinions that would “merely tell the jury what result to
reach.” United States v. Noel, 581 F.3d 490, 497 (7th Cir. 2009)
(quoting FED. R. EVID. 704 advisory committee’s note to 1972
No. 16-1603                                                13

proposed rules). That’s what Farrell’s opinion testimony
would have done here.
   Brown falls back on a general argument that because
Farrell is an expert on the use of force, his opinion about
objective reasonableness should have been admitted. But an
expert’s role is to “help the trier of fact to understand the
evidence,” FED. R. EVID. 702(a), not to draw conclusions for
the fact finder when no help is needed.
    Brown’s remaining arguments need only brief attention.
He contends that the judge acted inconsistently by excluding
his expert witness but allowing Hatch and Hyfantis to testify
as witnesses for the government. There was no inconsistency.
The judge prevented Hatch and Hyfantis from testifying
about the substance of the Chicago Police Department’s
instructional program on the Fourth Amendment. The judge
also precluded them from offering opinion testimony about
Brown’s police reports or the reasonableness of his use of
force.
    Finally, Brown argues that the judge abdicated her role as
an “impartial referee of the adversarial system” by excluding
his expert witness for reasons not raised by the government
in its motion in limine. This argument, too, is meritless. The
government advanced the relevant arguments, at least
generally, in its motion in limine and in its responses to
Brown’s own pretrial motions.
                                                   AFFIRMED.
