                              In the

United States Court of Appeals
               For the Seventh Circuit

No. 10-3801

E FRAIN SANCHEZ,
                                                  Plaintiff-Appellant,
                                  v.

C ITY OF C HICAGO, et al.,
                                               Defendants-Appellees.


             Appeal from the United States District Court
        for the Northern District of Illinois, Eastern Division.
               No. 08 C 2735—John W. Darrah, Judge.



   A RGUED JANUARY 10, 2012—D ECIDED N OVEMBER 2, 2012




  Before B AUER, R OVNER, and SYKES, Circuit Judges.
  R OVNER, Circuit Judge. Efrain Sanchez sued the City
of Chicago and two of its police officers, Rick Caballero
and Matthew Peterson, pursuant to 42 U.S.C. § 1983.
Sanchez alleged that Caballero and Peterson had
falsely arrested him, employed excessive force against
him during the arrest, and failed to intervene in the
misconduct of one another and other unnamed officers
in connection with the arrest—all in contravention of
2                                             No. 10-3801

his Fourth Amendment right to be secure in his person
from unreasonable seizures. See Graham v. Connor, 490
U.S. 386, 109 S. Ct. 1865 (1989). He also asserted state-
law claims for battery, respondeat superior, and indem-
nification, although he ultimately dismissed these
claims voluntarily. The federal claims were tried to a
jury, which found in the defendants’ favor. Sanchez
now appeals, contending that the district court com-
mitted various errors in connection with the trial.
Although the parties and the court at times appear to
have misapprehended the prospective liability of the
defendants for the acts of the unnamed officers, and
although the jury was not properly instructed as to the
potential liability of Officers Caballero and Peterson for
failing to intervene in the alleged wrongdoing of the
unnamed officers, we conclude that none of the errors
that Sanchez has preserved for appeal warrants reversal.
We therefore affirm the judgment.


                            I.
  The claims in this case arise from an encounter that
the Chicago police had with Sanchez and his brother in
the early hours of April 5, 2008. Sanchez testified that
he and his brother José and two of their friends, Israel
Cabral and Alex Castillo, were gathered outside of
José’s house awaiting the arrival of one or more women
they had met at a party earlier in the evening. Around
the time that one of these women, Valerie Rodriguez,
showed up, three unmarked police cars pulled up in
front of the house and officers demanded to know what
No. 10-3801                                                    3

the men were doing outside. José explained that they
had been waiting for Rodriguez. Evidently not satisfied
with the explanation, the officers ordered the men to
place themselves against one of the cars, empty their
pockets, and place the contents onto the top of the
car; the men were then subjected to a pat-down. José
protested that the officers had no right to search them.
Officer Caballero, whom José recognized from Cabal-
lero’s off-duty work as a security guard at a restaurant
where José worked, seized the keys to José’s house
(which José had removed from his pocket and placed on
top of the car) and walked toward the dwelling. Sanchez
protested that Caballero had no right to enter the house
without José’s permission. A swearing match ensued
between the officers and Sanchez, who was placed in
handcuffs. According to Sanchez, Caballero, just before
he entered José’s house, nodded his head at a group of
four or more officers who were standing near Sanchez;
those officers then began to force Sanchez into a nearby
alley. When Sanchez resisted, the officers kicked his
legs out from underneath him and forced him to the
pavement. Sanchez testified that the officers then began
to hit and kick him, ignoring the protests of José and
Rodriguez. José made a 911 call on his cell phone to
report that the police were beating his brother. Caballero,
who by this time had emerged from the house, picked
Sanchez up off the ground, and the police left the
scene without arresting anyone.1


1
  Sanchez would later testify that the officers never returned to
him the wallet, keys, or cell phone that they took from him
                                                   (continued...)
4                                                   No. 10-3801

  Caballero and Peterson gave a significantly different
account at trial. Caballero testified that he was on patrol
with Peterson when they saw Sanchez fighting with
his brother outside of José’s home: “What I observed
was the two brothers entangled in a combative wrestling
type hold.” R. 167-1 at 7. The officers stopped to
break up the fight. Sanchez, whose face was bloodied,
was placed in handcuffs, and all four of the men
present were patted down. Caballero said he was so
familiar with the Sanchez brothers that he did not ask
either of them to produce identification. The officers
decided there was no point in making an arrest and left
the scene. They later filled out contact cards docu-
menting the encounter which omitted any mention of
a fight. Caballero wrote as the reason for the encounter
that José had been “loitering”; whereas Peterson wrote
that Sanchez had “made a suspicious movement as if
to conceal something.” Id. at 24, 25-26.
  Sanchez’s second amended complaint sought relief
from Caballero, Peterson, “as yet unknown” Chicago
police officers, and the City itself. R. 96 at 1 ¶ 3. As relevant
to this appeal, the complaint alleged that the individual
police officers had violated Sanchez’s Fourth Amend-
ment rights by arresting him without probable cause,
employing excessive force in arresting him, and failing “to


1
  (...continued)
during the initial pat-down; and his second amended complaint
included allegations that the officers unlawfully seized his
property. However, as these allegations are not material to
the issues we address in this appeal, we need not discuss
them further.
No. 10-3801                                             5

intervene and lessen or prevent the illegal stop, search,
illegal seizure, and use of excessive force inflicted upon
plaintiff . . . .” Id. at 8 ¶ 35. A companion state claim
alleged that the officers were also liable for battery and
unlawful detention under the common law of Illinois.
The complaint also alleged that under Illinois law,
because the officers were acting within the scope of their
employment when they tortiously injured Sanchez, the
City was responsible for the acts of the officers pursuant
to the doctrine of respondeat superior.
   These claims were presented to a jury over the course
of a five-day trial. Before the jury retired to consider
its verdict, however, Sanchez voluntarily dismissed all
of the state-law claims. The jury found in favor of
Caballero and Peterson on each of the federal claims
against them. Sanchez now appeals, contending that
various errors in the instructions that the court gave to
the jury and in the admission of evidence entitle him
to a new trial.


                           II.
  Before we address the particular errors that Sanchez
raises on appeal, a few words are in order regarding
the interplay between the alleged actions of the unidenti-
fied police officers and the claims against Caballero,
Peterson, and the City. This was a recurring subject of
discussion and dispute among the parties and the court
below, and because it has a bearing on some of the argu-
ments that Sanchez has made on appeal, it behooves us
to clarify the extent to which the named defendants,
including the City, could be held liable for the acts of
6                                              No. 10-3801

any unidentified police officer who may have been re-
sponsible for the injuries of which Sanchez complained.
  Sanchez was never able to identify all of the officers
who he believed participated in the April 5 incident, nor
could he produce evidence as to which of the officers,
named or unnamed, allegedly dragged him to the alley-
way, tripped and shoved him to the pavement, and
kicked or otherwise struck him while he was on the
ground. As relevant here, the only two officers he was
able to identify were Caballero and Peterson. Both
officers acknowledged that they participated in the col-
lective pat-down of Sanchez and the three other men,
but neither could recall whether he had frisked and/or
handcuffed Sanchez. Sanchez believed that Peterson
may have been one of the officers who handcuffed him
(although he was not sure), and he testified that Caballero
was standing behind him when he was placed in hand-
cuffs. Sanchez could not say that either of those two
officers actually participated in the alleged alleyway
beating. By Sanchez’s account, Caballero had gone into
his brother’s apartment before the beating commenced
(albeit after Caballero nodded his head at the officers
who allegedly beat him) and emerged after the beating
was already in progress. Peterson, on the other hand, was
allegedly standing beside Sanchez during the beating,
but Sanchez could not rule Peterson in or out of the
group of officers who actually struck him. Apart from
the alleged beating, Sanchez asserted only that either
Caballero or Peterson handcuffed him at the beginning
of the encounter, and may have used excessive force
in doing that.
No. 10-3801                                                 7

  Although Sanchez was not able to identify the other
officers involved in the encounter, his claims against
both Caballero and Peterson, as well as the City, none-
theless were premised in significant part on the actions
of the unidentified officers. It goes without saying that
Caballero and Peterson would be liable to Sanchez for
their own acts: so if they deliberately and unnecessarily
kicked Sanchez, for example, they would be liable for
that use of excessive force. But Sanchez also contended
that even if officers other than Caballero and Peterson
committed such wrongful acts, Caballero and Peterson
could be liable for the failure to intervene and stop
their fellow officers from violating Sanchez’s rights. See
Miller v. Smith, 220 F.3d 491, 495 (7th Cir. 2000); Yang v.
Hardin, 37 F.3d 282, 285 (7th Cir. 1994); Byrd v. Brishke,
466 F.2d 6, 10-11 (7th Cir. 1972). In fact, as we have
noted, Sanchez’s second amended complaint included
an express claim against the named officers for their
failure to intervene in the alleged wrongdoing of their
brethren. R. 96 at 8 (Count IV). Furthermore, Sanchez
argued that Illinois law rendered the City liable under
the doctrine of respondeat superior for the wilful and
wanton acts of its police officers, even if those officers
remain unidentified. See Williams v. Rodriguez, 509 F.3d
392, 405 (7th Cir. 2007). Sanchez’s complaint, as we have
noted, thus included a respondeat superior claim. R. 96
at 12 (Count X).2


2
  Below, Sanchez also posited an indemnification theory under
which he could somehow pursue federal claims against the
unidentified officers for which the City would be liable to
                                                (continued...)
8                                                No. 10-3801

  Although these theories had ample legal support, the
district judge at first expressed a disinclination to allow
Sanchez to pursue relief against one or more of the
named defendants based on the acts of any unidentified
police officer. The judge’s primary concern appeared
to be that Sanchez not attempt to pursue a judgment
against any unnamed officer. See R. 166 at 4-6; e.g., Eison
v. McCoy, 146 F.3d 468, 471-72 (7th Cir. 1998) (plaintiff
cannot seek relief against officer whose identity is not
ascertained until after the statute of limitations has
run). But the judge also made more sweeping remarks
suggesting that he would not permit Sanchez’s coun-
sel to so much as mention any unnamed officer. R. 166
at 5-6. When Sanchez’s counsel attempted to explain
why, in his view, both the City and the named officers
could be liable based on wrongs committed by unnamed
police officers, he was cut off. Id. at 4-5.
   Later in the trial, however, the court signaled its under-
standing that the City could bear respondeat superior
liability under state law for the wrongful acts of its
police officers, and that Peterson and Caballero could be
liable under section 1983 for their failure to intervene
in the wrongful acts of their unnamed colleagues. The
judge, for example, indicated that he would instruct
the jury as to respondeat superior liability, and he


2
  (...continued)
him under state law. R. 169-1 at 69-73. The district court
rejected this possibility for want of any cited authority sup-
porting it. Id. at 73-74. Sanchez does not press the theory
on appeal, and therefore we need not consider it.
No. 10-3801                                                9

solicited proposed instructions as to the City’s liability
under state law for the acts of its unidentified officers.
R. 169-1 at 64. And the battery instruction subsequently
approved by the court indicated that the City could
be liable for the actions of unidentified police officers.
R. 155 at 29-30. The court also indicated that the verdict
form would inquire whether Caballero, Peterson, or one
or more unidentified officers had committed a battery
upon Sanchez. R. 169-1 at 92-93. Moreover, while dis-
cussing the jury instructions as to the section 1983
claims, the court appeared to agree with Sanchez’s
counsel that Caballero and/or Peterson could be
liable for the failure to intervene in the misconduct of
the unnamed officers:
    What Mr. Cerda is saying . . . [is] in this case personal
    involvement could extend to failing to intervene. So
    theoretically, the jury could convict these two
    officers of failing to intervene in the conduct of
    these unknown officers.
R. 169-1 at 101.
  Nonetheless, two developments at the close of the trial
reflected lingering confusion by the parties, if not the
court, as to liability premised on the actions of unidenti-
fied officers.
  First, on the last day of the trial, just before closing
arguments were to begin, Sanchez’s counsel announced
that he was voluntarily dismissing the state claims, in-
cluding the respondeat superior claim against the
City. R. 170 at 2. That announcement was followed by
an off-the-record discussion to which we, naturally, are
10                                              No. 10-3801

not privy. Id. What led counsel to make the decision to
withdraw the state claims is not at all clear, especially
given the court’s prior acknowledgment that the City
could be liable for any wrongful acts (including battery)
that were committed by an unidentified police officer. In
any event, the voluntary nature of the dismissal renders
it unnecessary for us to explore that subject further.
  Second, the final jury instruction as to Sanchez’s failure-
to-intervene theory gave the jury the mistaken
impression that neither Caballero nor Peterson could
be held liable under federal law for failing to intervene
in the use of excessive force by another officer unless
one or both of these named officers himself participated
in the use of such force. R. 155 at 28; R. 170 at 74-75.
Specifically, the instruction—adapted from Seventh
Circuit Pattern Civil Jury Instruction No. 7.16—advised
the jury that the plaintiffs must first prove that “[o]ne
or both of the Defendant Officers falsely detained
and/or used excessive force on Plaintiff[.]” R. 155 at 28;
R. 170 at 74-75. Compounding the problem, the instruc-
tion went on to state that Sanchez was also obligated
to prove that “[o]ne of the Defendant Officers knew
that another Defendant was falsely detaining Plaintiff
and was using, or was about to use, excessive force on
the Plaintiff[.]” R. 155 at 28; R. 170 at 75. The instruction
thus foreclosed the possibility of holding Caballero and
Peterson liable for failing to intervene in the wrongdoing
of other, unnamed officers so long as neither of them
participated in that wrongdoing, be it falsely detaining
Sanchez or using excessive force on him. Indeed, no-
where in the instruction are the unnamed officers even
No. 10-3801                                                 11

mentioned; the language we have just quoted refers
instead to the failure to intervene in the wrongdoing
of another defendant (or, as the instruction was read to
the jury “the other defendant,” R. 170 at 75). In sum,
relief on Sanchez’s failure-to-intervene theory was con-
ditioned on a finding that one of the two named
officers falsely detained and/or used excessive force on
Sanchez, and that the other named defendant failed to
intervene in that wrongdoing. The possibility that one
or more unnamed officers might have falsely detained
Sanchez and used excessive force on him, and that Cabal-
lero and/or Peterson failed to stop those officers from
abusing Sanchez, was never communicated to the jury.
  Surprisingly, this instruction was actually proposed
by Sanchez’s counsel, and was given over the objection
of the defendants. See R. 155 at 28; R. 169-1 at 90-91; R. 170
at 4. At oral argument, Sanchez’s counsel noted the error
in the failure-to-intervene instruction, but of course
Sanchez is in no position to complain now about an
instruction that his own counsel proposed and that was
given over the objection of the defendants-appellees.
See City of Springfield, Mass. v. Kibbe, 480 U.S. 257, 259, 107
S. Ct. 1114, 1115 (1987) (“We think . . . that there would
be considerable prudential objection to reversing a judg-
ment because of instructions that petitioner accepted,
and indeed itself requested.”); Doe by & through G.S. v.
Johnson, 52 F.3d 1448, 1460 (7th Cir. 1995) (“ ‘in a civil
case, a litigant may not attack an instruction of which
he was the proponent’ ”) (quoting Williams v. Boles, 841
F.2d 181, 184 (7th Cir. 1988)).
12                                                No. 10-3801

   As the foregoing issues have not been preserved
for review, we may conclude this discussion by
making the following points. First, in a section 1983
action alleging that police violated the plaintiff’s
Fourth Amendment rights by subjecting him to excessive
force, a defendant police officer may be held to account
both for his own use of excessive force on the plaintiff,
see, e.g., Phillips v. Community Ins. Corp., 678 F.3d 513, 519-
20 (7th Cir. 2012), as well as his failure to take rea-
sonable steps to attempt to stop the use of excessive
force used by his fellow officers, Lewis v. Downey, 581
F.3d 467, 472 (7th Cir. 2009); Abdullahi v. City of Madison,
423 F.3d 763, 773-74 (7th Cir. 2005); Miller v. Smith, supra,
220 F.3d at 495; Yang v. Hardin, supra, 37 F.3d at 285.
Second, it is possible to hold a named defendant liable
for his failure to intervene vis-à-vis the excessive force
employed by another officer, even if the plaintiff cannot
identify the officer(s) who used excessive force on him.
See Byrd v. Brishke, supra, 466 F.2d at 11 (“the plaintiff
was entitled to have his case against defendants
Moran, Pfeiffer, and Finnin submitted to the jury upon
his having offered testimony that he was beaten by un-
known officers in their presence”); see also Miller, 220
F.3d at 495 (plaintiff’s inability to identify which of
named defendants was his assailant did not preclude
liability on excessive force claim; “[i]f Miller can show
at trial that an officer attacked him while another
officer ignored a realistic opportunity to intervene, he
No. 10-3801                                                     13

can recover”) 3 Third, under Illinois law, a municipality


3
  On this point, the fact that Sanchez’s claims are based on the
Fourth Amendment distinguishes this case from Harper v.
Albert, 400 F.3d 1052, 1065-67 & n.19 (7th Cir. 2005). The
majority in Harper deemed a plaintiff’s inability to identify
the prison guard(s) who allegedly beat him to be fatal to a
failure-to-intervene claim against any named defendant
guard who allegedly observed the beating and did nothing
to stop it. Because the plaintiff in Harper was serving a prison
term, his excessive force claim was based not on the Fourth
but the Eighth Amendment. Id. at 1065. To prevail on an
Eighth Amendment claim that he was subject to cruel and
unusual punishment by means of excessive force, a plaintiff
must show that force was employed to maliciously and sadisti-
cally cause him harm rather than in a good faith effort
attempt to maintain or restore discipline. Id. (quoting Hudson
v. McMillian, 503 U.S. 1, 7, 112 S. Ct. 995, 999 (1992)). The
necessity to establish the mens rea of the individual who
used force on the plaintiff, in the majority’s view, compels
a plaintiff to identify who that individual was.
    In order for courts to satisfy the mandate to inquire into
    the state of mind of prison officials who have allegedly
    caused a constitutional violation, see Wilson [v. Seiter], 501
    U.S. [294,] at 299, 111 S. Ct. 2321 [(1991)], it is most im-
    perative that we are provided with “identified culprits”;
    for “[w]ithout minds to examine, we cannot conduct an
    individualized inquiry.” K.F.P. [v. Dane County], 110 F.3d
    [516,] at 519 [(7th Cir. 1997)].
Harper, 400 F.3d at 1065. Thus, the plaintiff’s inability in Harper
to identify his alleged assailant rendered him unable to
establish that a violation of the Eighth Amendment had oc-
                                                    (continued...)
14                                                    No. 10-3801




3
  (...continued)
curred, and in turn foreclosed him from arguing that a
named defendant was liable for the failure to intervene in
that violation.
     [A]bsent any evidence or even an allegation which could
     establish a constitutionally cognizable claim for excessive
     force against any of the defendants (e.g., identification of
     the individual guard(s) who used excessive force against
     him . . .) Harper cannot possibly establish bystander
     liability as to Townley [a named defendant] or anyone
     else for failure to intervene, and his claim must fail.
Id. at 1066 (citation and footnote omitted). But see id. at 1069-70
(Flaum, J., concurring in part and concurring in the judgment)
(finding plaintiff’s testimony that unnamed guards gratui-
tously inflicted force on him in presence of named defendant
guard sufficient to create jury issue on failure-to-intervene
claim).
  By contrast, a Fourth Amendment excessive force claim does
not depend on the subjective purpose for which an officer
employed force against the plaintiff, but rather on whether
the force employed was reasonable. As we explained in
Richman v. Sheahan:
     The Eighth Amendment is about punishment, so a punitive
     purpose must be shown in an excessive-force case
     litigated under that amendment—hence the language
     about malice and sadism that we quoted. The issue
     under the Fourth Amendment “is ‘whether the officers’
     actions [were] objectively reasonable in light of the facts
     and circumstances confronting them.’ ” Smith v. Ball State
     University, 295 F.3d 763, 770 (7th Cir. 2002), quoting Graham
                                                      (continued...)
No. 10-3801                                                           15

may be held liable for battery and other wilful and
wanton tortious acts committed by its police officers in
the course of their duties, even if the plaintiff has not
been able to identify those officers. Williams v. Rodriguez,
supra, 509 F.3d at 405; Gordon v. Degelmann, 29 F.3d 295,
299 (7th Cir. 1994); McCottrell v. City of Chicago, 481
N.E.2d 1058, 1060 (Ill. App. Ct. 1985); 745 Ill. Comp. Stat.
10/2-202. With those points clarified, we may now turn
to the issues that Sanchez has preserved for appeal.




3
    (...continued)
       v. Connor, 490 U.S. 386, 397, 109 S. Ct. 1865, 104 L. Ed. 2d 443
       (1989); see also Saucier v. Katz, 533 U.S. 194, 201-02, 121 S. Ct.
       2151, 150 L. Ed. 2d 272 (2001). The officers’ intent in using
       force is irrelevant in a Fourth Amendment case. Graham v.
       Connor, supra, 490 U.S. at 397, 109 S. Ct. 1865; Phelps v. Coy,
       286 F.3d 295, 299-301 (6th Cir. 2002). Only its reasonable-
       ness matters—which means whether it was excessive in
       the circumstances, because if it was, it was unreasonable—
       and reasonableness is the focus of the briefs of both sides
       in this case.
512 F.3d 876, 882 (7th Cir. 2008). Thus, in the Fourth Amendment
context, it is not essential that the identity of the individual
officer who applied the force be established. If the plaintiff
can establish that an unknown officer subjected him to
excessive force, he may seek to hold liable other, named
officers who were present, observed the use of excessive
force, had a reasonable opportunity to stop the use of
that force, and failed to intervene. See Miller, 220 F.3d at 495,
and Byrd, 466 F.2d at 11.
16                                             No. 10-3801

A. Personal Involvement Jury Instruction
  At the request of the defendants, the district court gave
the following instruction, adapted from Seventh Circuit
Pattern Civil Jury Instruction No. 7.02, to the jury:
     In order to hold Defendant Officers Rick Caballero
     or Matthew Peterson individually liable, Plaintiff
     must prove by a preponderance of the evidence
     that each was personally involved in the conduct
     Plaintiff complains about. You may not hold Defen-
     dants Caballero or Peterson liable for what other
     employees did or did not do.
R. 155 at 33; R. 170 at 75-76. Sanchez objected to this
instruction. He maintained that the instruction was
misleading, to the extent that it could be understood to
suggest that neither Caballero nor Peterson could be
held liable for excessive force perpetrated by another
officer unless he participated in the use of that force.
The district court at first was sympathetic to this
argument, and it invited Sanchez’s counsel to submit
language that would harmonize the personal involve-
ment instruction with the instruction as to Sanchez’s
failure-to-intervene theory of liability. R. 169-1 at 103.
But once defense counsel represented to the court that
one of the plaintiff’s attorneys, who evidently was no
longer involved in the case, previously had agreed to
the wording of the instruction, the court approved the
defense version of the instruction over the plaintiff’s
objection. Id. at 104. When the court addressed the jury
instructions for the final time on the last day of trial,
Sanchez’s counsel reminded the court that it had
No. 10-3801                                             17

invited the parties to confer overnight regarding any
outstanding disputes, and counsel indicated he was
proposing an additional sentence for the personal in-
volvement instruction that would inform the jury “that
the failure to intervene can[ ] constitute a form of
personal involvement.” R. 170 at 5. The court reiterated
that it had already overruled Sanchez’s objection to
this instruction, and once it confirmed that defense
counsel had not agreed to the additional language
that Sanchez proposed, the court rejected the proposed
modification. Id.
  A threshold argument that Sanchez makes on appeal
is that it was inappropriate to give the personal involve-
ment instruction in a case that does not involve super-
visory liability, but this is a nonstarter. It is true that
the personal involvement pattern instruction was
drafted with supervisory liability cases in mind; the
two cases cited in the committee comments to the
pattern instruction, Walker v. Rowe, 791 F.2d 507, 508
(7th Cir. 1986), and Duckworth v. Franzen, 780 F.2d
645, 650 (7th Cir. 1985), both address supervisory liabil-
ity. But supervisory liability is merely one form
of liability which is premised on a defendant’s failure
to intercede in the wrongdoing of another individual
and which thus presents the danger that the jury will
deem the defendant vicariously liable for the actions of
the other person, without regard to whether the de-
fendant actually had notice, opportunity, and the ability
to prevent the other person from inflicting harm. Liability
premised on a defendant police officer’s failure to in-
tervene in the actions of another officer presents the
18                                              No. 10-3801

same danger, and it is thus within a court’s discre-
tion to give the personal involvement instruction in a
failure-to-intervene case to address that danger. Indeed,
the committee comments themselves envision that the
personal involvement instruction may be given in such
a case: that is why the comments suggest that when
the jury will also be instructed on the failure to
intervene pursuant to Pattern Civil Jury Instruction
No. 7.16, the court consider adding language har-
monizing the two instructions. See Seventh Circuit
Pattern Civil Jury Instruction No. 7.02, committee com-
ments.
  Which brings us to Sanchez’s primary argument. The
language that Sanchez proposed adding to the personal
involvement instruction, in order to harmonize it with
the pattern instruction on the failure to intervene, was
appropriate. A layperson is unlikely to understand
that Officer A’s failure to intervene in the wrongdoing
of Officer B, despite A’s knowledge of and ability to
stop the wrongdoing, is a form of personal involvement
in B’s misconduct. Yet, that is the upshot of such cases
as Miller v. Smith, supra, 220 F.3d at 495, and it is the
foundation for the failure-to-intervene theory of liability.
Without language qualifying the personal involvement
instruction, a jury might believe, mistakenly, that so
long as Officer A does not himself use excessive force
on the plaintiff, he can have no liability for Officer B’s
use of excessive force. To address that potential prob-
lem, the committee comments suggest that the failure-to-
intervene instruction be given immediately after the
personal involvement instruction and that the word
No. 10-3801                                             19

“however” be added as a preface to the failure-to-intervene
instruction. In our view, a modest one-word addition
may not suffice to harmonize the two instructions, as
Pattern Instruction No. 7.16 simply delineates the ele-
ments of a failure-to-intervene claim, without any intro-
ductory language signaling that a defendant’s failure to
intervene in the wrongdoing of another officer qualifies
as a form of personal involvement in that wrongdoing. An
additional sentence explicitly advising the jury that a
defendant officer’s failure to intervene in the wrongful
conduct of another officer, despite a reasonable opportu-
nity do so, can be a form of personal involvement in
that wrongful conduct, would be prudent. The language
that Sanchez’s counsel proposed was in line with our
own suggestion, and the district court would have
done well to entertain it. The fact that a prior attorney
for Sanchez, who was no longer involved in the case,
had agreed to the version of the personal involve-
ment instruction that the court ultimately gave was
irrelevant, particularly in view of the fact that the prob-
lem presented was one of reconciling that instruction
with a separate instruction, and the modification that
Sanchez proposed was consistent with the committee
comments’ own recognition that the two instructions
need additional language to render them consistent
with one another. It is true, of course, that Sanchez’s
attorney had the opportunity to reconcile the two in-
structions in his closing argument to the jury. But
given the obvious tension between the personal involve-
ment instruction and the failure-to-intervene instruc-
tion, the court itself should have explained to the jury
20                                              No. 10-3801

that a defendant’s failure to intervene can be a form
of personal involvement in the wrongdoing of another
officer. An attorney’s effort to reconcile competing con-
cepts, however cogent, is likely to carry less weight with
a jury, particularly when, as here, the court instructs the
jury after rather than before the attorneys deliver their
closing arguments and the court admonishes the jury to
follow its instructions even if it disagrees with them.
See Seventh Circuit Pattern Civil Jury Instruction No. 1.01;
R. 170 at 65-66; see also R. 170 at 56 (in response to objec-
tion to argument that defense counsel made in closing,
court instructed jury: “You must follow the law in
the instruction[s], and to the extent that the law is mis-
stated by either lawyer here, you should disregard
that and follow the written instructions and my verbal
instructions. You all understand.”).
  But ultimately the real problem in this case was the
one posed by the language of the failure-to-intervene
instruction that Sanchez’s counsel proposed and that
the court gave over defense objection. As we have
noted, the modified language of the failure-to-intervene
instruction erroneously advised the jury that in order
to hold Caballero or Peterson liable for the failure to
intervene in the misconduct of another officer, one or
both of the named officers themselves must have partici-
pated in that misconduct. Additionally, the instruction
did not even mention the other, unnamed officers; the
instruction was premised entirely on the use of force
by at least one of the named defendants. Whatever
potential for misunderstanding that may have been
posed by the personal involvement instruction was
No. 10-3801                                             21

thus eclipsed by the flawed language of the failure-to-
intervene instruction. The latter instruction as given
expressly precluded the jury from holding Caballero
and Peterson liable for failing to stop the false detention
and/or battering of Sanchez unless one or both of
the defendants were themselves perpetrators of those
wrongful acts. Any modification to the wording of the
personal involvement instruction thus would have
done Sanchez no good whatsoever in view of the hope-
lessly defective language of the failure-to-intervene in-
struction.
   In short, any error in the court’s refusal to embrace
the language that Sanchez posed to harmonize the
personal involvement instruction with the failure-to-
intervene instruction was harmless. The real fault lies
with the failure-to-intervene instruction itself, which
was proposed by Sanchez. Having been the proponent
of the flawed failure-to-intervene instruction, Sanchez
is foreclosed from objecting to that instruction now.


B. Testimony of Investigator Brian Killen
  In response to a complaint filed by Sanchez, Chicago’s
Independent Police Review Authority (“IPRA”) conducted
an investigation of the April 5, 2008 incident and cleared
Caballero and Peterson of wrongdoing. Prior to trial, the
district court granted Sanchez’s motion in limine to
exclude any testimony about the IPRA’s findings. R. 179
at 25-26. At trial, however, the court permitted the City
to elicit testimony from IPRA investigator Brian Killen
that he had looked into an allegation that Caballero
22                                            No. 10-3801

had threatened to plant drugs on Sanchez if he did not
withdraw his complaint to the IPRA. The threat was
reportedly made to Sanchez’s brother José. Killen
testified that he spoke with José, who denied
that Caballero made such a statement to him or that
he had reported such a threat to his brother. Over
Sanchez’s objection, Killen was also permitted to testify
more generally that he found no evidence to support
the allegation that Caballero had threatened Sanchez:
“[D]id you find any evidence to support this alleged
threat to Efrain?” defense counsel inquired. “No sir,”
Killen answered. R. 169-1 at 38.
  From the dubious premise that Killen testified as an
expert, Sanchez argues that the district court erred in
admitting his testimony because it was not properly
disclosed in advance of the trial as required by Federal
Rule of Civil Procedure 26(a)(2) and therefore was
barred by Rule 37(c)(1). He argues secondarily that
Killen’s testimony improperly intruded upon the jury’s
determination as to whether the defendants violated
his civil rights.
  The district court did not abuse its discretion in
allowing this limited testimony. It is important to empha-
size first that Killen said nothing about the IPRA’s con-
clusion as to the April 5, 2008 incident; his testimony
was confined to the collateral allegation that Caballero
had threatened to retaliate against Sanchez if Sanchez
did not withdraw his complaint. So the testimony
did not invade the jury’s province to decide what
occurred on April 5, 2008. Second, Killen did not testify
No. 10-3801                                               23

as an expert. His testimony was limited to what he
was able to ascertain as a result of his own investiga-
tion into a particular allegation. Rather than opining,
based on a body of specialized knowledge and ex-
perience, whether Caballero made a threat or whether
Sanchez was dissembling, Killen simply reported that
he found no evidence to support the allegation. See, e.g.,
United States v. Christian, 673 F.3d 702, 709 (7th Cir. 2012)
(law enforcement official is giving lay opinion if he
is testifying to what he observed or to facts derived
from his investigation); DeBiasio v. Ill. Cent. R.R., 52 F.3d
678, 686 (7th Cir. 1995) (witness is not expert to extent
he testifies to matters within his personal knowledge).
Moreover, Killen’s testimony was offered for the pur-
pose of impeaching Sanchez’s credibility, so it was
not subject to the disclosure rules governing expert testi-
mony. Rule 26(a)(3)(A); Hammel v. Eau Galle Cheese
Factory, 407 F.3d 852, 869 (7th Cir. 2005) (citing DeBiasio,
52 F.3d at 686). Third, the objected-to portion of
Killen’s testimony could not have prejudiced Sanchez in
any meaningful way. Sanchez did not object to Killen’s
testimony that José, when questioned about Cabal-
lero’s alleged threat, denied having heard a threat by
Caballero or reporting such a threat to his brother. R. 169-1
at 30, 32, 33. That was the most damaging portion of
Killen’s testimony, as it directly refuted what Sanchez
had reported to the IPRA about the threat. Killen’s addi-
tional testimony that he otherwise found no evidence
to support the allegation that a threat had been made,
was essentially punctuation. To the extent Killen’s testi-
mony necessarily disclosed the fact of the IPRA’s investi-
24                                              No. 10-3801

gation into the April 5 incident, it is worth pointing
out that Sanchez’s own counsel had already elicited
testimony in the case about the IPRA investigation and
had introduced materials from that investigation. See, e.g.,
R. 166-1 at 107; R. 167-1 at 17-20; see also R. 166-1 at 6-7
(plaintiff’s opening statement).


C. Prior Arrests
  Over Sanchez’s objection, the district court permit-
ted the defense to establish, on cross-examination of
Sanchez, that he had been arrested on several occasions
in the ten years prior to the trial; the court did not
allow evidence concerning Sanchez’s prior convictions.
R. 167-1 at 91-92. The district court deemed Sanchez’s
arrest history relevant in view of his testimony, on
direct examination, that he had suffered emotional
distress as a result of the way in which he was treated
in the April 5 encounter. Sanchez testified that he had
become afraid of the police, that he had nightmares
about them, that he did not trust them, and became
nervous when he saw them. Id. at 78-80. In the court’s
view, that testimony gave rise to an inaccurate impres-
sion that Sanchez had no real prior experience with
law enforcement officials; and “common sense” suggested
that an individual who had many prior interactions
with the police would be less likely to suffer emotional
distress from an encounter like the one Sanchez described
than an individual who had never before had contact
with the police. Id. at 85, 86, 88-91. However, in order
to limit the prejudice that might result from dis-
No. 10-3801                                              25

closure of Sanchez’s arrest record, which was lengthy
(reportedly, Sanchez had a record of more than thirty
arrests in the ten years prior to trial), the court directed
defense counsel to ask Sanchez only whether he had
“several” prior arrests in those years. Id. at 92. Further-
more, immediately after Sanchez answered “yes” to this
question, id. at 96, the court instructed the jury that it
could consider this information “only insofar as it bears
on Mr. Sanchez’s testimony that he suffered extreme
emotional distress . . . because of the actions on the night
of April 5th, 2008, for that limited purpose only,” id.
The court then asked the jury members whether they
understood the court’s instruction, and all indicated
that they did. Id.
  At the conclusion of the trial, in his closing argument
to the jury, defense counsel returned to the subject of
Sanchez’s prior arrests, and based on that history
argued that it was unlikely Sanchez had suffered
genuine emotional distress:
    Is this really [a] babe in the woods? This innocent
    who has been traumatized? He’s been arrested more
    times than you can shake a stick at.
R. 170 at 48. Sanchez’s counsel immediately objected to
this characterization of Sanchez’s arrest record. The
court sustained the objection and instructed the jury to
disregard the statement, reminding the jurors, “You’ve
heard the evidence. He was arrested several times.” Id.
  Sanchez argues that the district court was wrong to
allow evidence concerning his prior arrests. He reasons
that his arrest history had little if any bearing on the
26                                             No. 10-3801

degree of emotional distress he suffered as a result of
the April 5, 2008 encounter, given that his claim was that
he was not simply wrongfully stopped and detained,
but beaten by police officers. He reasons further that
defense counsel’s intemperate remark in closing both
mischaracterized and gave emphasis to this evidence,
magnifying its prejudicial effect.
  Reasonable people might disagree as to the probative
worth of Sanchez’s arrest history. Given Sanchez’s al-
legation that he was manhandled (and worse) by the
officers who dealt with him on April 5, the fact that
he had been arrested on multiple occasions before—
presumably without excessive force—arguably does not
undermine his claim of trauma. There is, after all, a mate-
rial difference between being arrested and being sub-
jected to excessive force in the course of that arrest.
On the other hand, Sanchez’s claim was not limited to
the alleged use of excessive force; he also alleged that
he was falsely detained. Had the jury rejected his al-
legation that he was beaten, but agreed that he was
falsely detained, his prior arrest history arguably
might have been relevant to the jury’s assessment of
any emotional distress he suffered as a result of the
improper detention.
  In any case, we have a difficult time discerning how
Sanchez was materially prejudiced by the disclosure of
his prior arrests. The court, as we have noted, limited the
evidence to the fact that Sanchez had “several” prior
arrests, and gave a limiting instruction admonishing
the jury that it was to consider this evidence only insofar
No. 10-3801                                              27

as it shed light on the extent of any emotional harm
he experienced. Defense counsel indeed was out of line
when he stated in closing that Sanchez had more con-
victions “than you can shake a stick at,” but Judge
Darrah both sustained Sanchez’s objection and reminded
the jury what the actual evidence was as to the extent
of Sanchez’s arrest history. Thus, even if we were to
assume that the court erred in allowing limited evidence
of Sanchez’s arrest record, the error was harmless.


D. Gang-related Testimony
  Officers Caballero and Peterson were members of a
gang unit, but it was agreed among all parties that
their decision to stop and question the Sanchez brothers
on April 5, 2008, was not based on any suspicion that
either of the brothers was engaged in gang-related
activity. See R. 167-1 at 5; R. 169 at 14, 62, 65-66. During
pretrial discovery, it emerged that Sanchez did have
some history of gang affiliation. R. 137. In advance of
the trial, Sanchez moved to exclude any reference to
gang activity, and the district court granted that motion
to the extent of excluding any reference to Sanchez’s
gang activity. R. 179 at 18. Sanchez contends that the
defendants nonetheless went on to mention gangs fre-
quently during the trial, with one witness (Officer
Reynaldo Serrato) testifying that José Sanchez’s home
was known for having noise disturbances and gang-
related parties. R. 169 at 5-6. The district court sustained
Sanchez’s objection to this particular testimony, but in
Sanchez’s view the bell could not be unrung; and he
28                                              No. 10-3801

contends that when defense counsel remarked in closing
argument that Sanchez had more arrests “than you
can shake a stick at,” the jury must have surmised
that Sanchez “was a gang banger who should not be
awarded any damages.” Sanchez Br. 27.
  This argument makes too much of very little testi-
mony connecting Sanchez with gang activity. The word
“gang” was uttered on a number of occasions during
the trial, but in the great majority of instances in the
context of identifying the individual defendants’ assign-
ment within the Chicago police force. This was not
prohibited by the district court’s in limine ruling, and
plaintiff’s counsel was equally responsible with defense
counsel in eliciting this information. See, e.g., R. 166-1
at 3, 70, 78; R. 167-1 at 3, 4-5. As evidence of an officer’s
experience, assignment, and qualifications, the mere
mention that the officer is a gang specialist or assigned
to a gang unit typically is appropriate and harmless. See,
e.g., Anderson v. Sternes, 243 F.3d 1049, 1053-54 (7th
Cir. 2001). Indeed, these references were not objected
to by Sanchez. The only potentially problematic refer-
ence was Officer Serrato’s reference to gang-related
parties occurring at or near José’s home; but the
district court sustained the objection to that remark and
instructed the jury to disregard it. We presume that the
jury follows such instructions. Wilson v. Groaning, 25
F.3d 581, 587 (7th Cir. 1994) (coll. cases). As Sanchez
himself concedes, “[a]t trial, there was no suggestion
from anyone that Mr. Sanchez was engaged in gang
activity at the time of the incident, or that the officers’
No. 10-3801                                             29

treatment of him was based on gang affiliation.” Sanchez
Br. 27. In this context, Serrato’s isolated remark,
which the court instructed the jury to disregard, was not
sufficiently serious to have prejudiced Sanchez.


E. Prior Fight Between Sanchez Brothers
  Over objection, the district court allowed the defense
to ask Sanchez if he had fought with his brother prior
to April 5, 2008, and, more specifically, whether Caballero
had ever broken up a fight between them prior to April 5.
Sanchez denied that they had ever fought, be it on
April 5 or on any other occasion. R. 167 at 38-39. The
defense posed that question to Sanchez in anticipation
of Caballero’s testimony that he had witnessed a fight
between the brothers on an occasion prior to the April 5
incident. Subsequently, when Caballero was on the
witness stand, Sanchez’s counsel asked him why he
had not indicated on the contact card he completed
after the April 5 incident that Sanchez and his brother
were fighting. Caballero answered:
   In this case Efrain and his brother were fighting.
   I have seen them fighting before, seen Efrain
   numerous times, and I’ve let him go. He’s begged me
   to let him go. And at this point I did the same thing.
R. 167-1 at 30. Caballero was then asked (by plaintiff’s
counsel) about the prior fight Caballero claimed to have
witnessed, and Caballero said that he previously had
encountered Sanchez and his brother engaged in a
shoving match. “We broke them up and they went on their
30                                              No. 10-3801

way,” Caballero explained. Id. at 31. Defense counsel
then explored the subject further when Caballero was re-
called in the defense case. R. 169 at 64-65. Sanchez
argues that the court erred in permitting the defense to
open this subject by allowing the initial questions of
Sanchez, which in turn led to Caballero’s testimony
about the prior shoving match.
  The court allowed this evidence as relevant to explain
why Caballero and Peterson did not arrest Sanchez or
his brother on April 5 nor document a fight between
the two brothers on the contact cards they filled out
after the incident, despite their testimony that they had
observed the brothers in a physical altercation and that
Sanchez was bleeding as a result of the fight. This was
a subject that Sanchez’s counsel had touched upon
in his opening statement to the jury and on which he
had questioned both officers. R. 166-1 at 5-6; id. at 74-75,
88, 90-91; R. 167-1 at 25, 26-27. Clearly one inference
that Sanchez wanted the jury to draw was that the fight
was a fabrication; otherwise, the logic goes, the officers
surely would have documented the fight if not arrested
one or both of the brothers.
   Sanchez’s concern is that the jury likely relied on
the prior fight not as evidence of what motivated the
defendants not to arrest him or his brother on April 5
if they had in fact been fighting as the defense claimed,
but rather as confirmation of the defense account
that there was a fight between the brothers on April 5.
In other words, Sanchez posits that this testimony con-
stituted proof of a prior bad act (the fight), offered to
No. 10-3801                                             31

show a character trait (fighting, especially with one’s
brother) with which Sanchez was acting in conformity
on the night in question. See Fed. R. Evid. 404(a)(1).
  Because this evidence was at least minimally relevant
to explain why the defendants might have decided legiti-
mately not to arrest the Sanchez brothers on April 5,
and thus had a bearing on something other than their
propensity to fight, it was within the district court’s
discretion to admit the evidence. In any event, we find
it difficult to believe that the testimony prejudiced
Sanchez. The testimony that the brothers had fought
before came from the same, self-interested source (Cabal-
lero) who asserted that the brothers were fighting on
April 5. The jury was thus unlikely to rely on the testi-
mony regarding the prior fight as independent verifica-
tion of Caballero’s disputed contention that the brothers
were involved in a fight on April 5. Moreover, coming
to blows with one’s sibling, although it may technically
be illegal, is an experience known to, if not shared by,
many individuals, and the disclosure of such a history
is not inherently prejudicial as would be the disclosure
of other prior crimes. Caballero himself remarked during
his testimony, “[M]yself, I fought with every one of my
brothers, so I know how that goes, so I didn’t think it
was a reason to lock anybody up.” R. 169 at 72; see
also R. 167-1 at 30 (“I fought my brothers hundreds of
times . . . .”).


F. Cumulative Effect of Disputed Evidentiary Rulings
  Sanchez has argued that even if any of the evidentiary
rulings that he contests were not prejudicial individually,
32                                             No. 10-3801

together they deprived him of a fair trial. We have con-
sidered this argument but, upon review of the record,
find it to be without merit.4


                           III.
  For the foregoing reasons, we A FFIRM the judgment of
the district court.




4
  In his statement of facts, Sanchez has suggested that the
district court erred in admitting his original and amended
complaints into evidence for impeachment purposes over his
objection. Sanchez Br. 15. However, he has not developed
this argument in the body of his opening brief or, for that
matter, in his reply brief. We consider the argument waived
for lack of development. E.g., Trentadue v. Redmon, 619
F.3d 648, 654 (7th Cir. 2010).


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