United States v. Williams, 900 F.3d 486

Jaboree Williams was convicted of multiple offenses arising from a sex‑trafficking scheme in which he recruited, controlled, and violently abused women forced into prostitution.
The government disclosed FBI agent Amy Mentzel as an expert, but its Rule 16(a)(1)(G) summary listed only broad topics (e.g., recruiting, advertising, coercion) without detailing the substance of her testimony.
Williams moved to exclude Mentzel pretrial for inadequate disclosure and as impermissible character evidence; the district court deferred and later overruled his objections and admitted her testimony.
Mentzel testified about common pimp/sex‑trafficker methods, coercion, terminology, money handling, and control tactics, but did not offer opinions about Williams specifically.
Substantial lay and documentary evidence — victim testimony, admissions, photos of injuries, ads linked to Williams, and messages — supported the government’s case; the jury convicted on almost all counts.

Issue	Plaintiff's Argument (Williams)	Defendant's Argument (Government)	Held
Adequacy of expert disclosure under Rule 16(a)(1)(G)	Notice was too vague (mere list of topics); summary required actual expected testimony	Rule 16 summary need only list topics for background/non‑opinion experts	Court: Government likely violated Rule 16 because summary must summarize expected testimony, not just list topics, but error was harmless
Prejudice from inadequate disclosure	Vague notice prevented preparation, rebuttal expert, and effective objections	Williams was not unduly surprised and suffered no demonstrable prejudice	Court: No prejudice shown; any deficiency was harmless given overwhelming other evidence
Admissibility as impermissible character/group evidence under Rule 404	Mentzel’s testimony effectively showed a typical sex‑trafficker character, implying Williams’s propensity	Testimony described modus operandi, not character; used to contextualize acts, not to prove propensity	Court: Testimony was admissible as modus operandi/background, not forbidden character evidence; Rule 404 inapplicable
Effect of expert testimony on verdict	Expert improperly bolstered prosecution and affected juror assessment of victims	Expert did not address witness credibility; victims’ testimony and documentary evidence were decisive	Court: Even excluding Mentzel, jury would likely convict; error harmless

United States v. Duvall, 272 F.3d 825 (7th Cir.) (Rule 16 requires a summary of expected expert testimony, not a mere list of topics)
United States v. Thornton, 642 F.3d 599 (7th Cir.) (no prejudice where defendant not unduly surprised by disclosure)
United States v. Saunders, 826 F.3d 363 (7th Cir.) (harmless‑error standard asks whether jury would find prosecution significantly less persuasive without the evidence)
United States v. Romero, 189 F.3d 576 (7th Cir.) (expert testimony about common offender techniques can be admissible to explain modus operandi, not as character evidence)
United States v. Doe, 149 F.3d 634 (7th Cir.) (expert testimony illuminating modus operandi of criminal enterprises is not impermissible propensity evidence)