Supreme Court allows general expert testimony about what ‘most people’ in a role know in Diaz v. United States

Jim Chance introduced Diaz v. United States as the episode's second case, which examines Federal Rule of Evidence 704(b) and whether an expert may testify about what people in a given role commonly know or do without invading the jury's province to determine a defendant's specific mental state.

Evan Lee explained the facts: Diaz was arrested transporting illegal drugs across the border and said she did not know there were drugs hidden in the vehicle; the prosecution presented an expert who testified that most couriers in these schemes know they are transporting drugs because smugglers generally avoid using genuinely 'blind' couriers. Diaz argued that such testimony effectively told the jury the defendant knew, violating Rule 704(b).

The Supreme Court, in a 6–3 decision, held that the expert's testimony did not run afoul of Rule 704(b) because the expert described what most people in comparable roles do or know, not that the particular defendant had the requisite mental state. The majority construed the 704(b) exception narrowly, allowing general background testimony to aid jurors' understanding while preserving the jury's role to decide whether a defendant possessed the specific intent or knowledge required.

Justice Ketanji Brown Jackson wrote a notable concurrence emphasizing reciprocity: if prosecutors may present general expert evidence about what most people in a role know, defenses should be equally able to call experts to present an opposing general view. By contrast, as Evan Lee summarized, Justice Gorsuch's dissent warned the majority created difficult line-drawing problems, arguing that testimony about 'what most people know' often amounts to an implicit conclusion about a particular defendant's state of mind and urging lower courts to use other evidentiary rules like relevance and prejudice to exclude problematic testimony.

Panelists said the decision leaves trial judges with practical tools—Daubert reliability review, relevance and prejudice balancing under Rule 403—to limit unreliable or unfair expert testimony. Laurie Levinson cautioned that judges must continue to scrutinize experts' methods and qualifications, and reminded listeners that the decision permits but does not require admission of general expert background evidence.

The podcast ended with the observation that Diaz narrows Rule 704(b)'s reach but places a premium on rigorous judicial assessment of expert methodology and on ensuring balanced presentation when experts speak to common practices in criminal enterprises.