Oral Argument: Diaz v. United States | Case No. 23-14 | Date Argued: 3/19/24 | Date Decided: 6/20/24

Link to Docket: Here.

Background:

Federal Rule of Evidence 704(b) provides: "In a criminal case, an expert witness must not state an opinion about whether the defendant did or did not have a mental state or condition that constitutes an element of the crime charged or of a defense. Those matters are for the trier of fact alone." Fed. R. Evid. 704(b).

Question Presented: In a prosecution for drug trafficking-where an element of the offense is that the defendant knew she was carrying illegal drugs-does Rule 704(b) permit a governmental expert witness to testify that most couriers know they are carrying drugs and that drug-trafficking organizations do not entrust large quantities of drugs to unknowing transporters?

Holding: Expert testimony that "most people" in a group have a particular mental state is not an opinion about "the defendant" and thus does not violate Federal Rule of Evidence 704(b).

Result: Affirmed.

Voting Breakdown: 6-3. Justice Thomas delivered the opinion of the Court, in which Chief Justice Roberts and Justices Alito, Kavanaugh, Barrett, and Jackson joined. Justice Jackson filed a concurring opinion. Justice Gorsuch filed a dissenting opinion, in which Justices Sotomayor and Kagan joined.

Link to Opinion: Here.

Oral Advocates:

• For Petitioner: Jeffrey L. Fisher, Stanford, Cal.
• For Respondent: Matthew Guarnieri, Assistant to the Solicitor General, Department of Justice, Washington, D.C.