The question
Whether expert testimony that most people in a group have a particular mental state violates Federal Rule of Evidence 704(b), which bars an expert in a criminal case from stating an opinion about whether the defendant did or did not have a mental state that is an element of the crime or a defense.
Diaz v. United States
What the Court decided
The Court affirmed the Ninth Circuit and upheld Diaz's conviction. Expert testimony that most people in a group have a particular mental state is not an opinion about the defendant and therefore does not violate Federal Rule of Evidence 704(b). The agent testified that most drug couriers know they are transporting drugs, which does not necessarily describe Diaz's own mental state, so the ultimate question of what Diaz knew was left to the jury.
How the justices split
6-3The Court's opinion
Thomas, joined by Roberts, Alito, Kavanaugh, Barrett, Jackson
Rule 704(b) is a narrow exception to Rule 704(a) and bars only expert conclusions about whether the defendant had a mental state that is an element of the crime or a defense. The agent testified about the knowledge of most drug couriers, not about whether Diaz herself knowingly transported the drugs. An opinion about most couriers is not an opinion about all couriers, so the testimony left the ultimate issue of Diaz's mental state to the jury and did not violate Rule 704(b).
Concurring
Jackson, writing alone
Jackson joined the Court's opinion in full and wrote separately.
Dissenting
Gorsuch, joined by Sotomayor, Kagan
Gorsuch, joined by Sotomayor and Kagan, filed a dissenting opinion disagreeing with the majority's application of Rule 704(b) to the expert testimony.
How we know · 5 sourced claims
Expert testimony that most people in a group have a particular mental state is not an opinion about the defendant and does not violate Rule 704(b).
Source: syllabus, Held
Rule 704(b) is an exception to Rule 704(a) and prohibits only expert conclusions as to the defendant's own mental state.
Source: syllabus, Held
The agent testified that most drug couriers know they are transporting drugs, which does not necessarily describe Diaz's mental state.
Source: syllabus, Held
Thomas delivered the opinion of the Court, joined by Roberts, Alito, Kavanaugh, Barrett, and Jackson.
Source: syllabus, lineup
Gorsuch filed a dissenting opinion joined by Sotomayor and Kagan.
Source: syllabus, lineup