On June 20, 2024, the U.S. Supreme Court decided Diaz v. United States, No. 23-14, holding an expert’s opinion that “most people” in the defendant’s situation have a particular mental state is not an inadmissible opinion “about whether the defendant did or did not have a mental state” under Federal Rule of Evidence 704(b).

When Delilah Diaz tried to drive into the United States from Mexico, an officer stopped the car and found 54 pounds of methamphetamine hidden inside secret compartments. The federal government charged Diaz with “knowingly” importing methamphetamine. Diaz maintained that the car belonged to her boyfriend and that she didn’t know about the drugs. As the case proceeded to trial, the government offered expert testimony that “most” drug couriers are aware that they are carrying drugs. Diaz argued that this expert evidence was inadmissible under Federal Rule of Evidence 704(b), which states that “[i]n 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.”

A federal district court ruled the evidence admissible. The government presented its expert testimony at trial, and Diaz was convicted. The Ninth Circuit affirmed.

The Supreme Court affirmed, ruling that an “expert’s conclusion that ‘most people’ in a group have a particular mental state is not an opinion about “the defendant” and thus does not violate Rule 704(b).’” The Court explained that the government’s expert did not testify that “Diaz herself knowingly transported methamphetamine.” His testimony left it to the jury to decide whether Diaz was like the majority of couriers, who know that they are transporting drugs, or not. The Court declined to adopt Diaz’s argument that the expert “functionally” gave an opinion about her mental state. That would only be true, the Court said, if the government’s expert testified that “all” couriers know they are transporting drugs.”

Justice Thomas delivered the opinion of the Court, in which Chief Justice Roberts, and Justices Alito, Kavanaugh, Barrett, and Jackson joined in full. Justice Jackson filed a concurring opinion. Justice Gorsuch filed a dissenting opinion, in which Justices Sotomayor and Kagan joined.

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