Episode description

Welcome to Supreme Court Opinions. In this episode, you’ll hear the Court’s opinion in Diaz v United States.

In this case, the court considered this issue: Under Federal Rule of Evidence 704(b), may a governmental expert witness testify that couriers know they are carrying drugs and that drug-trafficking organizations do not entrust large quantities of drugs to unknowing transporters to prove that the defendant knew she was carrying illegal drugs?   

The case was decided on June 20, 2024.

The Supreme Court held that 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). Justice Clarence Thomas authored the 6-3 majority opinion of the Court.

Federal Rule of Evidence 704(b) is a narrow exception to the general admissibility of ultimate issue opinions established by Rule 704(a). It only prohibits expert opinions about whether a specific defendant had a particular mental state that is an element of the crime or defense. It does not bar testimony about the mental states of people in general or groups that may include the defendant. Moreover, the language of Rule 704(b), particularly the word “about,” supports the conclusion that the rule targets specific conclusions about a defendant's mental state, not just any testimony related to mental states. This interpretation is further supported by the context of Rule 704(a), which allows opinions embracing ultimate issues, making Rule 704(b) a narrow exception to that broader rule.

Justice Kentanji Brown Jackson authored a concurring opinion to emphasize that, as Congress designed it, Rule 704(b) is party agnostic.

Justice Neil Gorsuch authored a dissenting opinion, in which Justices Sonia Sotomayor and Elena Kagan joined. Justice Gorsuch argued that the majority’s interpretation allows prosecutors to put an expert on the stand to testify as to what “most” people like the defendant think when they commit a legally proscribed act and then merely convince the jury that the defendant is like “most” people and convict.

The opinion is presented here in its entirety, but with citations omitted. If you appreciate this episode, please subscribe. Thank you.