The 6-3 majority concluded with the following in relevant part: Diaz’s counterarguments are unpersuasive. She first argues that Agent Flood functionally stated an opinion about whether she knowingly transported drugs when he opined that most couriers know that they are transporting drugs. But an opinion about most couriers is not an opinion about all couriers. Agent Flood asserted that Diaz was part of a group of persons that may or may not have a particular mental state. The ultimate issue of Diaz’s mental state was thus left to the jury’s judgment.
Diaz next relies on dictionary definitions of “about” to argue that Rule 704(b)’s phrase “state an opinion about” includes all testimony that “concerns” whether the defendant had a particular state of mind. That text’s surrounding context, however, makes clear that Rule 704(b) addresses only conclusions as to the defendant’s mental state.
Rule 704(a) further confirms the narrow scope of testimony prohibited by Rule 704(b). Because Rule 704(b) is an “exception” to Rule 704(a), Rule 704(b) can only be understood to cover a subset of the testimony that Rule 704(a) expressly allows, which is opinion testimony that includes ultimate issues. Diaz’s reading would have the exception swallow the rule.
Justice Thomas was joined by Chief Justice Roberts and Justices Alito, Kavanaugh, Barrett, and Jackson. Justice Gorsuch filed a dissenting opinion in which he was joined by Justices Sotomayor and Kagan. The dissent noted that this opinion provides the Government with a powerful new tool in which they could put forth probabilistic testimony from an expert witness who acts as a mind reader. Then, they can urge the jury to conclude that the defendant acted consistently with the way that most people probably act. Under those circumstances, the defense should be allowed to highlight that the expert witness did not offer an opinion about the particular defendant. However, the defense might be barred from making this argument since expert testimony concerning the particular defendant is not allowed under the evidence rules.
