Supreme Court, by 6-3 vote, rejects claim that mental state expert testimony violated FRE 704(b)

The Supreme Court this morning handed down a short opinion in Diaz v. United States, No. 23-14 (S. Ct. June 20, 2024) (available here).  Justice Thimas authored the opinion for the Court, which starts this way:

Federal Rule of Evidence 704(b) prohibits expert witnesses from stating opinions “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.”  In this drug-trafficking prosecution, petitioner argued that she lacked the mental state required to convict because she was unaware that drugs were concealed in her car when she drove it across the United States-Mexico border.  At trial, the Government’s expert witness opined that most drug couriers know that they are transporting drugs.  Because the expert witness did not state an opinion about whether petitioner herself had a particular mental state, we conclude that the testimony did not violate Rule 704(b).  We therefore affirm.

Justice Jackson authored a concurrence which explains and stresses that “the type of mental-state evidence that Rule 704(b) permits can prove essential not only for prosecutors, but for defendants as well.”

Justice Gorsuch authored a dissent joined by Justices Sotomayor and Kagan that starts this way:

Federal Rule of Evidence 704(b) prohibits an expert witness from offering an opinion “about whether the defendant did or did not have [the] mental state” needed to convict her of a crime.  “Those matters,” the Rule instructs, “are for the trier of fact alone.” Following the government’s lead, the Court today carves a new path around that command. There’s no Rule 704(b) problem, the Court holds, as long as the government’s expert limits himself to testifying that most people like the defendant have the mental state required to secure a conviction.

The upshot?  The government comes away with a powerful new tool in its pocket.  Prosecutors can now put an expert on the stand — someone who apparently has the convenient ability to read minds — and let him hold forth on what “most” people like the defendant think when they commit a legally proscribed act.  Then, the government need do no more than urge the jury to find that the defendant is like “most” people and convict.  What authority exists for allowing that kind of charade in federal criminal trials is anybody’s guess, but certainly it cannot be found in Rule 704.