Diaz v. United States

Delilah Diaz was stopped at the U.S.-Mexico border with methamphetamine hidden in her car. She claimed ignorance, arguing she thought the car contained a different substance. During the trial, a government expert witness testified that drug couriers typically know they are transporting drugs. Diaz was convicted. She appealed, arguing the expert's testimony violated Federal Rule of Evidence 704(b), which forbids experts from opining on whether a defendant had the specific mental state constituting an element of the crime. The Supreme Court reversed the Ninth Circuit's decision, ruling that the expert's testimony did not violate Rule 704(b) because it described typical courier knowledge rather than directly stating Diaz herself knew.

The U.S. Supreme Court held, in a 6-3 decision authored by Justice Clarence Thomas, that the expert's testimony did not violate Rule 704(b). The Court reasoned that the expert spoke about the typical mental state of a hypothetical courier, not directly about Diaz's own knowledge, and such testimony assists the jury without usurping its role. Diaz's conviction was upheld.

The central dispute was whether an expert witness's testimony that "most people" in a defendant's situation would know they were transporting drugs violates Federal Rule of Evidence 704(b), which prohibits experts from stating an opinion about whether the defendant did or did not have a mental state that constitutes an element of the crime charged.

The ruling clarifies the boundaries of expert testimony in criminal trials, particularly in drug trafficking cases. By permitting experts to testify about the general knowledge of hypothetical "most people" in a defendant's position, the decision potentially makes it easier for prosecutors to prove a defendant's criminal intent or knowledge circumstantially. Critics argue this effectively erodes Rule 704(b)'s protections, allowing expert opinions on ultimate issues to slip through a semantic loophole, while proponents contend it provides necessary context for juries evaluating complex criminal operations.