Diaz v. United States

Delilah Diaz was stopped at the U.S.-Mexico border driving a vehicle with about 25 kilograms of methamphetamine hidden in the door panels. She claimed she did not know the drugs were present. At trial, a Homeland Security Investigations agent testified as an expert that large-scale drug trafficking organizations generally do not entrust multi-kilogram loads to unknowing couriers. The jury convicted Diaz of importing methamphetamine, and the Ninth Circuit affirmed. The Supreme Court granted review to decide whether this expert testimony violated Federal Rule of Evidence 704(b), which prohibits experts from opining on a defendant's mental state. On June 20, 2024, the Court held 6-3 that the testimony described typical courier behavior and did not express an opinion about Diaz's own knowledge or intent, so the evidence was admissible and the conviction was affirmed.

The Supreme Court affirmed the Ninth Circuit in a 6-3 decision, holding that the expert testimony was admissible because it described general courier behavior and did not opine on Diaz's personal knowledge or intent. Diaz's conviction stands.

Whether expert testimony that most drug couriers know they are transporting drugs violates Federal Rule of Evidence 704(b) by stating an opinion about the defendant's mental state.

The ruling clarifies and potentially expands the use of expert testimony about criminal organizations' methods, making it easier for prosecutors to prove knowledge or intent in drug trafficking cases. Critics argue it may risk undermining the presumption of innocence by allowing general patterns to imply a specific defendant's guilty knowledge.