Justices to Resolve Split on Drug Trafficking Expert Witnesses

The US Supreme Court agreed to resolve a dispute among border jurisdictions on when expert witnesses can testify in drug trafficking cases.

The case granted on Monday involves Delilah Diaz, who was sentenced to 84 months in prison after she was caught with $368,550 worth of methamphetamine at the Southern border. Diaz said she didn’t know there were drugs in the car and that she borrowed the vehicle from her boyfriend.

The issue for the justices relates to expert witnesses. In particular, Federal Rule of Evidence 704(b) prohibits expert witnesses in criminal cases from stating an opinion about the defendant’s mental state at the time of the crime.

The government’s expert witness testified that drug couriers typically know they are carrying drugs and that traffickers are particularly unlikely to use “blind mules” when importing large quantities of drugs.

The US Court of Appeals for the Ninth Circuit, which covers California and Arizona, affirmed Diaz’ conviction. The court said the testimony was appropriate so long as the expert didn’t opine on the defendant’s specific state of mind, but instead provided a general opinion about couriers and drug traffickers.

The Fifth Circuit, covering Texas—and thus a large part of the Southern border—has come to the opposite conclusion.

The Supreme Court agreed to resolve the dispute.

The case is Diaz v. United States, U.S., No. 23-14.