Diaz v. United States
Issues
Does Federal Rule of Evidence 704(b) allow an expert witness to testify that most drug couriers carry drugs knowingly, and that drug-trafficking organizations do not trust unknowing couriers with large quantities of drugs, when knowledge is an element of the offense?
This case asks the Supreme Court to decide whether Federal Rule of Evidence 704(b) prohibits expert testimony about the mental states of a group that the defendant is a member of. Diaz was convicted of knowingly importing drugs into the United States after a government expert testified that “in most circumstances, the driver knows they are hired . . . to take the drugs from point A to point B.” Diaz argues that such testimony is impermissible under Rule 704(b) because it is functionally equivalent to stating an opinion that she had a certain mental state. The United States counters that the rule only restricts an expert from explicitly concluding or directly opining on whether a defendant had a certain mental state. The outcome of this case will affect how the government goes about proving its criminal cases, how effectively parties can rebut expert testimony, and how juries make decisions about mental states.
Questions as Framed for the Court by the Parties
Whether in a prosecution for drug trafficking — where an element of the offense is that the defendant knew she was carrying illegal drugs — Federal Rule of Evidence 704(b) permits a governmental expert witness to testify that most couriers know they are carrying drugs and that drug-trafficking organizations do not entrust large quantities of drugs to unknowing transporters.
Delilah Guadalupe Diaz lives in Moreno Valley, California. Brief for Respondent, the United States at 5. One night in August 2020, she drove across the border from Mexico into the United States. Id. She told a border patrol officer who inspected the car that it was her boyfriend’s.
Additional Resources
- Paul Shechtman, ‘Diaz’: SCOTUS to Consider Expert Law Enforcement Testimony, New York Law Journal (Jan. 26, 2024).
- Caitlin Williams, US Supreme Court Agrees to Consider Scope of Expert Witness Testimony in Drug Trafficking Case, JURIST (Nov. 13, 2023).