## Facts

- Delilah Guadalupe Diaz drove a vehicle from Mexico into California and was stopped at a border checkpoint.
- A Customs and Border Protection officer became suspicious after hearing a “crunch-like” sound when Diaz rolled down the window.
- Officers used a density-measuring device on the door panels and discovered nearly 28 kilograms of methamphetamine concealed inside the vehicle.
- Diaz told officers she did not know about the drugs and explained she had stayed in Mexico to visit her boyfriend, who lent her a car to return to the United States.
- The United States charged Diaz with importation of methamphetamine under 21 U.S.C. §§ 952 and 960, requiring proof that she acted knowingly.
- The government sought to present expert testimony from a Homeland Security Investigations agent that traffickers generally do not entrust large quantities of drugs to unknowing couriers and that most couriers know they are transporting drugs.
- Diaz moved to exclude the testimony under Federal Rule of Evidence 704(b); the district court limited the testimony but allowed the “most couriers know” and “traffickers rarely use unwitting couriers” opinions.
- The jury convicted Diaz, and she was sentenced to seven years’ imprisonment.
- The Ninth Circuit affirmed, reasoning the expert did not explicitly state an opinion about Diaz’s own mental state.
- The Supreme Court affirmed the Ninth Circuit’s judgment.

## Issues

1. Whether Federal Rule of Evidence 704(b) bars a government expert from testifying, in a case where knowledge is an element, that most drug couriers know they are carrying drugs and traffickers generally do not use unknowing couriers for large shipments.
2. Whether an expert opinion stated at a group level constitutes an impermissible opinion about “the defendant’s” mental state within the meaning of Rule 704(b).

## Decision

- The Supreme Court affirmed the judgment upholding Diaz’s conviction.
- The Court held Rule 704(b) prohibits only expert opinions that state whether the defendant herself had the required mental state.
- The Court ruled that testimony about the typical knowledge of “most couriers,” and trafficking organizations’ usual practices regarding couriers, is not an opinion about the defendant’s mental state and therefore is not barred by Rule 704(b).
- The Court noted that other evidentiary rules (including Rules 401, 403, and 702) remain available to limit unreliable or unfairly prejudicial expert testimony.

## Legal Principles

- Rule 704(b) is a narrow exception to Rule 704(a)’s general rule allowing opinion testimony that embraces an ultimate issue.
- In a criminal case, an expert may not state an opinion about whether the defendant did or did not have a mental state that constitutes an element of the charged offense or a defense.
- Expert testimony describing typical conduct or typical mental states of a group (e.g., “most couriers know”) does not violate Rule 704(b) unless it explicitly addresses the particular defendant’s mental state.
- Even when Rule 704(b) does not bar the evidence, trial courts may exclude or limit expert testimony under Rules 401 and 403 (relevance and unfair prejudice) and Rule 702 (reliability and fit).

## Conclusion

The Court upheld admission of generalized expert testimony that most drug couriers know they carry drugs and that traffickers typically avoid unwitting couriers, holding that Rule 704(b) bars only opinions directly stating whether the defendant herself had the requisite mental state.


United States v. Diaz, 602 U.S. ___ (2024)