Diaz v. United States

Supreme Court of the United States · 2024 · Evidence

EvidenceExpert testimonyFederal Rule of Evidence 704(b)Ultimate issueMental stateFed. R. Evid. 704(a)Fed. R. Evid. 704(b)ultimate issue

Facts

Diaz drove a car into the United States from Mexico, and officers discovered more than 54 pounds of methamphetamine hidden in the car. Because the charged offenses required proof that she knowingly transported drugs, Diaz defended on the ground that she did not know the drugs were concealed in the vehicle. Before trial, she objected under Federal Rule of Evidence 704(b) to proposed expert testimony from Agent Flood that drug traffickers generally do not entrust large quantities of drugs to unknowing couriers. The district court barred absolute testimony that all couriers know, but allowed Flood to testify that in most circumstances drivers know they are transporting drugs, and he so testified at trial.

Issue

Does Federal Rule of Evidence 704(b) prohibit an expert in a criminal case from testifying that most people in a group that includes the defendant have a particular mental state, when the expert does not directly opine on whether the defendant herself had that mental state?

Rule

In a criminal case, Rule 704(b) prohibits only expert opinions that state a conclusion about whether the defendant did or did not have a mental state or condition that constitutes an element of the crime charged or of a defense. An expert's opinion that most members of a group have a particular mental state is not an opinion about the defendant unless it necessarily includes a conclusion that the defendant herself had that mental state.

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One of 10 multiple-choice questions for this case. Pick an answer to see why.

In a federal firearms trafficking prosecution in El Paso, the defendant, Nora Velasquez, claims she did not know the rifles in her van were stolen. The government calls a qualified expert who testifies that most people paid to move large loads of stolen firearms know what they are transporting, but the expert never states an opinion about Nora herself.

Under the majority's interpretation of Federal Rule of Evidence 704(b), should the trial court exclude the testimony?

Explanation. Rule 704(b) is a narrow exception to Rule 704(a). It bars only expert opinions in criminal cases that state a conclusion about whether the defendant did or did not have the mental state constituting an element of the crime or defense. Testimony that most members of a group know something does not necessarily describe the defendant's own mental state, so it is not barred on that ground. (Derived from Diaz v. United States (n.d.).)