A drug-trafficking defendant argued that an expert violated Federal Rule of Evidence 704(b) by testifying that most couriers know they are transporting drugs. The Supreme Court held that the testimony was permissible. Rule 704(b) bars an expert from stating an opinion about whether the particular defendant possessed the mental state constituting an element or defense; it does not prohibit testimony about the mental states generally associated with a class of persons. Because the expert did not directly opine that Diaz herself knew about the drugs, admission of the testimony did not violate the Rule. The judgment was affirmed.