Diaz v. United States

The Supreme Court held that expert testimony stating "most people" in a certain group possess a particular mental state does not violate Federal Rule of Evidence 704(b), provided the expert does not explicitly offer an opinion about the specific defendant's mental state. The case arose from a drug trafficking prosecution where the defendant claimed she was unaware of the drugs in her vehicle, and the government introduced expert testimony that drug couriers generally know they are carrying contraband.