Petitioner Delilah Diaz was stopped at a port of entry on the United States-Mexico border. Border patrol officers searched the car that Diaz was driving and found more than 54 pounds of methamphetamine hidden in the vehicle. Diaz was charged with importing methamphetamine in violation of 21 U. S. C. §§952 and 960, charges that required the Government to prove that Diaz “knowingly” transported drugs. In her defense, Diaz claimed not to know that the drugs were hidden in the car. To rebut Diaz’s claim, the Government planned to call Homeland Security Investigations Special Agent Andrew Flood as an expert witness to testify that drug traffickers generally do not entrust large quantities of drugs to people who are unaware they are transporting them. Diaz objected in a pretrial motion under Federal Rule of Evidence 704(b), which provides that “[i]n a criminal case, an expert witness must not state an opinion 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.” The court ruled that Agent Flood could not testify in absolute terms about whether all couriers knowingly transport drugs, but could testify that most couriers know they are transporting drugs. At trial, Agent Flood testified that most couriers know that they are transporting drugs. The jury found Diaz guilty, and Diaz appealed, challenging Agent Flood’s testimony under Rule 704(b). The Court of Appeals held that because Agent Flood did not explicitly opine that Diaz knowingly transported methamphetamine, his testimony did not violate Rule 704(b). Held: Expert testimony that “most people” in a group have a particular mental state is not an opinion about “the defendant” and thus does not violate Rule 704(b). Diaz v. United StatesRead by RJ Dieken