Diaz v. United States
Headline: Court allows experts to say most drug couriers know they are carrying drugs, ruling that such group-based testimony does not itself state a defendant’s mental state and is admissible at trial.
Holding: Expert testimony that "most people" in a group have a particular mental state is not an opinion about the individual defendant and therefore does not violate Federal Rule of Evidence 704(b).
- Allows experts to describe typical behavior of groups like drug couriers.
- Permits testimony that "most" group members have a certain mental state.
- Keeps final judgment about a defendant’s mindset with the jury.
Summary
Background
Delilah Diaz, a U.S. citizen, was stopped at a United States–Mexico port of entry where officers found about 54 pounds of methamphetamine hidden in the car she was driving. Diaz said she did not know the drugs were in the vehicle. She was charged with importing methamphetamine, a crime the Government must prove was done "knowingly." The Government planned to call Homeland Security agent Andrew Flood to testify that drug traffickers generally do not use unknowing couriers, and the defense objected under the rule that bars experts from stating whether a defendant had a particular mental state.
Reasoning
The central question was whether an expert saying "most" couriers know they are carrying drugs is the same as an expert saying the defendant herself had that knowledge. The Court explained that Rule 704(b) forbids experts from giving opinions about whether a particular defendant had the mental state required for a crime, but it does not bar experts from testifying about how people in a larger group typically behave. Because Agent Flood testified about the knowledge of most couriers rather than stating an opinion about Diaz specifically, the Court held his testimony did not violate the rule and left the ultimate judgment about Diaz’s state of mind to the jury.
Real world impact
Going forward, experts may describe typical practices or likely mental states of groups (for example, drug couriers) but may not directly opine that a specific defendant had the required mental state. Juries remain responsible for deciding whether the particular defendant acted knowingly. This decision affects how prosecutors and defenses present expert evidence in criminal trials.
Dissents or concurrances
Justice Jackson wrote separately to emphasize that the rule applies to both sides and that expert group evidence can help juries; Justice Gorsuch dissented, warning that permitting "most people" testimony risks undermining the rule and shifting the jury’s role.
Ask about this case
Ask questions about the entire case, including all opinions (majority, concurrences, dissents).
What was the Court's main decision and reasoning?
How did the dissenting opinions differ from the majority?
What are the practical implications of this ruling?