Diaz v. United States
Headline: Court allows experts to say most drug couriers know they carry drugs, ruling that such group-based testimony does not itself declare whether a specific defendant knew, affecting evidence in criminal trials.
Holding: The Court held that an expert's testimony that most people in a relevant group have a particular mental state is not an opinion about the defendant and therefore does not violate Rule 704(b).
- Allows experts to testify about what most people in a group think, not about the defendant.
- Permits prosecutors and defense to offer group-based mental-state expert evidence.
- Trial judges can exclude or limit such testimony under other evidence rules.
Summary
Background
A woman, Delilah Diaz, was stopped at a United States–Mexico port of entry and officers found about 54 pounds of methamphetamine hidden in the car she was driving. She was charged with importing drugs, a crime the Government must prove was done "knowingly." Diaz claimed she did not know the drugs were in the car and offered a “blind mule” defense. The Government planned to call a Homeland Security agent as an expert to say drug traffickers generally do not use unknowing couriers, and the district court allowed the agent to testify that most couriers know they are transporting drugs.
Reasoning
The central question was whether an expert may say most people in a class have a particular mental state without violating Rule 704(b) (the rule that bars experts from stating whether a defendant had the mental state required for a crime). The Court held that an expert opinion about what most couriers know is not the same as an opinion about whether Diaz herself knew, so the testimony did not violate Rule 704(b). The majority emphasized the narrow scope of the rule and left the ultimate question of Diaz’s knowledge for the jury to decide.
Real world impact
The decision lets experts testify about common practices or typical mental states of a group, while preserving the jury’s role in deciding any particular defendant’s state of mind. The Court noted other evidence rules and careful cross-examination remain available to limit unreliable or prejudicial expert testimony. This ruling affirmed Diaz’s conviction.
Dissents or concurrances
Justice Jackson concurred, stressing the rule applies to both sides and can aid juries; Justice Gorsuch dissented, warning group testimony risks usurping the jury and undermines Rule 704(b).
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