Diaz v. United States
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Overview
Overview
The Supreme Court held 6-3 that a government expert witness in a federal drug trafficking prosecution may testify that most people in the defendant's circumstances knowingly transport drugs without violating Federal Rule of Evidence 704(b)'s prohibition on expert opinions about a defendant's mental state, as long as the expert does not directly opine on this particular defendant's intent. Justice Kavanaugh wrote for the majority; Justice Jackson dissented, joined by Justices Sotomayor and Gorsuch.
The Facts
Facts
Delilah Diaz was charged with importing methamphetamine after border agents discovered drugs hidden in a vehicle she was driving; her defense was that she did not know the drugs were there. The government presented a Homeland Security expert who testified that in his experience the vast majority of drug couriers know they are transporting controlled substances. Diaz challenged this testimony as barred by Rule 704(b) because it functionally addressed the central mental-state question and directed a guilty verdict.
The Issue
Issue
Whether Federal Rule of Evidence 704(b)'s prohibition on expert opinions about a criminal defendant's mental state bars an expert from testifying that people in the defendant's general circumstances typically possess the required mental state, when the expert does not expressly attribute that state of mind to this particular defendant, with Diaz arguing the testimony functionally resolved the ultimate issue and the government arguing 704(b) bars only direct attribution of intent to the specific defendant.
The Rules
Rule
Federal Rule of Evidence 704(b) prohibits expert witnesses from directly opining on whether a criminal defendant did or did not have the mental state required for the offense; the majority held this bar applies only to direct opinions about the particular defendant's intent and does not prohibit testimony about the general knowledge patterns of people in comparable factual circumstances.
The Application
Analysis
The Homeland Security expert's testimony that the vast majority of drug couriers know they are transporting controlled substances satisfied Rule 704(b) because the expert never directly opined that Diaz specifically possessed this knowledge. Although Diaz contended the testimony effectively resolved the ultimate mental-state question by creating a powerful inferential path to guilt, the majority held that Rule 704(b) bars only express attribution of mental state to the defendant on trial, not general testimony about knowledge patterns in comparable circumstances. Under this holding, the government could present expert profiling evidence that strongly suggests Diaz's awareness of the drugs without technically violating the prohibition, provided the expert maintained the formal distinction between what most couriers know and what this particular defendant knew.
The Conclusion
Conclusion
Prosecutors may elicit expert testimony that persons in circumstances similar to the defendant's generally possess knowledge of criminal activity, so long as the expert does not directly attribute intent to the specific defendant. Courts need only confirm that the expert stopped short of expressly stating what this defendant knew or intended. The ruling permits expert profiling evidence that closely approaches the ultimate issue without technically crossing the line Rule 704(b) draws.
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