602 U.S. 526
SCOTUS2024Background
- Delilah Diaz was stopped at a U.S.-Mexico border entry point while driving her boyfriend’s car; officers found over 54 pounds of methamphetamine hidden in the car.
- Diaz was charged with importing methamphetamine, requiring the government to prove she “knowingly” transported the drugs.
- Diaz claimed she was unaware of the drugs, asserting a “blind mule” defense (unknowing courier).
- The government planned to call a Homeland Security agent, Andrew Flood, to testify as an expert that most drug couriers know they are transporting drugs.
- Diaz objected, arguing this expert testimony would violate Federal Rule of Evidence 704(b), which bars expert opinions on whether the defendant had the requisite mental state.
- The district court permitted testimony that most couriers know, but not that all do; Diaz was convicted, appealed, and the Ninth Circuit affirmed, finding no Rule 704(b) violation.
Issues
| Issue | Plaintiff's Argument | Defendant's Argument | Held |
|---|---|---|---|
| Whether expert testimony that "most couriers know" violates Rule 704(b) by implying the defendant's mental state | Such testimony is functionally an opinion about Diaz and invades the jury’s role | Testimony does not address Diaz’s specific mental state, but only group tendencies | Testimony about "most couriers" is not about the defendant and does not violate Rule 704(b) |
| Interpretation of "about" in Rule 704(b) | "About" covers any testimony that concerns or references the defendant’s mental state | "About" should be read narrowly to mean only testimony expressing a conclusion regarding the defendant’s actual mental state | Rule 704(b) covers only opinions that expressly state the defendant’s mental state, not generalized testimony |
| Scope of permissible expert testimony under Rules 704(a) and 704(b) | Rule 704(b) should bar such expert group-based testimony since it concerns the ultimate issue | The rules allow expert opinions on general group behaviors but bar only conclusions on the specific defendant | Rule 704(b) is a narrow exception and does not bar all testimony related to mental state—only opinions about the defendant’s mental state |
| Whether the court’s interpretation undermines Rule 704(b)’s purpose | Allowing group-based testimony will gut 704(b) and let experts unduly influence juries | Such testimony does not usurp the jury’s function or opine directly on the ultimate issue | Generalized group opinions do not violate 704(b); the jury retains its fact-finding role |
Key Cases Cited
- United States v. Detroit Timber & Lumber Co., 200 U.S. 321 (clarifying that the syllabus is not part of the Court’s opinion)
- United States v. Spaulding, 293 U.S. 498 (explained the role of the ultimate issue rule and witness testimony)
- Miranda v. Arizona, 384 U.S. 436 (established the Miranda rights; referenced regarding Diaz’s waiver of rights)
- Francis v. Franklin, 471 U.S. 307 (clarified that the jury must determine state of mind independently)
- Morissette v. United States, 342 U.S. 246 (discussed the fundamental role of mens rea in criminal law)