Diaz v. United States
602 U. S. ____ (2024) (2024)
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Rule of Law:
An expert witness's testimony that "most" people in a defendant's position have a particular mental state does not violate Federal Rule of Evidence 704(b)'s prohibition on stating an opinion "about whether the defendant" had that mental state.
Facts:
- In August 2020, Delilah Diaz drove a car from Mexico into the United States at a port of entry.
- A border patrol officer found the car's rear window difficult to operate and heard a "crunch-like sound" inside the door.
- A subsequent search revealed 56 packages of methamphetamine, weighing over 54 pounds, hidden in the car's door panels and under the trunk carpet.
- Diaz claimed she was driving her boyfriend's car and did not know there were drugs hidden inside, later giving contradictory details about the boyfriend.
- For her defense at trial, Diaz argued she was a "blind mule," meaning she was unaware she was transporting drugs.
Procedural Posture:
- Delilah Diaz was charged with importing methamphetamine in the U.S. District Court for the Southern District of California, a federal trial court.
- The government notified its intent to have an expert testify that drug traffickers do not typically use unknowing couriers.
- Diaz filed a pre-trial motion to exclude this testimony under Federal Rule of Evidence 704(b).
- The district court ruled that the expert could not testify in absolute terms ('all couriers know') but could testify that 'most' couriers have knowledge.
- Following a trial, a jury found Diaz guilty.
- Diaz, as appellant, appealed her conviction to the U.S. Court of Appeals for the Ninth Circuit, an intermediate federal appellate court.
- The Ninth Circuit affirmed the conviction, holding that the expert's testimony was permissible because it was not an 'explicit opinion' on Diaz's specific state of mind.
- The U.S. Supreme Court granted a writ of certiorari to review the Ninth Circuit's decision.
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Issue:
Does an expert witness violate Federal Rule of Evidence 704(b) by testifying that most drug couriers know they are transporting drugs, when the defendant's knowledge is an element of the charged crime?
Opinions:
Majority - Justice Thomas
No. An expert does not violate Federal Rule of Evidence 704(b) by testifying that most drug couriers know they are transporting drugs because such testimony is about a group, not an explicit opinion about 'the defendant' herself. Rule 704(b) is a narrow exception to Rule 704(a), which generally permits ultimate issue testimony. The rule specifically proscribes opinions 'about whether the defendant' had the requisite mental state. Agent Flood's testimony was about the general practices of drug trafficking organizations and the knowledge of 'most' couriers, not about Diaz's personal knowledge. This testimony is distinct from an opinion about 'all' couriers, which would necessarily include the defendant. By testifying about 'most' couriers, the expert provided evidence for the jury to consider while leaving the ultimate inferential step—deciding whether Diaz was like the majority or the minority of couriers—to the jury alone.
Concurring - Justice Jackson
No. Rule 704(b) is a narrow, party-agnostic rule that correctly permits expert testimony about the likelihood of a mental state based on group characteristics, which can be crucial for both the prosecution and the defense. This interpretation allows prosecutors to present evidence about the common practices of criminal organizations, but it also preserves a critical tool for defendants. For instance, a defendant could offer expert testimony about the typical mental states of individuals with schizophrenia or battered woman syndrome to help jurors understand a condition and challenge assumptions about mens rea. The rule strikes a proper balance, allowing probative expert testimony while reserving the ultimate judgment about the specific defendant's mental state for the jury. Other evidentiary rules (e.g., Rules 403 and 702) and procedural safeguards like cross-examination and jury instructions remain available to prevent misuse of such testimony.
Dissenting - Justice Gorsuch
Yes. An expert witness violates Federal Rule of Evidence 704(b) by testifying that most people like the defendant possess the required mental state because such testimony is an opinion 'about' the defendant's mens rea, a matter reserved exclusively for the jury. The word 'about' should be read broadly to mean 'concerning' or 'in reference to,' and testimony that 'most' people in the defendant's position have a certain mental state is clearly 'about' the defendant's mental state. The majority’s distinction between 'most' and 'all' is a linguistic trick that creates a new path around the Rule's clear command. This decision invites a 'battle of experts' speculating about a defendant's thoughts, diminishes the jury's central role, and allows the government to bolster its case with what amounts to 'junk science' about mind-reading.
Analysis:
This decision clarifies the scope of FRE 704(b), establishing that testimony about the general tendencies of a group ('most people') does not constitute a prohibited opinion on the specific defendant's mental state. This provides prosecutors with a clear method for using law enforcement experts to rebut 'blind mule' or other lack-of-knowledge defenses by describing common criminal practices. Conversely, the ruling confirms that defense attorneys can similarly use experts to explain group characteristics, such as those related to mental health conditions or patterns of abuse, to cast doubt on the defendant's mens rea. The Court's narrow interpretation prevents Rule 704(b) from becoming a broad prohibition on any testimony that touches on a defendant's likely state of mind, preserving it as a tool for both sides.
