Diaz v. United States
In brief
Experts can describe what most people in a relevant group know, but they cannot directly tell a jury that this defendant had the required mental state.
- Where it stands
- Decided
Decided 6-3 · June 20, 2024 · Opinion by Justice Thomas
- What it’s about
- The Supreme Court held that expert testimony stating "most people" in a certain group possess a particular mental state does not violate Federal Rule of Evidence 704(b), provided the expert does not explicitly offer an opinion about the specific defendant's mental state.
- Who it affects
- Federal prosecutors may use expert testimony about what people in a group generally know, while juries still decide what a particular defendant knew.
Summary: written with AI from the case record.
What it's about
The case arose from a drug trafficking prosecution where the defendant claimed she was unaware of the drugs in her vehicle, and the government introduced expert testimony that drug couriers generally know they are carrying contraband.
The decision defines the boundary between helpful expert testimony and the jury's role in deciding a criminal defendant's state of mind.
Question presented
Under Federal Rule of Evidence 704(b), may a governmental expert witness testify that couriers know they are carrying drugs and that drug-trafficking organizations do not entrust large quantities of drugs to unknowing transporters to prove that the defendant knew she was carrying illegal drugs?
What the Court decided
Holding
Expert testimony that “most people” in a group have a particular mental state is not an opinion about “the defendant” and thus does not violate Rule 704(b). Affirmed.
- Result
- Affirmed
The vote
- Joined the judgment
- Concurred in a separate opinion
- Dissented
- Wrote an opinion
- Concurring · 1agreed with the result, wrote separately
- Jacksonwrote separately
From the opinions
“Expert testimony that “most people” in a group have a particular mental state is not an opinion about “the defendant” and thus does not violate Rule 704(b).”
“There’s no Rule 704(b) problem, the Court holds, as long as the government’s expert limits himself to testifying that most people like the defendant have the mental state required to secure a conviction.”
What's next
Lower federal courts will apply this distinction when deciding whether to admit expert testimony in criminal trials. Prosecutors and defense lawyers will shape expert questions to avoid direct opinions about the individual defendant's mental state.
Why it matters
For example, in a drug-trafficking trial, an expert may describe couriers' typical knowledge but may not say the defendant knew about the drugs.
Documents
Docket activity
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AI analysis generated: Case Briefing
AI analysis generated: Decision Record
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Brief added: Questions Presented
More questions
- What did the Supreme Court rule in Diaz v. United States?
- The Court said experts may testify about what most people in a group know. They may not directly state that the defendant had the required mental state.
- Who won Diaz v. United States?
- The United States won. The Court affirmed the Ninth Circuit's judgment by a 6-3 vote.
- What does Diaz v. United States mean for drug-courier trials?
- Prosecutors may offer expert testimony that couriers generally know they carry contraband. The jury must still decide whether the particular defendant knew.
- Is Rule 704(b) still the law after Diaz v. United States?
- Yes. Rule 704(b) still bars experts from giving an opinion about whether the defendant had a required mental state.
- What happens next after Diaz v. United States?
- Lower courts will use the Court's distinction when evaluating expert testimony. Lawyers will likely focus on whether testimony addresses a group or the individual defendant.
Sources
Primary materials plus reporting. Best-effort analysis: this explainer relies on a mix of primary materials and trusted secondary sources. Official filings and opinions remain authoritative.