Diaz v. United States
The Supreme Court ruled 6–3 that a government expert did not break the rules on expert testimony by telling a jury that most drug couriers knowingly transport drugs, because he never said this particular defendant was one of them — leaving that conclusion to the jury.
The decision gives federal prosecutors a tool to use group-based expert testimony about criminal intent in drug trafficking cases, so long as the expert stops short of directly opining that the specific defendant on trial had the required state of mind.
How it got here: A federal district court in California convicted Diaz; the Ninth Circuit affirmed; the Supreme Court granted certiorari and affirmed again.
The Case in Depth
What happened
Delilah Diaz, an American citizen, was caught at the U.S.-Mexico border driving a car with more than 54 pounds of methamphetamine hidden in the door panels and trunk, valued at over $368,000. She claimed she had no idea the drugs were there — a defense sometimes called a "blind mule." The government countered by calling a Homeland Security agent with nearly 30 years of experience as an expert, who told the jury that in most cases drug couriers know they are being paid to transport drugs.
The question before the Court
Does federal evidence law prohibit a government expert from telling jurors that most drug couriers know what they're carrying, when the only trial issue is whether this particular defendant knew?
The Court's answer
No — the testimony did not violate the rule.
Federal Rule of Evidence 704(b) bars expert witnesses in criminal cases from stating an opinion about whether "the defendant" had the mental state required to commit a crime. The Court ruled that this ban covers only opinions specifically about the individual defendant — not broader observations about what people in a similar situation generally know or intend. Because the agent said only that "most" couriers are aware they are transporting drugs, he left the ultimate question — whether Diaz specifically knew — entirely to the jury. That was enough to stay within Rule 704(b), and Diaz's conviction stands.
Curious how the Court got there? See the step-by-step legal reasoning →
Why it matters
Federal prosecutors handling drug trafficking cases can now routinely call expert witnesses to testify that most couriers know what they are carrying, even when a defendant claims ignorance. Defense lawyers will need to rely on cross-examination and other evidentiary challenges — not Rule 704(b) — to push back on that kind of testimony, affecting how knowledge is proved in thousands of federal drug prosecutions each year.
What changes now
Diaz's 84-month prison sentence stands. Federal prosecutors can now present experts in drug cases who testify that most couriers knowingly transport drugs, even when a defendant claims to have been deceived. Both Justice Jackson's concurrence and Justice Gorsuch's dissent flag that defendants can still challenge such testimony under other evidence rules — governing relevance, unfair prejudice, and expert reliability — though those questions remain open for future cases.
What this does not decide
The ruling addresses only Rule 704(b) and does not decide whether testimony like Agent Flood's could be excluded under other evidence rules governing relevance, unfair prejudice (Rule 403), or expert reliability (Rule 702). It also does not resolve how probabilistic an expert's opinion must be — whether "generally," "almost always," or "99%" — before it crosses a line.
Concurrences and dissents
Concurrence — Justice Jackson
Justice Jackson joined the majority in full but wrote separately to emphasize that Rule 704(b) is evenhanded: neither prosecutors nor defense lawyers can have an expert opine directly on what the defendant was thinking, but both sides may use group-based mental-state evidence. She illustrated how defendants benefit — for example, experts on schizophrenia, battered woman syndrome, or unknowing couriers. She also cautioned that other safeguards, including vigorous cross-examination and Rules 402, 403, and 702, remain available to check misuse of such testimony, and urged trial judges to give careful jury instructions when this kind of evidence is admitted.
Dissent — Justice Gorsuch
Justice Gorsuch argued the majority created a loophole that effectively guts Rule 704(b). In his view, the word 'about' in the Rule is broad — it covers any opinion concerning or referring to the defendant's mental state, whether stated directly or through probabilistic group statistics. Because Diaz's knowledge was the only issue at trial, an expert saying most couriers in her exact position knowingly transport drugs was functionally opining on her guilt. He warned the ruling invites competing expert 'mindreaders' on both sides, diminishes the jury's role, and risks convicting people on speculation rather than hard facts and reasonable inferences.
How the Court got there
The legal reasoning, step by step
- Rule 704(b) is a narrow exception Congress carved out after the 1981 Hinckley trial, where competing experts claimed to know exactly what the defendant was thinking when he shot President Reagan. Because it is an exception to the broader Rule 704(a) — which freely allows expert opinions on ultimate issues a jury must decide — Rule 704(b) can only cover a subset of what 704(a) permits, not a wider category.
- The text of Rule 704(b) targets opinions about 'the defendant' specifically. By its terms it prohibits conclusions about a particular person's mental state as an element of the crime or defense — not general observations about how people in a situation tend to behave. The word 'about' in the Rule, read in context, means a direct conclusion on that precise topic, not any testimony that merely relates to or concerns the defendant's state of mind.
- Agent Flood testified that 'in most circumstances' drug couriers know they are hired to transport drugs. That is a statement about the majority of a group — one that includes Diaz but also includes couriers who do not know. Whether Diaz fell into the knowing majority or the unknowing minority was left entirely for the jury to determine. Her individual mental state was never decided by the expert.
- The majority drew a key distinction: if an expert says 'all' people in the defendant's situation have the required mental state, that necessarily covers the defendant and violates Rule 704(b). But 'most' people is different — it asserts only a probability about a group, leaving the defendant's individual membership in the knowing or unknowing category unresolved and reserved for the jury.
- Reading Rule 704(b) as broadly as Diaz proposed — to cover any testimony that 'concerns' or 'refers to' the defendant's mental state — would make the exception swallow the general rule in 704(a). Under that reading, 704(b) would end up prohibiting more testimony than even the old common-law 'ultimate issue' rule that 704(a) was enacted to abolish, an outcome the text and structure of Rule 704 cannot support.