Diaz v. United States
The Court ruled that a federal evidence rule barring experts from directly opining on a defendant's mental state does not cover an expert's testimony that 'most' members of a broader group share that state — leaving the ultimate judgment about the specific defendant to the jury.
The decision gives prosecutors (and defendants) a new avenue for mental-state expert testimony in criminal trials, while both a concurrence and the dissent flagged that other evidence rules can still be used to challenge such testimony.
How it got here: A federal district court in California convicted Diaz; she appealed to the Ninth Circuit, which affirmed; she then asked the Supreme Court to review the expert-testimony question and the Court agreed to hear it.
The Case in Depth
What happened
Delilah Diaz, a U.S. citizen, was stopped at the U.S.-Mexico border driving a car that turned out to contain more than 54 pounds of methamphetamine hidden in the door panels and trunk. She was charged with knowingly importing drugs but claimed she had no idea the drugs were there. At trial, the government called a federal agent as an expert who testified that in most circumstances drug couriers know they are transporting drugs. Diaz argued that testimony crossed a legal line by effectively telling the jury she must have known about the drugs.
The question before the Court
Does a rule barring expert witnesses from opining on a defendant's mental state also bar an expert from testifying that "most" drug couriers know they are transporting drugs?
The Court's answer
No — Rule 704(b), which bars expert opinions "about whether the defendant" had the mental state required for conviction, does not prohibit an expert from testifying about the typical mental state of a broader group. The Court read the rule's reference to "the defendant" to mean conclusions specifically about that individual person. An expert who says only that most members of a group knowingly commit a charged act has not crossed that line — because most is not all, and the defendant may or may not fall within the majority.
The Court drew a sharp contrast: if an expert had testified that all drug couriers know they are transporting drugs, that conclusion would necessarily cover the defendant too, violating the rule. But by saying most couriers know — while acknowledging some do not — the expert left the ultimate question of Diaz's own state of mind for the jury to resolve on its own.
Curious how the Court got there? See the step-by-step legal reasoning →
Why it matters
Prosecutors in federal criminal cases can now use expert witnesses to testify about the typical mental states of groups — such as drug couriers — without running afoul of the rule against opining on the defendant's own state of mind. Defense attorneys can use the same avenue. District judges must rely on other evidence rules to screen out unreliable or unfairly prejudicial expert testimony of this kind.
What changes now
Diaz's conviction and 84-month prison sentence stand. Going forward, federal prosecutors — and defense attorneys — may use expert witnesses to testify about typical mental states within a relevant group, as long as they stop short of directly opining on the specific defendant's state of mind. Courts and litigants will look to other evidence rules, including those governing relevance, unfair prejudice, and expert reliability, to police the boundaries of this kind of testimony.
What this does not decide
The ruling addresses only Rule 704(b) and does not decide whether testimony like Agent Flood's could still be excluded under other evidence rules barring irrelevant, unduly prejudicial, or unreliable expert testimony. It also does not resolve where the line falls if an expert claims an even higher percentage — say, 99% — of a group shares the relevant mental state.
Concurrences and dissents
Concurrence — Justice Jackson
Justice Jackson joined the majority in full but wrote separately to stress that Rule 704(b) is 'party agnostic': prosecutors and defendants alike may call experts to testify about the typical mental states of relevant groups, and defendants in particular can benefit — for example, by presenting expert evidence on mental health conditions or battered woman syndrome that contextualizes a defendant's behavior without directly opining on her state of mind. She also noted that risks of overreliance on expert testimony are real but are addressed by safeguards in other evidence rules and by careful jury instructions from trial judges.
Dissent — Justice Gorsuch
Justice Gorsuch argued that Agent Flood's testimony did violate Rule 704(b) because the word 'about' — meaning 'concerning' or 'in reference to' — covers any opinion bearing on whether the defendant had the required mental state, whether that opinion is definitive or merely probabilistic. Testifying that most couriers knowingly transport drugs is, in his view, plainly an opinion about Diaz's mental state since the entire point was to help prove she knew. He would have required the government to prove knowing conduct through circumstantial evidence and reasonable inferences, as prosecutors routinely do, rather than through what he called 'expert' mind-reading testimony.
How the Court got there
The legal reasoning, step by step
- The Court began with the structure of Rule 704 itself. Rule 704(a) generally allows expert opinions on so-called 'ultimate issues' — the very questions the jury must resolve, like whether someone acted knowingly. Rule 704(b) is a narrow exception to that broad permission. Because it is an exception, Rule 704(b) can cover only a subset of what Rule 704(a) expressly allows — not a broader swath of testimony than the rule it modifies.
- The text of Rule 704(b) limits the ban to opinions 'about whether the defendant did or did not have a mental state.' The words 'the defendant' anchor the prohibition to conclusions about a specific individual, not general statements about how groups of people tend to behave. The surrounding language reinforces this: the rule targets opinions that reach a conclusion on whether a precise fact is true about that person, not opinions that merely concern or relate to the topic of mental states.
- Agent Flood testified that 'in most circumstances, the driver knows they are hired to take the drugs from point A to point B.' That is a statement about a group of drug couriers as a whole — not a conclusion about Diaz's state of mind specifically. Because some couriers are unknowing, the opinion left open whether Diaz was among the knowing majority or the unknowing minority.
- The Court drew a clarifying contrast using a hypothetical: an expert who testifies that all people in a defendant's position acted with a certain mental state necessarily includes the defendant in that description, effectively stating an opinion on the defendant's mental state in violation of the rule. But 'most' — unlike 'all' — does not sweep in every member of the group, so it does not necessarily reach a conclusion about the defendant herself.
- With the expert having testified only about the typical pattern, the ultimate question — whether Diaz specifically knew about the drugs — remained entirely for the jury to answer. The jury was aware both that unknowing couriers exist and that Diaz presented evidence suggesting she could be one of them. Preserving that jury role is precisely what Rule 704(b) is designed to protect, and the Court found that protection intact here.