Expert Testimony on What “Most People” Know

Under Federal Rule of Evidence 704, can an expert testify as to what “most people” would know in a given situation? Yes, the U.S. Supreme Court ruled today in Diaz v. United States, No. 23-14. The decision was 6-3, but no, not on “party” lines.

Once upon a time, the standard rule of evidence was that expert witnesses could not opine on the “ultimate issue” that the jury needs to decide. When the Federal Rules of Evidence were initially adopted (based on an American Law Institute proposal that is also the basis of the California Evidence Code), Rule 704 wiped the old rule out with no exceptions. Many states then copied the federal rules.

Subsequently, FRE 704 was amended to make an exception as subdivision (b): “EXCEPTION: In a criminal case, an expert witness must not state an opinion about whether the defendant did or did not have a mental state or condition that constitutes an element of the crime charged or of a defense. Those matters are for the trier of fact alone.”

Note the “did or did not.” The exception works both ways.

So, can an expert testify that in a given set of circumstances most people would or would not know a particular fact, without opining whether this specific defendant did or did not? That seems like an easy question. The general rule says yes, and the exception does not cover this territory.

In the particular case, Delilah Diaz was a “mule” carrying drugs across the Mexican border, 54 pounds of meth. The prosecution’s expert on drug smuggling operations proposed to testify that the smugglers generally would not trust large, valuable quantities of drugs to people who did not know they were carrying such. “The court agreed with Diaz that Agent Flood could not testify in absolute terms about whether all couriers knowingly transport drugs. But, insofar as Agent Flood planned to testify only that most couriers know they are transporting drugs, the court concluded that his testimony was admissible.”

The opinion of the Court by Justice Thomas decides, as described above, that such testimony does not come within the subdivision (b) exception to the general rule of Rule 704. A passage on page 9 warrants close attention by those who wish to introduce or object to such evidence in a federal criminal case.

That argument (of the petitioner and dissent) mistakenly conflates an opinion about most couriers with one about all couriers. A hypothetical helps explain why this distinction matters under Rule 704(b). Take for example an expert who testifies at an arson trial that all people in the defendant’s shoes set fires maliciously (the mental state required for common-law arson). Although the expert never spoke the defendant’s name, the expert nonetheless violated Rule704(b). That is because the expert concluded that the defendant was part of a group of people that all have a particular mental state. The phrase “all people in the defendant’s shoes” includes, of course, the defendant himself. So, when the expert testified that all people in the defendant’s shoes always set fires with malicious intent, the expert also opined that the defendant had that mental state. The expert thus stated an opinion on the defendant’s mental state,an ultimate issue reserved for the jury, in violation of Rule 704(b).

Here, by contrast, Agent Flood asserted that Diaz was part of a group of persons that may or may not have a particular mental state. Of all drug couriers—a group that includes Diaz—he opined that the majority knowingly transport drugs. The jury was then left to decide: Is Diaz like the majority of couriers? Or, is Diaz one of the less-numerous-but-still-existent couriers who unwittingly transport drugs? The ultimate issue of Diaz’s mental state was left to the jury’s judgment. As a result, Agent Flood’s testimony did not violate Rule 704(b).

Justice Jackson joined the opinion in full and added a concurring opinion noting that the rule works both ways, and defendants sometimes need such testimony also.

Justice Gorsuch dissented, joined by Justices Kagan and Sotomayor. He believes this is an evasion of the rule with little real difference.

This opinion is binding precedent only in federal court, but it would likely be persuasive in any state that has adopted the 704(b) exception. Of course, in states that have the original, exceptionless Rule 704 or an equivalent, none of this matters.