Diaz v. United States: SCOTUS Blesses FRE 704(b) Expert Opinions About What “Most People” Do

Last Thursday, the Supreme Court’s 6-3 decision in Diaz v. United States clarified Federal Rule of Evidence 704(b)’s prohibition on expert testimony about a criminal defendant’s state of mind. While Rule 704(a) generally permits experts to opine on ultimate issues in a case, Rule 704(b) creates an exception that forbids expert witnesses in a criminal trial from “stat[ing] 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.” In Diaz, the Court resolved a circuit split by concluding that expert testimony that “most people” in a group have a particular mental state is not an opinion about the defendant’s own state of mind, and therefore not barred under Rule 704(b). Because Rule 704(b) is “party agnostic” (as Justice Jackson emphasized in her concurring opinion), Diaz ensures that both the prosecution and the defense may use such expert opinions to corroborate their respective arguments about a defendant’s mental state. As a result, future evidentiary battles likely will shift to focus on whether the opinion should be barred as inappropriate expert testimony under Rule 702 and Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993), as irrelevant under Rule 402, and or as unduly prejudicial or confusing under Rule 403.

The petitioner, Delilah Diaz, was charged with “knowingly” importing methamphetamine after she tried to enter the U.S. from Mexico while driving a car with several dozen pounds of the drug hidden inside. The government at trial sought to refute Diaz’s defense that she was a “blind mule,” a colloquial term for an unsuspecting drug courier. Prosecutors proffered a Department of Homeland Security Special Agent as an expert on the use of drug couriers by Drug Trafficking Organizations (DTOs). Over the defendant’s objections, the trial court allowed the agent to testify that, “in most circumstances, the driver knows they are hired” to transport drugs, and that DTOs do not use generally unsuspecting couriers due to the risk of the drugs not making it to the intended market. Ultimately, the jury convicted Diaz and the Ninth Circuit affirmed, holding that Rule 704(b) does not apply to expert testimony that “does not provide an explicit opinion on the defendant’s state of mind.” United States v. Diaz, No. 21-50238, 2023 WL 314309, at *2 (9th Cir. Jan. 19, 2023) (quotation marks omitted).

The Supreme Court granted certiorari to resolve Rule 704(b)’s scope. Justice Thomas’ opinion for the Court concluded that, because Rule 704(b)’s prohibition “applies only to opinions about the defendant,” “[a]n expert’s conclusion 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).” United States v. Diaz, No. 23-14, slip op. at 7, 11 (2024). The Court distinguished between an opinion that “all” drug couriers know that they are transporting drugs and an opinion that “most” drug couriers know that they are doing so. In the Court’s view, an opinion about “all” drug couriers would describe an entire class to which the defendant belonged, thus necessarily stating an opinion on the defendant’s state of mind, while an opinion about “most” drug couriers only defines a class of people while leaving the jury to decide whether the defendant was like “most” couriers. Id. at 9-10. Because the testifying agent’s opinion concerned “most” couriers, the Court held that his testimony did not violate Rule 704(b). But, as Justice Jackson’s concurring opinion and Justice Gorsuch’s dissent (joined by Justices Kagan and Sotomayor) emphasized, the Court’s analysis of Rule 704(b) leaves the prosecution and defense free to invoke other rules of evidence to combat such expert testimony when appropriate.

Diaz’s reading of Rule 704(b) carries significant implications for white collar cases that turn on whether the defendant acted “knowingly” or “willfully,” such as securities and health care fraud prosecutions. Under Diaz, prosecutors and defense counsel alike can seek to offer expert testimony about the state of mind of the majority of a class of people who are similarly situated to the defendant. For example, in a securities fraud prosecution of a corporate officer for knowingly omitting material information from corporate disclosures, appropriately qualified experts might be noticed to testify about whether other similar corporate officers are aware of the accuracy of the information that they are disclosing, or about whether such officers are familiar with federal reporting requirements.

Conversely, counsel opposing the introduction of such Diaz-style opinions should consider early Rule 702 and Daubert challenges to the reliability of the expert’s opinion and underlying methodology. Even under Diaz, when an expert seeks to opine that “most of” or “the majority of” a class of people possess a certain state of mind, the testimony’s proponent should still be required to show that the opinion rests on the expert’s comprehensive experience with or study of that class. Counsel opposing the testimony should explore the possibility that the proffered expert has only interacted with too small a subset of the class to state an opinion on “most” members of that class, thus casting doubt on the reliability and sufficiency of the opinion being placed before the jury.

Even if a Daubert challenge proves unsuccessful, defense counsel (and prosecutors) can still argue for exclusion as irrelevant under Rule 402, or even if relevant, for exclusion under Rule 403 because the danger of unfair prejudice or the danger of jury confusion substantially outweighs the evidence’s probative value. Although Diaz unsuccessfully raised these objections at trial, defendants in other cases featuring different facts, law, and expert opinions still can argue that evidence about the general mindset of “most” people in a class says nothing about a defendant’s own mind or risks confusing the jury by conflating “most” of that class’ mental state with the defendant’s own.

Despite Diaz’s significant implications in criminal cases for both the prosecution and defense, the pre-existing landscape of other evidentiary rules suggests that advocates and courts still can seek to preclude improper expert testimony in a manner consistent with the Supreme Court’s guidance. We at Enforcement Edge will continue to monitor how courts analyze Diaz and these kinds of expert opinions going forward. Please reach out to the authors of this post or members of Arnold & Porter’s White Collar Defense & Investigations group if you have any questions about this case or about expert testimony during criminal trials more generally.

© Arnold & Porter Kaye Scholer LLP 2024 All Rights Reserved. This blog post is intended to be a general summary of the law and does not constitute legal advice. You should consult with counsel to determine applicable legal requirements in a specific fact situation.