The Supreme Court recently addressed whether, in a criminal expert case, an expert may render an opinion that most people in a group like that of the defendant have a particular mental state. In Diaz v. United States, the majority found that such testimony does not violate Federal Rule of Evidence 704(b) because it does not state an opinion about the defendant’s state of mind.

Diaz presented a typical fact pattern for federal drug prosecutions on the southern border. At approximately 2:00 a.m. on August 17, 2020, Petitioner Delilah G. Diaz, a U.S. citizen, crossed the border into the United States from Mexico at the San Ysidro Port of Entry, driving a Ford Focus. Diaz told the customs inspector that she was driving alone to San Diego and that her boyfriend owned the car. The inspector opened the rear door and tried to roll the manual window down. He felt some resistance and heard a “crunching” sound. Diaz was sent to secondary inspection where Homeland Security and Investigations agents searched the car. The search revealed that there was approximately 54.5 pounds of pure methamphetamine hidden in the car doors and quarter panels. They also found two cell phones. Diaz agreed to speak with the HSI agent after waiving her Miranda rights. Diaz claimed she was returning from visiting her boyfriend in Mexico. Importantly, Diaz denied any knowledge of the drugs hidden in the car.

Diaz, facing a statutory minimum of not less than ten years in prison, went to trial. She asserted the common “blind mule” defense: she claimed to have no knowledge of the drugs in the car. At trial, the prosecution presented the testimony of HSI Special Agent Flood. Agent Flood testified that, in most circumstances, “the driver knows” they are transporting drugs at the time. He further explained that the risks of product loss meant drug traffickers were less likely to use “blind mules.” The jury convicted Diaz. The Ninth Circuit Court of Appeals upheld the conviction.

Justice Thomas, writing the opinion for the Court, which Chief Justice Roberts and Justices Alito, Kavanaugh, Barrett, and Jackson joined, held that expert testimony about the mental state of “most people” in a group is not testimony about the defendant. Because such testimony is not about the defendant on trial, admitting it does not violate Rule 704(b).

Federal Rule of Evidence 704 addresses expert testimony about ultimate issues. Ultimate issues are matters for the trier of fact to determine. The rule at common law categorically prevented a witness from testifying about any conclusion about an ultimate issue. Rule 704 departs from the common law tradition. Subsection (a) of the Rule is clear that opinion testimony embracing an ultimate issue is not objectional on its face. Subsection (b), however, restores some of the common law protections by creating an exception applicable to criminal cases. But according to the Diaz majority, the exception is not a return to the complete ban on any testimony encompassing an ultimate issue. Rather, the Rule excludes testimony directly related to the defendant’s state of mind.

The majority looks to the language of the Rule. The Rule 704(b) exception states, in part, “In a criminal case, an expert must not state an opinion about whether the defendant did nor did not have a mental state or condition that constitutes an element of the crime charged or of a defense.” The majority’s position is that the Rule restricts the exception to testimony about “the defendant’s” mens rea. It further explains that, as originally drafted, Rule 704 “made it clear” that the rule banning ultimate issue testimony did not apply in federal court. The exception in subsection (b) was promulgated after the acquittal for John Hinckley, Jr., who shot President Reagan. The Rule must be interpreted within this context, giving the “the defendant” language import because it constitutes a narrow exception to FRE 704. The majority did not accept that the 704(b) exception was a fuller restoration of the common law tradition. At bottom, the majority trusts the trier of fact to understand that expert testimony about the knowledge of “most people” in a group is not testimony about the defendant’s intent. In this way, the majority argues its holding reinforces the fundamental role of a jury of peers.

Justice Gorsuch authored the dissent, which was joined by Justices Sotomayor and Kagan. The dissent claims that the majority, in fact, “carves a path around 704(b)’s mandate that expert testimony about the defendant’s state of mind or an essential element of the charged crime must be ‘for the trier of fact alone.’” It comments that the majority’s holding is a “charade” and gives the government a new tool: it allows the government to put on testimony about the intent of people like the defendant, and then simply argue at closing that the defendant is like those people. In this way, the majority undermines a fundamental tenet of criminal jurisprudence: to convict, the government must prove the defendant had the requisite scienter for the crime. Proof of the act alone is not enough to convict.

Turning to the language to the Rule, the dissent finds that the 704(b) exception bars expert testimony “about whether the defendant” has the requisite mental state. This language, according to the dissent, expands the exception not only to expert testimony that is directly referring to the defendant’s intent, but also to that which is deterministic or probabilistic about the group into which the defendant falls. Here, the dissent is concerned about undermining the government’s burden of proof and undermining the jury’s role as the trier of fact. The dissenters’ opinion seeks to balance these concerns while staying true to the text of 704(b).

The dissent also seeks to prevent government overreach. In criminal cases, there is rarely a declaration of intent by the defendant. Rather, the government relies on circumstantial evidence to meet its burden on proof on the intent element of the charged crime. In this case, Diaz’s story was fraught with inconsistencies and dubious explanations, and the government was free to argue that these proved she had knowledge. Instead, the government decided to “gild the lily” by calling Special Agent Ford to testify as “an ‘expert’ in mindreading,” according to the dissent. Additionally, the dissent raises concerns about the nature of such expert testimony and how it could evolve, commenting that “perhaps there is no more junky science than mental telepathy.” The dissent suggests that there are other rules of evidence that may prevent the type of testimony the majority upholds. But, at its core, the dissent’s interpretation of the 704(b) exception aims to preserve the function of the jury, maintain the language of the Rule, and prevent government overreach.

Justice Jackson, in her concurrence, recognizes that the language of Rule 704(b) is narrow in its application to expert testimony about the defendant’s mental state. However, she notes that the Rule is not limited to use by the government. Both parties may put on expert testimony regarding whether a group of persons into which the defendant falls “mostly” have the requisite intent. Interestingly, Justice Jackson notes that Diaz presented testimony from a specialist in automobiles. The specialist testified that someone driving the same model car as Diaz would “almost certainly not know that it contained drugs.” In this way, Justice Jackson sees that the Rule as interpreted by the majority promotes the adversarial process and aids the jury in deciding defendant’s mental state.

Justice Jackson’s opinion provides an insightful balance to the opposing opinions. Importantly, Justice Jackson takes care to recognize the concerns expressed by the dissent related to risks of “overreliance” on expert testimony inherent in criminal cases. She notes that other rules may serve as barriers to improper expert testimony offered under 704(b). Justice Jackson, like the dissent, points to other rules of evidence as tools to exclude improper testimony, like vigorous cross-examination and effective closing arguments. And lastly, Justice Jackson invokes the important role of the district court judges in protecting the role of the jury.

The holding of the majority, the concerns expressed by the dissent, and Justice Jackson’s insights provide litigators and judges with important guidance on the use of experts under FRE 704(b). The door is open for both the government and the defense to try to admit expert testimony that a defendant is or is not part of a group that is likely to have the requisite criminal intent. However, Justice Jackson and the dissenters alert judges and litigators to the risks associated with this type of testimony. What unifies the opinions, however, is a concern for preserving the fundamental role of the jury, despite the differing perspectives of the members of the Court.

Note from the Editor: The Federalist Society takes no positions on particular legal and public policy matters. Any expressions of opinion are those of the author. We welcome responses to the views presented here. To join the debate, please email us at [email protected].