Supreme Court Expands Federal Rule of Evidence 704(b) to Allow Expert Testimony Regarding Issue of Knowledge or Intent to Commit the Crime

bundles of drugs, suitcase full of money, and a gun sitting on a table - drug trafficking concept

On June 20, 2024, the US Supreme Court issued its ruling in Diaz v. United States, upholding the conviction of a California woman who claimed she was a “blind mule,” a term used for someone who transports drugs unwittingly.

The defendant, a US citizen named Delilah Guadalupe Diaz, was stopped in August 2020 after crossing into the US from Mexico at the San Ysidro port of entry. A search of her vehicle revealed almost 55 pounds of methamphetamine with a street value of $368,500.

In her defense, Diaz claimed her boyfriend lent her the car when she was returning to the US from a trip to Mexico and that she had no knowledge of the illegal drugs.

Prosecutors claimed it was impossible that she would be unaware of thousands of dollars worth of methamphetamine in her car.

Diaz’s appeal focused on Federal Rule of Evidence 704(b), which states that “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.” The rule was passed in the 1980s in response to public outrage after John Hinckley, Jr. was found not guilty of the attempted assassination of President Ronald Reagan based on an insanity defense.

At Diaz’s trial, a Department of Homeland Security agent testified that most drug cartels do not allow blind mules to transport large quantities of drugs, and therefore, most mules know that they are transporting illegal substances.

Diaz’s lawyer argued, unsuccessfully, that the testimony should have been excluded because it was the “functional equivalent” of telling the jury that Diaz knew about the drugs.

Diaz was found guilty and sentenced to seven years in prison. Diaz appealed her case to the US Supreme Court.

Justice Clarence Thomas, writing for the majority, ruled that the Homeland Security Agent "did not state an opinion about whether [Diaz] herself had a particular mental state," but was merely speaking to what usually happens in similar situations. As a result, his testimony did not violate the evidence rule.

In his dissenting opinion, joined by Justices Sotomayor and Kagan, Justice Neil Gorsuch wrote that the ruling gives prosecutors a “powerful new tool” that allows them to call witnesses to hint at a defendant’s guilt as part of their expert testimony.

Historical Context of Rule 704(b)

Federal Rule of Evidence 704(b) was introduced in 1984 in response to public outrage over the acquittal of John Hinkley, Jr. by reason of insanity following his attempted assassination of President Reagan in 1981. Expert testimony featured prominently in the trial, as psychiatric experts offered competing opinions on Hinkley’s sanity, which was the ultimate issue the jurors were asked to decide. Many viewed the conflicting testimony as being confusing for the jury.

In response, Congress passed Rule 704(b), which prevents expert witnesses from offering opinion testimony on whether a defendant had a mental state or condition that constituted an element of the crime or a defense.

The rule was originally implemented to preclude expert testimony in cases involving the insanity defense but has been expanded to prohibit expert opinions in any case where the defendant’s mental state is an issue.

In Diaz, the Supreme Court ruled that testimony from the Homeland Security agent did not violate the rule because he “did not express an opinion about whether Diaz herself knowingly transported methamphetamine.”

Key Takeaways from Diaz v. United States

While the Court described its ruling as “narrow,” its implications could have far-reaching effects, influencing how prosecutors use expert testimony in trials. Oral arguments revealed key issues in the Diaz case and how it could impact expert testimony and the application of Rule 704(b).

The Issue of Probability

During oral argument, the Justices raised concerns about how experts might be called to testify about the probability that a defendant could have a particular mental state. Presenting evidence that “most drug couriers know they are carrying drugs” introduces an opinion on probability while avoiding offering an opinion that bears directly on the defendant’s mental state.

Probability testimony can be problematic because it implies a particular mental state without stating it directly. This kind of testimony can lead the jury to find a defendant guilty based on probabilities rather than on specific evidence.

Interplay Between Rule 704(b), Rule 702, and Daubert

Several justices explored the relationship between Rule 704(b) and Rule 702, which governs expert testimony generally. Some expressed concern about the reliability of expert testimony that expresses an opinion on a defendant’s mental state and suggested the issue could be better addressed through Rule 702 and Daubert rather than through Rule 704.

Rule 702 governs the admissibility of expert testimony and requires that such testimony be based on sufficient facts or data, be the product of reliable principles and methods, and that the expert has reliably applied the principles and methods to the facts of the case.

The Daubert standard requires that expert testimony be both relevant and reliable. Under Daubert, judges act as gatekeepers, ensuring the testimony is scientifically valid and applies to the facts at issue. A criminal defense lawyer could use Daubert to try to keep out opinion testimony about a defendant’s mental state by challenging the reliability and relevance of the expert’s methods.

While Rule 704(b) prohibits experts from expressing an opinion about a defendant’s mental state, Rule 702 ensures that any expert testimony admitted is both relevant and reliable. In the future, challenging expert testimony under Rule 702 and Daubert may be the best way to challenge an expert’s opinion on a defendant’s mental state.

The Issue of Defense Experts

During oral arguments, it was suggested that defendants could hire their own experts to counter expert testimony introduced by the prosecutor. However, prosecutors generally have more resources available than defendants. This disparity in resources creates an imbalance in the quality and quantity of testimony presented. Additionally, in indigent defense cases, judges are often reluctant to approve expenses for defense experts, further highlighting this power imbalance.

Justice Gorsuch’s Dissent

In his dissent, Justice Gorsuch raised important questions about the role of expert testimony and the balance of power in criminal trials. He described the Homeland Security Agent as “an expert on the minds of drug couriers (yes, really)” and warned that the “problem of junk science in the courtroom is real and well documented.” He also wrote that the ruling usurps the jury’s role in determining the defendant’s state of mind, writing, “On that particular issue, Congress has concluded that jurors need no help from experts.”

The Impact of Diaz on Expert Testimony in Criminal Defense Cases

The Court’s decision in Diaz v. United States will have important implications for the use of expert testimony in criminal trials. While the majority decision is expressly “narrow” and attempts to distinguish between general statements and those offered to prove a defendant’s specific mental state, Justice Gorsuch’s dissent highlights important concerns about prosecutorial power and the power imbalance inherent in the US criminal justice system.

In the future, criminal defense lawyers will need to carefully scrutinize expert testimony under Rule 702 and Daubert and push judges to exercise their gatekeeping function and keep “junk science” out.

Hope Lefeber is a federal criminal defense attorney in Philadelphia and New York City. She has over 30 years of experience representing people accused of crimes in federal court and has earned a reputation as a fierce defender of the rights of the accused.

Categories: Drug Crimes