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Diaz v. United States: Gorsuch’s Dissent Takes Center Stage

On June 20, 2024, the United States Supreme Court issued a 6-3 ruling in the case of Diaz v. United States, upholding the conviction of Delilah Guadalupe Diaz for drug trafficking. While the majority opinion, authored by Justice Clarence Thomas, affirmed the lower court’s decision, it’s Justice Neil Gorsuch’s forceful dissent that has captured significant attention.

Diaz at a Glance

Diaz was convicted of importing over 54 pounds of methamphetamine across the U.S.-Mexico border. The key issue in her appeal centered on the testimony of a government expert witness, who stated that “most drug couriers know they are carrying drugs.” Diaz argued that this testimony violated Federal Rule of Evidence 704(b), which prohibits expert witnesses from opining on a defendant’s mental state in criminal cases. The Rule provides:

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.

The Majority Opinion

Justice Thomas, writing for the majority, concluded that the expert’s testimony did not violate Rule 704(b) because it spoke about drug couriers in general, rather than Diaz specifically. The Court held that an opinion about “most people” in a group is not equivalent to an opinion about the defendant’s mental state.

Gorsuch’s Dissent: A Powerful Critique

Justice Gorsuch, joined by Justices Sotomayor and Kagan, penned a scathing dissent that has garnered significant attention. He argued that the majority’s decision effectively gives prosecutors a “powerful new tool” that undermines the intent of Rule 704(b).

“Prosecutors can now put an expert on the stand, someone who apparently has the convenient ability to read minds, and let him hold forth on what ‘most’ people like the defendant think when they commit a legally proscribed act. Then, the government need do no more than urge the jury to find that the defendant is like ‘most’ people and convict.”

Implications for Prosecutorial Power

Gorsuch’s dissent raises important questions about the balance of power in criminal trials and the role of expert testimony. He argues that the majority’s interpretation of Rule 704(b) allows prosecutors to circumvent the rule’s intent, potentially infringing on defendants’ rights and the proper role of juries in determining mental state.

Broader Context: Prosecutorial Discretion and Power

This case touches on ongoing debates about the extent of prosecutorial power in the American justice system. Critics have long argued that prosecutors wield too much influence, from charging decisions to plea bargaining tactics. Gorsuch’s dissent adds fuel to this discussion, suggesting that the Court’s decision further tips the scales in favor of prosecutors.

Looking Ahead

While Diaz v. United States specifically addresses the interpretation of Rule 704(b), its implications could be far-reaching. The decision may influence how expert testimony is used in criminal trials and could potentially lead to calls for legislative action to clarify or modify the rule.

As the legal community digests this decision, it’s clear that Justice Gorsuch’s dissent will continue to be a focal point in discussions about prosecutorial power, expert testimony, and the rights of criminal defendants.

What We Learned from the Opinion

The Supreme Court’s decision in Diaz v. United States provides a detailed interpretation of Rule 704(b) and its application. The majority opinion highlights the distinction between general statements about a group and specific statements about the defendant’s mental state. The Court emphasized that an expert’s testimony about “most people” in a category does not directly speak to the defendant’s mental state, thereby not violating Rule 704(b).

Justice Thomas pointed out that the rule is a narrow exception to the general allowance of expert testimony on ultimate issues. The decision underscores the intent of Rule 704(b) to prevent experts from directly opining on the defendant’s mental state while allowing testimony that helps the jury make its own determination.

Key Takeaways from Oral Arguments

The oral arguments in Diaz v. United States provided additional insights into the complexities of applying Rule 704(b). Key takeaways include:

Probability Issue

The probability issue discussed during the oral arguments raised concerns about how experts might testify regarding the likelihood that certain defendants have the requisite mental state based on general probabilities. For example, stating that “most drug couriers know they are carrying drugs” introduces a probabilistic opinion that skirts close to directly opining on the defendant’s mental state.

This probabilistic testimony can be problematic as it implies that the defendant likely knew about the drugs, even if it does not state this directly. This kind of testimony can unduly influence the jury, leading them to infer the defendant’s guilt based on general probabilities rather than specific evidence. This ultimately became an issue crystalized in the dissent.

Much of the discussion focused on whether probabilistic statements about mental states (e.g. “80% of people in this situation know X”) should be treated differently than absolute statements under Rule 704(b).

Relationship with Rule 702

Several justices explored how Rule 704(b) interacts with Rule 702, which governs expert testimony more broadly. Some suggested that concerns about reliability of expert testimony on mental states could potentially be addressed through Rule 702 and Daubert gatekeeping, rather than Rule 704(b). Ultimately, the Court focused on the more specific rule found in 704(b).

Framework Evidence

The Court discussed the value of “framework evidence” – general testimony about how drug trafficking organizations operate. There was debate over where to draw the line between permissible framework evidence and impermissible opinions on a defendant’s mental state.

Historical Context of Rule 704(b)

Rule 704(b) was introduced in 1984 as part of the Insanity Defense Reform Act, motivated by public outrage over John Hinckley Jr.’s acquittal by reason of insanity following his attempted assassination of President Reagan in 1981.

Historically, the Federal Rules of Evidence, adopted in 1975, had eliminated the common law “ultimate issue rule,” which barred experts from testifying on ultimate issues to be decided by the jury. Rule 704 initially allowed such testimony if it met other admissibility standards.

During the Hinckley trial, psychiatric experts provided conflicting opinions on Hinckley’s sanity, the ultimate legal issue for the jury. This conflicting testimony was seen as confusing for jurors.

In response, Congress added Rule 704(b) to prevent expert witnesses from offering opinions on whether a defendant had the mental state or condition constituting an element of the crime or defense. This aimed to prevent experts from directly testifying about the ultimate issue of a defendant’s sanity or insanity.

Originally targeting psychiatric testimony in insanity defense cases, Rule 704(b) has since been broadly applied by courts to prohibit expert opinions on a defendant’s mental state in any crime where it is an element.

Rule 704(b) represents a partial revival of the ultimate issue rule, specifically for expert testimony on a defendant’s mental state, reversing the trend towards allowing such testimony since the adoption of the Federal Rules in 1975.

The legislative history shows that Congress intended to eliminate the “confusing spectacle” of experts offering contradictory conclusions on the ultimate legal issue for the jury.

Overall, Rule 704(b) reflects the ongoing tension between allowing helpful expert testimony and preserving the jury’s role in determining ultimate issues, particularly concerning a defendant’s mental state in criminal cases.

Application of the Diaz v. United States Opinion

Relationship with Rule 702

Rule 702 of the Federal Rules of Evidence governs the admissibility of expert testimony, requiring 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 relationship between Rule 702 and Rule 704(b) is crucial in this context.

While Rule 704(b) specifically prohibits experts from stating opinions about a defendant’s mental state, Rule 702 ensures that any expert testimony admitted is both relevant and reliable. Going forward, Rule 702 and Daubert challenges may be the best vehicles to exclude this type of evidence.

Daubert Issue

The Daubert standard is a rule of evidence regarding the admissibility of expert witnesses’ testimony in federal court. Established by the Supreme Court in the 1993 case Daubert v. Merrell Dow Pharmaceuticals, it requires that expert testimony be both relevant and reliable. Under Daubert, judges act as gatekeepers, ensuring that the methodology underlying the testimony is scientifically valid and applicable to the facts at issue. This standard might keep out opinion testimony about a defendant’s mental state by scrutinizing the reliability and relevance of the expert’s methods, thus preventing speculative or unfounded opinions from influencing the jury.

Justice Gorsuch and others pointed out that allowing experts to testify about general probabilities or classes of individuals without sufficient scrutiny under the Daubert standard could lead to the admission of unreliable and prejudicial evidence in court.

Expert Testimony and Specific Defendants

In this case, the expert testimony initially referred to a class of individuals but then discussed exceptions to that class, effectively ruling them out and making the testimony apply specifically to Diaz. This approach, referred to as “mental gymnastics,” allows prosecutors to present damaging evidence that appears to be about a general class but is effectively directed at the defendant.

This method can be a powerful tool in the hands of prosecutors, as it allows them to introduce seemingly general expert opinions that strongly suggest the defendant’s guilt. Such testimony can mislead the jury into believing that the expert has indirectly confirmed the defendant’s mental state, thus undermining the protective intent of Rule 704(b).

What about Defense Experts?

During the oral arguments, it was suggested that the defense could hire their own experts to counter the prosecution’s expert testimony. However, this idea has significant flaws:

  1. Judicial Disinclination to Fund Experts: Under Ake v. Oklahoma, indigent defendants are entitled to access to expert witnesses, but judges often show reluctance in approving funds for defense experts. Studies have found that indigent defendants often struggle to obtain expert assistance, even when it is critical to their defense. This suggests judicial reluctance to approve funding.
  2. Disparate Standards for Defense Expert Appointments: Defense attorneys report difficulties in securing funds for experts, having to overcome high bars set by judges to prove the necessity of expert assistance. The Fifth Circuit, for example, required showing both “substantial need” and that the defendant would be prejudiced without the expert.
  3. Narrow Interpretation of Ake: Some courts have interpreted Ake narrowly, only applying it to psychiatric experts or capital cases, further limiting indigent defendants’ access to other types of experts.
  4. Resource Disparity: Prosecutors generally have more resources at their disposal compared to public defenders or appointed defense attorneys. This resource disparity can lead to an imbalance in the quality and quantity of expert testimony presented by both sides.

Duties of a Defense Lawyer Under Ake and Diaz

After Diaz, the duties of a defense lawyer have become more crucial than ever in ensuring fair trials. Under Ake v. Oklahoma, defense attorneys must:

  • Vigorously Advocate for Expert Assistance: Defense lawyers must make strong, well-documented requests for expert assistance, emphasizing the necessity of such experts for an adequate defense. Defense lawyers should be prepared to educate judges on why their defense requires the ability to counter any State’s mental state expert, and how the defense would not be able to adequately prepare if they waited until the deadline for the State to give notice of their experts before trial.
  • Seek Timely Appointments: Lawyers must act promptly to request experts early in the case to ensure there is sufficient time for preparation and testimony.
  • Challenge Prosecutors’ Experts Under Daubert and Rule 702: Defense attorneys must rigorously challenge the admissibility of the prosecution’s expert testimony. Through a 702 hearing, that the testimony meets the standards set forth in Daubert v. Merrell Dow Pharmaceuticals, Inc. and Rule 702, which require that expert testimony be both relevant and reliable. By scrutinizing the prosecution’s experts through these lenses, defense lawyers can prevent unreliable or prejudicial testimony from influencing the jury.

The Supreme Court’s decision in Diaz v. United States has pivotal implications for the use of expert testimony in criminal trials. While the majority upheld the conviction by distinguishing general statements from specific mental state opinions, Justice Gorsuch’s dissent highlights a critical concern: this ruling potentially grants prosecutors excessive power, allowing them to bypass Rule 704(b)’s intent and undermine defendants’ rights.

Moving forward, it is essential to scrutinize expert testimony under Rule 702 and the Daubert standard to ensure its reliability and relevance. Defense attorneys must be vigilant in challenging such testimony and advocating for equal access to expert witnesses.

Varghese Summersett is a premier criminal defense firm based in Fort Worth, Texas. Our attorneys focus exclusively on criminal law and represent clients charged with crimes at both the state and federal level. We handle everything from DWI to capital murder to white collar crime. Collectively, our attorneys bring together more than 100 years of criminal law experience and have tried more than 550 cases before Texas juries. All of our senior attorneys served as former state or federal prosecutors and four are Board Certified in Criminal law, the highest designation an attorney can reach. We are the firm people turn to when the stakes are high and they are facing the biggest problem in their lives. - Contact Varghese at  
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