How the Supreme Court’s Diaz Decision Could Change Criminal Defense Strategies in Drug Trafficking Cases

The U.S. Supreme Court decided the case of Diaz v. United States in June 2024, fundamentally altering the way expert testimony is treated in criminal trials. This decision has far-reaching implications, particularly for defendants in drug trafficking cases. But its effects extend to other types of criminal prosecutions. This blog explains the Diaz ruling and how it can affect your criminal trial. 


At Stechschulte Nell, Attorneys at Law in Tampa, Florida, we emphasize the importance of public awareness regarding legal changes and how they impact individual cases. The Diaz decision is particularly notable due to its potential impact on the strategies we and other defense attorneys use to prepare for and conduct criminal trials. If you are facing either a federal or state criminal charge, contact Stechschulte Nell for a full review of your case and become an important participant in your own defense. 



The Facts of Diaz v. United States 


The case began in 2020 when Delilah Guadalupe Diaz was stopped at a border checkpoint returning to California from Mexico. U.S. border agents discovered nearly 28 kilograms of methamphetamine hidden in her car. Diaz claimed she was unaware of the drugs and that she had borrowed the car from her boyfriend. 


During her trial, the prosecution introduced expert testimony from a Homeland Security agent who stated that drug traffickers generally do not use unknowing couriers. This testimony implied that Diaz, by virtue of her circumstances, probably knew about the drugs. Diaz’s defense lawyer argued that the Homeland Security agent’s testimony violated Federal Rule of Evidence 704(b), which prohibits experts from testifying about a defendant’s mental state. Remember that in this and other crimes, a defendant’s knowledge and present intent are essential elements of the crime that the prosecution must prove beyond a reasonable doubt to get a conviction.  


Traditional Application of Federal Rule of Evidence 704(b) 


Rule 704(b) traditionally was interpreted to restrict expert witnesses from offering direct opinions on whether a defendant possessed a particular mental state or condition that was an element of the crime. This rule seemed to guarantee that a defendant’s mental state was to be determined solely by the jury.  


Before the Diaz decision in June of 2024, the standard practice allowed experts to provide background information and general knowledge but were barred from making statements that could be interpreted as direct opinions on the defendant’s mental state. This limitation was intended to prevent undue influence on the jury’s decision-making process. The rule maintained a clear boundary between expert analysis and the jury’s ultimate factual conclusions. 


Diaz v. United States, The Supreme Court’s Decision 


The Supreme Court’s ruling in Diaz v. United States declares that expert testimony about the mental states of certain groups, even if it indirectly suggests something about the particular defendant’s state of mind. The Supreme Court held that such testimony does not violate Rule 704(b) as long as it does not explicitly state an opinion on the mental state of the individual defendant on trial. 


Justice Clarence Thomas wrote the majority opinion in the Diaz case. Thomas emphasized that expert testimony about the behaviors and practices of drug traffickers was already generally permissible. This testimony, according to the Court, provided the jury with useful and valuable context without directly infringing on the jury’s role to determine the specific mental state of the defendant. 


Impact on Defense Strategies 


The Diaz decision is likely to significantly alter defense strategies in criminal cases, especially those involving drug trafficking charges. Traditionally, defense lawyers could successfully challenge the admissibility of expert testimony that appeared to comment about the defendant’s mental state. Now, after the Diaz decision, keeping out harmful prosecution expert testimony about what “most drug dealers” or “most embezzlers” will be more challenging. 


Defense attorneys must now prepare to counter expert testimony that broadly generalizes about the mental states of individuals in similar situations to their clients. This requires a more detailed approach, focusing the jury’s attention on the individual circumstances and specific facts of the case to differentiate the defendant from the generalizations made by experts. 


Defense lawyers might also need to employ their own experts more strategically. The defense could call experts to provide alternative interpretations or to challenge the prosecution’s experts, ensuring that the jury receives a balanced perspective. For instance, defense experts might testify about scenarios where individuals genuinely are unaware of illegal activities occurring around them, thereby mitigating the impact of the prosecution’s generalized claims. This could involve defendants with certain mental illnesses or other disabilities. It might also allow defense lawyers to call experts to explain why a victim of battered spouse syndrome might not leave an abusive relationship before engaging in some defensive violence. 


Diaz Decision’s Implications for Other Criminal Cases 


While the Diaz case directly concerns drug trafficking, its implications extend to other areas of criminal law where expert testimony is pivotal. In fraud cases, for example, experts often testify about the typical knowledge or intent of individuals engaged in certain financial transactions. Similarly, in conspiracy cases, experts might speak about the common knowledge shared by members of a conspiracy. 


The Diaz ruling allows such testimony as long as it remains general and does not explicitly attribute a mental state to the defendant. This broader interpretation of Federal Rule of Evidence 704(b) means that expert witnesses can provide more descriptive contextual information that prosecutors can use to build stronger cases based on general behaviors and practices. 


However, defense attorneys can also use this ruling in the defendant’s favor. By calling defense experts to testify about general behaviors and practices that support the defense’s narrative, they can provide the jury with alternative perspectives that might explain the defendant’s actions without implying criminal intent. 


The Role of Expert Testimony as a Result of Diaz 


Expert testimony is a powerful tool in criminal trials, often shaping the jury’s understanding of complex issues. 


  • Thorough pre-trial motions to limit the scope of expert testimony can still be effective. While Diaz allows broader expert opinions, experienced defense attorneys can argue to exclude specific portions of testimony that we believe cross the line into direct commentary on the defendant’s mental state. 
  • Cross-examination remains key. Effective cross-examination can expose weaknesses or biases in expert testimony and undermine the prosecution’s case. At Stechschulte Nell, defense lawyers should be well-prepared to challenge the assumptions and methodologies behind the prosecution’s expert opinions. 
  • Jury instructions are more important than ever. Defense attorneys must ensure that jury instructions relating to how the jury should evaluate expert testimony make clear that the expert’s opinion is not proof of the defendant’s mental state. The jury must understand that such testimony can inform but not dictate their conclusions about the defendant’s mental state. 


The Supreme Court’s decision in Diaz v. United States is a significant change in the way criminal trials may be conducted. By allowing expert testimony about the general mental states of groups, the ruling provides prosecutors with a potent tool to argue their cases. However, it also opens the door for defense attorneys to use similar strategies, calling our own experts to provide counter-narratives. 


Get Experienced Criminal Defense Representation in Florida Federal or State Court 


At Stechschulte Nell, Attorneys at Law in Tampa, Florida, we believe it is essential for the public to understand these legal nuances and their potential impact on individual cases. The Diaz decision underscores the evolving nature of legal standards and the importance of hiring experienced defense attorneys to vigorously defend your legal rights and apply their knowledge of the law and experience in court to protect the clients’ rights effectively. 


Contact our attorneys today for a case review at 813-280-1244. 

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