Any day now, The U.S. Supreme Court is expected to make an important decision related to the Fourth Amendment about drug traffickers. The case awaiting a decision is Diaz v. United States.

On August 17, 2020, Delilah Guadalupe Diaz was on her way back to California from Mexico. Upon reaching the border, border agents heard a crunching sound when she lowered her car window. This caused the border patrol to inspect the car’s door panels with a density measuring device. This revealed 27.98 kilograms of methamphetamine hidden within the panels. Diaz maintained that she was unaware of the presence of the drugs, explaining that she had traveled to Mexico with her daughter, who had returned home earlier. Diaz stayed behind to visit her boyfriend and was using his car for her return journey. Her boyfriend had informed her that he would pick up the car from her within a couple of days.

The government charged Diaz with importing methamphetamine, in violation of the Controlled Substances Act. A key element of this charge was proving that Diaz knew she was transporting drugs. To establish this, the prosecution brought in an expert who testified that “narcotic traffickers do not entrust large and valuable quantities of narcotics to unknowing couriers.” Diaz contested the admissibility of this expert testimony, arguing it would violate Federal Rule of Evidence 704(b) by directly addressing the ultimate issue of her knowledge. However, the district court admitted the evidence, the jury convicted Diaz, and she was sentenced to seven years in prison. On appeal, the U.S. Court of Appeals for the Ninth Circuit upheld the conviction.

Federal Rule of Evidence 704(b) states, “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 main question in this case is whether, under Federal Rule of Evidence 704(b), the governmental expert witness is allowed to testify that couriers know they are carrying drugs because drug-trafficking organizations do not entrust valuable quantities of drugs to unknowing transporters as proof that Diaz was aware that she was carrying drugs. Rule 704(b) prevents an expert witness from assigning a mental condition that constitutes a crime (mens rea) to the defendant.

On March 19, Supreme Court Litigator Jeffrey L. Fisher made the argument that although the expert witness’ testimony did not specifically mention the defendant, it groups her in with a class of people (drug couriers) and assigns a mental state to them, the defendant included. However, several Supreme Court Justices argued against Fisher, saying Rule 704(b) could be skirted as long as some of the language of the testimony was adjusted, so the testimony would end up being used by the government anyway. Much of the argument surrounded the contention whether the expert witness’ testimony grouped the defendant with drug couriers who were aware of the drugs she was carrying, or if the testimony was simply a probabilistic opinion that most drug couriers are aware of the drugs they carry.

The upcoming decision on Diaz v. United States could have many implications. The Fourth Amendment to the U.S. Constitution protects people from unreasonable searches and seizures by the government, and a ruling in favor of Diaz could reinforce Fourth Amendment protections and preserve the individual rights of those who are unknowingly involved in a crime. Upholding Diaz’s argument could place stricter limits on the use of expert testimony in criminal cases, particularly regarding the mental state of defendants. This would help prevent the potential misuse of expert opinions that effectively determine the defendant’s guilt, maintaining the jury’s obligation to determine what the true facts of the case are. Further, a decision that limits the use of generalized expert testimony about drug couriers could protect defendants from being unfairly profiled and convicted based on broad, probabilistic statements rather than specific evidence related to their individual circumstances.

This case could set a vital precedent for handling similar situations in the future, potentially leading to more rigorous standards for evidence and testimony in criminal trials. A ruling in Diaz’s favor would protect individual rights and prompt law enforcement agencies to adopt more precise and respectful methods for conducting searches and gathering evidence, while also empowering defendants to more effectively challenge evidence that unlawfully convicts them.