In Diaz v. United States, the United States Supreme Court held that expert witness testimony that speaks generally about the mental state of a group of people does not violate Federal Rule of Evidence 704(b).
Facts of the Case
Diaz was a drug trafficking case. Border patrol officers at the U.S.-Mexico border found 56 packages of methamphetamine hidden in the door panels and trunk of the car the defendant was driving. The defendant said she was not aware of any drugs, and that she was driving her boyfriend’s car. She was, however, unable to provide any information about her boyfriend or about two cellphones that were found in her car.
At trial, the Government called a Homeland Security Investigations Special Agent as an expert witness to testify about “the common practices of Mexican drug-trafficking organizations.” The agent testified that generally, drug traffickers know that they are transporting drugs. The agent explained that this is because drug-trafficking organizations are “often unwilling to take . . . chances with hundreds of thousands of dollars on the line.” However, on cross-examination, the agent conceded that “drug-trafficking organizations sometimes use unknowing couriers.”
The defendant was found guilty by a jury.
Federal Rule of Evidence 704(b)
Federal Rule of Evidence 704 governs expert testimony about the “ultimate issue” in a case. While 704(a) states that such an opinion is “not objectionable,” Rule 704(b) lays out an 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 Appeals
The defendant first appealed to the Ninth Circuit Court of Appeals, arguing that because an element of each of the charges against her was that she “knowingly” transported drugs, the agent’s testimony improperly violated Federal Rule of Evidence 704(b).[1] The Ninth Circuit disagreed, and the United States Supreme Court granted certiorari.[2]
The Supreme Court’s opinion, authored by Justice Thomas,[3] began by explaining the history of “the ultimate-issue rule” and that the reasoning behind it is to prevent “witnesses from taking over the jury’s role.” Interestingly, the Court explained that the 704(b) exception was created shortly after John Hinckley, Jr. was tried for the attempted assassination of President Ronald Reagan and found not guilty by reason of insanity. At Hinckley’s trial, both sides brought expert witnesses to opine on his sanity. Congress then created the 704(b) exception in 1983 “to eliminate the confusing spectacle of competing expert witnesses testifying to directly contradictory conclusions as to the ultimate legal issue to be found by the trier of fact.”[4]
The Supreme Court ultimately held that the expert opinion in Diaz did not violate Rule 704(b) because it was “about the knowledge of most drug couriers” and not about the defendant specifically. The Court reasoned that although the agent’s testimony explained that the majority of drug couriers do so knowingly, “[t]he ultimate issue of Diaz’s mental state was left to the jury’s judgment.”
Concurrence: Justice Jackson
Justice Jackson pointed out that defendant relied on similar expert testimony, as she “called an automobile specialist who testified that a driver of her particular car would almost certainly not know that it contained drugs.” She also explained that similar expert witness testimony could be used in other cases to help a jury understand the general mental effects of domestic abuse, as well as assist defendants with various other defenses, “including duress and self-defense.”
[1] United States v. Diaz, No. 21-50238, 2023 WL 314309 (Jan. 19, 2023); see 21 U.S.C. § 960(a)(1) (prohibiting a person from “knowingly or intentionally” importing a controlled substance).
[2] Diaz v. United States, 601 U.S. —, 144 S.Ct. 392 (2023) (mem. op.).
[3] Justice Jackson wrote a concurring opinion; Justices Gorsuch, Sotomayor, and Kagan dissented.
[4] S. Rep. No. 98-225, p. 230 (1983).
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