“Most people” Agree With This Blog Post—a Review of the Supreme Court’s Recent Decision in Diaz v. United States

Federal Rule of Evidence 704(b) precludes an expert witness in a criminal case from stating an opinion as to 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.” It adds, “Those matters are for the trier of fact alone.”

On June 20, 2024, the U.S. Supreme Court ruled in Diaz v. United States that Rule 704(b) does not, however, preclude an expert from testifying about a particular mental state that “most people” in a group share because it is not an opinion about “the defendant’s” mental state.

Ms. Diaz was caught with over $300,000 worth of methamphetamine hidden in her car door panel as she tried to drive from Mexico into the United States.  The key issue at her trial was whether she knew the drugs were there.  If she didn’t, no crime was committed.

Defense lawyers often argue this “blind mule” defense, which holds the government to its burden of proving a courier-defendant like Ms. Diaz acted with the requisite mental state for drug trafficking: it’s not a crime to unknowingly transport drugs; the government has to prove the defendant knew what they were doing.  Put another way, if there’s a kilo hidden in the wheel well of a rental car when you rent it, how are you supposed to know that?

The defense presumes that a drug cartel, or some other criminal opportunist, might hide contraband in an innocent person’s car.  The criminal then follows the “blind mule” through the port of entry (observing from a safe distance, no doubt), and then retrieves the erstwhile smuggled drugs once they’re safely past the border.

Often, in an attempt to prove a guilty mindset, prosecutors offer circumstantial evidence from which the jury may derive a reasonable inference that the defendant knew what they were doing.  Circumstantial evidence of a guilty mindset might look like this: proof that a murder suspect bought a pistol the same day she Googled “how to hide a body in the woods”—that is pretty strong evidence the crime wasn’t accidental.

In Diaz, the defendant gave a post-Miranda statement so full of holes it should have easily been enough for a conviction:  Ms. Diaz claimed the car was her “boyfriend’s” but then also claimed she’d only met that person two to three times; she claimed to not know his phone number or where he lived; she admitted that she owned one of the two phones that were found in the car, and claimed didn’t know how to unlock the other, which was given to her by a “friend” whom she refused to name.  A prosecutor would normally believe this was plenty of circumstantial evidence (not to mention the direct evidence—the 54 pounds of meth) for a conviction.

But the federal prosecutors in San Diego weren’t satisfied with what looked like an easy win.  So eager were they to rebut the “blind mule” defense that they had a federal agent testify in court as a supposed expert on the mindset of drug couriers.  This “expert” agent testified that, in most circumstances, the driver of a car carrying drugs knows (1) the drugs are in the car and (2) that they’ve been hired for that purpose.

There are a number of problems with this kind of testimony.  First, the extent to which anyone can claim to know the “typical mindset” of a person in Ms. Diaz’s position is suspicious in and of itself.  There is no “typical” defendant in any kind of case.  And there’s really no scientific rigor for evaluating what are likely anecdotal stories from conflicted and terrified witnesses.

Second, and the main problem for the purposes of Diaz v. United States, is that the Federal Rules of Evidence explicitly prohibit expert testimony on a defendant’s mindset.  Not so, anymore, so long as the experts’ opinions are couched in terms of “most people,” not specifically “this person.”

The Supreme Court’s decision allows this testimony so long as the purported expert doesn’t weigh in on what the specific defendant was thinking at the time of the crime.  The phrase “most people in this situation would have known what they were doing” is fine; the phrase “the defendant probably knew what they were doing” is not.

Justice Gorsuch’s dissent rightly argues this is a distinction without a difference.  Issues of mens rea—that is the defendants’ mindset—are reserved for the jury and the high risk that “junk science” will be used to assess someone’s mindset is exactly why Congress enacted Rule 704(b) in the first place—to prevent untestable opinion testimony on an ultimate issue reserved for the jury.

As Justice Gorsuch notes, defense attorneys will still have many avenues for objecting to this kind of evidence.  For one thing, evidence as to what “most people” think and know is likely not relevant to what “this defendant” thought and knew, and so is inadmissible under Rule 402.  Moreover, even if evidence of what “most people” know is relevant, it’s more prejudicial than probative as to what “this defendant” knew and so can be objected to pursuant to Rule 403.  But if the evidence is allowed, as Justice Gorsuch writes, it comes with the imprimatur of the government and that alone is a strong inducement for a jury to convict.  They might think, “surely the government’s expert witness can’t be wrong about the socio-psychology of drug couriers.”  “Why would the government have an expert on that if it wasn’t real science?”

Diaz v. United States allows government witnesses to speak generally about prototypical persons’ mindsets so long as they don’t claim to know what the defendant herself was thinking.  It’s a dangerous precedent because, in practical terms, this forces defense attorneys to present affirmative evidence of their client’s mindset to counter the government expert.  If “most people” think one way, the defense has to argue why their case, and their client, is different. This saddles the defense with a burden of proof contrary to the Constitution.  Defense attorneys should be fiercely protective against junk science, and particularly adverse to government witnesses who claim to know what “most people” think in any situation.