SCOTUS Decision in Diaz v. U.S. is Wrong, but it Doesn't Depend on Mind Reading

When Delilah Diaz was stopped at the Mexico/U.S. border, federal agents found 54 pounds of meth in the car she was driving. Under questioning, she denied knowledge of the drugs, saying that the car belonged to her boyfriend. At Diaz's trial, the government called Homeland Security official Andrew Flood as an expert witness. Flood didn't know and hadn't interviewed Diaz. He was called as an expert on drug trafficking and testified that "in most circumstances, the driver knows they are hired . . . to take the drugs from point A to point B," thus undercutting Diaz's "blind mule" defense.

Flood's testimony was permitted over an objection by Diaz, who cited Federal Rule of Evidence 704(b). It states, in full: "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."

Yesterday, in Diaz v. United States, the Supreme Court upheld Diaz's conviction. Flood was not testifying about Diaz's mental state, the Court said in an opinion by Justice Thomas, but about the mental states of most people who are found in possession of drugs. Thus there was no violation of Rule 704(b).

The 6-3 SCOTUS ruling is not persuasive. The reason why Flood's testimony was relevant at all was because Diaz's mental state was contested--indeed, was the only contested issue in the case. As Justice Gorsuch explained in a forceful dissent joined by Justices Sotomayor and Kagan, under the majority's approach, Rule 704(b) can always be circumvented by experts who limit their testimony to "most people." The majority opinion reads Rule 704(b) as essentially useless.

Unfortunately, the dissent also contains a line of criticism that is unwarranted and confusing. At various points, Justice Gorsuch expresses skepticism that there can even be such a thing as an expert in the mental states of drug mules. He notes that the government routinely proves knowledge or intent on the part of drug courier defendants through circumstantial evidence.  He then says: "There was no need to gild the lily by calling to the stand an 'expert' in mindreading." Later in the dissent, Justice Gorsuch adds: "No one, at least outside the fortuneteller's den, can yet claim the power to conjure reliably another’s past thoughts."

The problem with that line of mockery is that it seemingly aims to demonstrate that, quite apart from Rule 704, Flood shouldn't have been permitted to testify as an expert under the general expert witness testimony provision of Federal Rule of Evidence 702. But that's wrong.

To be sure, there is no such thing as an expert in mindreading or fortunetelling. However, the prosecution didn't offer Flood as an expert on what goes on inside the minds of drug mules. The government offered Flood as an expert on how drug gangs move their product.

As a preliminary matter, a Homeland Security or other law enforcement official can be an expert in how drug gangs operate in virtue of working in the area. Rule 702 expressly allows that one way in which someone can become an expert is via "experience." For example, as a classic scene from the cinematic masterpiece My Cousin Vinny illustrates, one can develop expertise in general automotive knowledge by working as a mechanic.

Likewise, by working in drug interdiction, one can gain an expertise in how drug gangs operate. Suppose that Diaz had offered her own expert--a former DEA agent I'll call Shmood. Imagine that Shmood testifies that the gangs that smuggle meth across the border frequently enlist young men whose girlfriends drive their cars without knowledge that they contain drugs. Perhaps the drug gangs believe women are less likely to appear suspicious and that, not knowing that they are carrying drugs, they will not appear nervous. Such testimony would be very helpful to jurors who, lacking familiarity with the specifics of the meth trade, might have assumed that most drug mules would know that they were acting as drug mules.

If that works for Shmood (and it does), the opposite works for Flood. Jurors in the actual Diaz case might have thought it quite plausible that someone could be driving a car that contains drugs without knowing so. However, expert testimony could inform them that the drug cartels expressly instruct their mules not to relinquish control of their vehicle to any third parties until the delivery is complete.

Accordingly, Justice Gorsuch is wrong to suggest that Flood was not properly qualified as an expert. Rule 702 was satisfied. Justice Gorsuch ought to have confined his otherwise persuasive dissent to explaining why Flood's testimony should have been excluded under Rule 704(b).