Diaz v. U.S.: Expert testimony on “most” people’s mental state does not violate Rule 704(b) – Jackson and Gorsuch suggest ways for defense to use or limit ruling
By Greg Mermelstein, Deputy Director & General Counsel, Missouri Public Defender
Expert testimony that “most” drug couriers know they are transporting drugs does not violate Rule 704(b)’s prohibition on testimony about a defendant’s mental state, because the expert testimony is not about whether the defendant herself had a particular mental state, the U.S. Supreme Court held June 20 in Diaz v. United States.
When Delilah Diaz, a U.S. citizen, attempted to re-enter the country
from Mexico, police searched the car she was driving at the border crossing. They discovered 54 pounds of methamphetamine hidden in the car’s door panels and under the carpet in the trunk.
She was charged with importing methamphetamine. The charge required the Government prove she “knowingly” transported drugs.
Diaz asserted a “blind mule” defense at trial that she did not know there were drugs in the car.
Over Diaz’s objection, the Government presented expert testimony from a Homeland Security Special Agent, who testified drug trafficking organizations “generally” do not entrust large quantifies of drugs to people who are unaware they are transporting them, due to their monetary value.
The Special Agent testified that “in most circumstances, the driver knows they are hired … to take the drugs from point A to point B.”
Diaz presented expert testimony from an auto mechanic, who testified there was “no way for someone to suspect or know that there were drugs hidden” in the car.
The jury found Diaz guilty.
The Ninth Circuit affirmed.
Holding
The Supreme Court, in a 6-3 opinion by Justice Thomas, held the Special Agent’s testimony did not violate Rule 704(b).
When Rule 704 was enacted in 1975, it allowed all expert opinions on “ultimate issues” in a case, the Court said.
But in the wake of the not-guilty-by-reason-of-insanity verdict in John Hinkley’s trial for attempted assassination of President Reagan – where experts for both sides testified as to Hinkley’s particular mental state – Congress carved out an exception to “ultimate issue” testimony by adopting Rule 704(b).
That Rule prohibits expert witnesses from stating opinions “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.”
“Rule 704(b) applies only to opinions about the defendant”, the Court said. Because the Agent “did not express an opinion about whether Diaz herself knowingly transported methamphetamine, his testimony did not violate Rule 704(b).”
The Agent’s testimony about “most” drug couriers “does not necessarily describe Diaz’s mental state”, the Court said.
“After all, Diaz may or may not be like most drug couriers”, the Court said. “Diaz herself made this point at trial.”
Diaz presented her own expert who testified there was “no way for someone” to know there were drugs the car, the Court noted.
“The jury was thus well aware that unknowing couriers exist and that there was evidence to suggest Diaz could be one of them”, the Court said. “It simply concluded that the evidence as a whole pointed to a different conclusion”.
“The ultimate issue of Diaz’s mental state was left to the jury’s judgment”, the Court concluded. “As a result, [the Agent’s] testimony did not violate Rule 704(b).”
Justice Jackson says defense can benefit from Rule 704(b)
Justice Jackson concurred “in full”, but wrote separately to note the Rule is “party agnostic” and defendants can benefit from it.
Diaz could have offered expert testimony on the “prevalence and characteristics of unknowing drug couriers”, Jackson said.
In other types of cases, psychiatrists will be able to testify that persons with schizophrenia will “generally … not appreciate the wrongfulness of their conduct”, she said. Such “expert evidence could help jurors better understand a defendant’s condition and thereby call into question a mens rea that might otherwise be too easily assumed.”
And she said experts could testify about “typical mental states of those with battered woman syndrome, helping jurors to better understand how those experiencing it respond to aggression or react to violence.”
Dissenters suggest ways to limit testimiony
Justice Gorsuch, joined by Justices Sotomayor and Kagan, dissented.
The ruling gives the government “a powerful new tool” that allows prosecutors to call an expert “who apparently has the convenient ability to read minds” about what “most” people like the defendant think when they do a proscribed act, Gorsuch said.
“What authority exists for allowing that kind of charade in federal criminal trials is anybody’s guess, but certainly it cannot be found in Rule 704”, he said.
However, Gorsuch saw “reason for hope” that the Court’s holding can be limited.
Gorsuch offered several examples.
Under Rule 402, evidence at trial must be “relevant”, he said. “Yet, if the government is right that an expert opinion about the mental state of ‘most’ people like the defendant is not ‘about’ the defendant’s mental state, it is hard to see how that opinion might be relevant.”
Rule 403 excludes relevant evidence when its probative value is outweighed by the danger of unfair prejudice, Gorsuch noted.
Testimony about what “most” people think “bears minimal probative value when the question at issue is what this individual thinks”, Gorsuch said. “Meanwhile, the danger of unfair prejudice can run very high” when jurors are “told by an expert that birds of a feather are flocked together.”
Finally, Gorsuch said that as part of its “gatekeeping” function, a federal court must ensure expert testimony is “reliable, grounded on widely accepted principles, and will assist the trier of fact to understand the evidence.”
“I struggle to see how a witness claiming to offer an opinion about another person’s (or class of persons’) thoughts at a particular moment in the past can meet any of those standards”, Gorsuch said.
Gorsuch said the above limitations would make the Court’s holding “immaterial in practice”.