Supreme Court Tests Bounds of Expert Testimony in Criminal Cases

U.S. Supreme Court in Washington on March 22, 2024. Madalina Vasiliu/The Epoch Times

The Supreme Court pressed attorneys on March 19 to specify how much expert witnesses can comment on the knowledge of particular groups of people in criminal wrongdoing.

Oral argument in Diaz v. United States focused on the legal principle of mens rea, or the idea that a criminal defendant understood they were breaking the law or engaging in wrongdoing while committing a crime.

Delilah Diaz was convicted for importation of methamphetamine despite claiming she didn’t know nearly 28 kilograms were hidden in her boyfriend’s car, which she was driving across the Mexican border in 2020. Homeland Security agent Andrew Flood had testified that in most cases, drivers who are carrying drugs know they have been hired for that purpose.

Although he didn’t directly comment on Ms. Diaz’s knowledge, she alleged that his testimony violated the federal rules of evidence by inferring that she would have known. More specifically, Federal Rule of Evidence 704(b) provides that “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.”

That law emerged out of the trial of John Hinckley Jr., who attempted to assassinate former President Ronald Reagan. He was ultimately found not guilty by reason of insanity following testimony from psychiatric experts.

In Diaz v. United States, Ms. Diaz was being tried under the Controlled Substances Act, which requires the accused to have knowledge they’re importing drugs.

Stanford Law Professor Jeffrey Fisher, who represented Ms. Diaz on March 19, told the court that Mr. Flood’s testimony “expresse[d] an opinion, a probabilistic opinion instead of an absolute opinion, but an opinion about whether the defendant had the requisite—requisite mens rea.”

His main argument was that comments like Mr. Flood’s establish “a class of individuals, including the defendant” and assign mens rea to that class.

Assistant to the Solicitor General Matthew Guarnieri derided this argument for “prohibit[ing] a novel and amorphous category of what [Ms. Diaz] has called class-wide mens rea testimony.” The “simple reason” Mr. Flood’s testimony didn’t violate Rule 704(b) was that it didn’t express any opinion about Ms. Diaz, Mr. Guarnieri told the court.

Hypotheticals

During oral argument, the justices fielded multiple hypotheticals testing the soundness and outlines of each side’s interpretations of 704(b).

“So suppose that the expert gets on the stand and says: In my experience, drug traffickers always inform their carriers before they head out about the nature of the scheme,” Justice Kagan told Mr. Fisher.

Mr. Fisher responded that that example would “probably barely fall outside of our rule.”

Some of the arguments surrounded the idea that an expert may say that 100 percent of people in a particular situation would act a particular way, meaning the jury would have to assume if the defendant was part of that group, they too would act that way.

When Justice Kagan asked Mr. Guarineri about that scenario, he said that raised a “reliability issue” for the witness.

Both Chief Justice John Roberts and Justice Samuel Alito indicated that other aspects of criminal law would prevent inferences about culpability from being taken too far in court.

The defense, Justice Roberts said, “could presumably stand up and ask the expert, Mr. Expert, are you saying that in every case, someone knows that the drugs are in the car? He, presumably, would say no. And are you saying—are you stating an opinion about whether this individual had—knew she had drugs in the car? He would have to say no, right?”

Mr. Fisher responded that “what the drafters of the rule cared about was not an absolute versus probabilistic opinion. What they cared about was the expert assigning a mens rea to the defendant or a class of people like the defendant. ”

Justice Alito, meanwhile, suggested that trial judges could make their own determinations about the testimony. He pointed to Rule 403, which allows the court to exclude certain evidence.

Justice Sonia Sotomayor seemed the most sympathetic to Mr. Fisher’s position.

“Justice Alito is inviting, I think, even more chaos in the—among courts,” she said. “If you’re going to rely on 403 or 702 or any individualized decision making by judges, you’re just throwing this up into the air with no clarity.”

In January, the Supreme Court heard another oral argument regarding expert testimony but as it related to the Constitution’s Confrontation Clause, which states that criminal defendants have the right “to be confronted with the witnesses against him.” That case—Smith v. Arizona—featured a man whose trial included testimony from a forensic scientist who spoke about drug testing by a different scientist.