Supreme Court weighs whether Rule 704(b) bars class or probabilistic expert testimony on mens rea

The Supreme Court heard argument Tuesday in Diaz v. United States over whether Federal Rule of Evidence 704(b) bars expert testimony that assigns or implies a defendant's mens rea by speaking about a class of people or giving probabilistic statements.

Mister Fisher, counsel for the petitioner, told the justices that Agent Flood's testimony in the case—that "in most circumstances, people like the defendant know they have drugs in the car when they cross the border"—crossed the line the rule was meant to draw. "That testimony is an opinion about whether the defendant had the requisite mens rea," Fisher argued, saying the rule should forbid not only absolute statements but also probabilistic class assertions that functionally assign mens rea to the defendant.

The government's lawyer, Mister Garnieri, urged the Court to read the rule more narrowly. "By its plain terms, Rule 704(b) comes into play only when the expert is offering an opinion about the defendant's own mental state," he told the justices, arguing that general framework or modus operandi evidence that helps the jury understand how drug-trafficking operations run is typically permissible and can be addressed under other safeguards if reliability is in doubt.

The bench treated both proposals to extensive questioning. Several justices tested hypotheticals the parties had briefed — an expert saying "80% of the time," or "83 of 83 cases," or describing a narrowly defined class that plainly includes the defendant — to probe where the line should fall. Justice Kagan asked whether the government was hedging in its brief about "the 80% case." Justice Alito and Justice Gorsuch pressed on how courts could draw administrable lines if probabilistic testimony were allowed; Justice Sotomayor asked whether permitting broad probabilistic statements would simply "bring us right back where we started" after the reform that followed the Hinckley trial.

Counsel for the petitioner framed the dispute as a choice between two doctrines: the Ninth Circuit's approach, which the petitioner says has allowed problematic testimony, and a Fifth Circuit formulation that permits inferential modus operandi evidence while excluding testimony that assigns mens rea to the defendant or a class including the defendant. The government emphasized textualism and history, noting that 704(b) was adopted in response to Hinckley and is a limited exception to the general rule permitting expert testimony on ultimate issues.

Justices repeatedly raised reliability and gatekeeping concerns. Several said that Rule 702 (Daubert-related reliability) and Rule 403 (prejudice-versus-probative-value balancing) can address extreme or misleading expert statements, while others worried that leaving too much to those doctrines would create inconsistent outcomes and difficult line-drawing tasks for trial judges.

Both sides acknowledged that the case before the Court is easier in some respects: agent Flood's testimony, as described in the briefs, included categorical and probabilistic language that the petitioner says directly implicated the defendant. "The jury can go back to the jury room and say, 'looks like Agent Flood thinks people like this generally have the mens rea. We'll go along with that,'" Fisher said in rebuttal, urging the Court to preserve the jury's fact-finding role on mens rea.

The Court took the case under advisement after rebuttal. The justices did not announce a decision at argument; the case was submitted.