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

Summary

At oral argument in Diaz v. United States, the justices probed whether Federal Rule of Evidence 704(b) prohibits expert testimony that gives probabilistic or class-based statements—such as "most people like the defendant"—about whether a defendant knew about hidden drugs, with counsel debating textual limits and practical consequences.

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…

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

Summary

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