The Limitations of Federal Rule of Evidence 704(b) & How Prosecutors Frequently Circumvent It

Federal Rule of Evidence 704(b) provides that 

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.

Under this Rule, expert witnesses are not allowed to testify that the defendant had a particular mens rea (e.g., “The defendant possessed the drugs with intent to distribute.”). But, as opinions such as the recent one by the Sixth Circuit in United States v. Xu, 2024 WL 4002876 (6th Cir. 2024), make clear, experts come come awfully close to saying this very thing.

In Xu, Yanjun Xu was charged with conspiracy to commit economic espionage and conspiracy to steal trade secrets from multiple aviation companies over a five-year period. After he was convicted, the defendant appealed, claiming that expert testimony by a former intelligence officer violated Federal Rule of Evidence 704(b) because the expert opined on his mental state. The Sixth Circuit disagreed, concluding that:

After examining the dialogue as a whole, the context of Olson’s testimony makes it clear that he did not opine on whether Xu possessed the requisite intent to steal trade secrets. Rather, Olson, as a former intelligence officer, explained how Xu’s actions toward Zheng were “consistent with,” “typical of,” and “indicative of” a covert intelligence-gathering operation based on common tradecraft principles and techniques….When experts testify, district courts do not require them to preface every answer to avoid a Rule 704(b) violation. Instead, it is the role of the court to ascertain whether an expert’s testimony crosses the line of reaching an ultimate issue….Under any applicable standard, the district court did not err in admitting Olson’s testimony, and Xu has not produced any evidence to the contrary.

This is pretty standard practice, whether you agree with it or not (and I’m not sure I do). An expert can’t testify that a defendant had a particular mental state, but can testify that their actions were “consistent with,” “typical of,” or “indicative with” a particular mental state.

-CM