In August 2017, the Missouri General Assembly amended its expert testimony statute, Section 490.065. The newly-enacted Section 490.065.2(3)(b) states, “In a criminal case, an expert witness shall 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.” Section 490.065.2(3)(b) is identical to Federal Rule of Evidence 704(b) (“Rule 704(b)”). This change is significant because issues in criminal cases, such as deliberation with respect to homicide and the affirmative defense of insanity, frequently implicate defendants’ mental states. In addition, Rule 704(b) and its state-law counterparts have drawn significant scholarly criticism as unduly restricting helpful expert testimony. For example, one scholar argues Rule 704(b) produces “counterproductive” and “troubling” results by “requir[ing] the jury, as the finder of fact, to reach a conclusion as to the defendant’s mental state without the benefit of the most useful testimony the expert could offer.”

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