In a criminal case, what is prohibited for an expert to testify?

• A. The expert may not testify about any mental state of the defendant.
• B. The expert must not state an opinion about whether the defendant had a mental state or condition that constitutes an element of the crime; those matters are for the trier of fact.
• C. The expert may only testify about mental state if the defendant consents.
• D. The expert must testify first about the underlying facts.

Answer: (B)

In this area, the important idea is that a medical or mental-health expert can explain factors that might affect a person’s capacity or awareness, but they cannot decide legal questions for the jury. The mental state or condition that constitutes an element of a crime is a legal determination for the trier of fact to decide. An expert may describe findings about mental illness, cognitive impairments, or how a condition could influence behavior, but they may not give an opinion concluding that the defendant did or did not have the specific mental state required by the crime. That judgment must come from the jury based on the law and the evidence.

That’s why the best choice is the one stating that the expert must not state an opinion about whether the defendant had a mental state or condition that constitutes an element of the crime; those matters are for the trier of fact. The other options go too far or misstate the role of expert testimony: a defendant’s mental state can be discussed in terms of diagnosis or impairment, consent is not a rule that governs this, and there’s no requirement that the expert testify first about underlying facts.