In some cases, evidence of a mental disease, defect or disorder may bear on whether the defendant actually formed a required mental state. California has abolished diminished capacity as a defense; the question is not whether a diagnosis made the person generally unable to act, but whether evidence supports a failure to form a particular mental state that the charged crime requires. The rule does not apply to every charge or every theory. (Penal Code § 28)

For a qualifying charge, section 28 permits the evidence solely on whether the defendant actually formed the required specific intent, premeditation, deliberation or malice aforethought. Map the charge to its exact mental-state element and connect the evidence to the defendant’s condition and conduct at the relevant time. A diagnosis by itself does not answer that question. The evidence remains subject to ordinary relevance and other evidentiary limits. (Penal Code § 28)

There is also a limit on expert testimony. In the guilt phase, a mental-health expert may not give the ultimate opinion that the defendant did or did not have the required mental state; that decision belongs to the factfinder. The expert may explain relevant clinical information and evidence within the permitted scope. (Penal Code § 29; People v. San Nicolas)

Match any instruction to the charged mental state

CALCRIM 3428 limits the jury’s consideration to the charged mental state supported by the evidence and leaves the prosecution with its burden to prove that element beyond a reasonable doubt. The instruction is given on request, not automatically, and should be tailored to the actual charge. The jury—not an expert—decides whether the required state of mind was formed. (CALCRIM No. 3428 (2026); People v. Saille)

This is not the legal-insanity question, a competency-to-stand-trial issue, or a claim that mental illness automatically excuses conduct. Before trial, identify the elements, contemporaneous records and observations, expert opinions within section 29, and the specific instruction counsel may request. If a defense expert will testify, a separate statutory examination question may arise; do not confuse that issue with the admissibility of mental-illness evidence itself.

For a broader overview of criminal trials, see the firm’s practice page. For help evaluating mental-state evidence, contact the Law Offices of David S. Chesley at (800) 755-5174.