What is the test for admitting evidence of mental illness in a murder case?

The ruling was an overly restrictive reading of the statutory limitations on admission of evidence of mental illness. Expert opinion on whether a defendant had the capacity to form a mental state that is an element of a charged offense or actually did form such intent is not admissible at the guilt phase of a trial. (People v. Smithey (1999) 20 Cal.4th 936, 960-961, 86 Cal.Rptr.2d 243, 978 P.2d

[23 Cal.4th 579]

1171.) Sections 28²² and 29²³ permit introduction of evidence of mental illness when relevant to whether a defendant actually formed a mental state that is an element of a charged offense, but do not permit an expert to offer an opinion on whether a defendant had the mental capacity to form a specific mental state or whether the defendant actually harbored such a mental state. An expert's opinion that a form of mental illness can lead to impulsive behavior is relevant to the existence vel non of the mental states of premeditation and deliberation regardless of whether the expert believed appellant actually harbored those mental states at the time of the killing.

[23 Cal.4th 579]