California Code, Penal Code - PEN § 28

(a) Evidence of mental disease, mental defect, or mental disorder shall not be admitted to show or negate the capacity to form any mental state, including, but not limited to, purpose, intent, knowledge, premeditation, deliberation, or malice aforethought, with which the accused committed the act. Evidence of mental disease, mental defect, or mental disorder is admissible solely on the issue of whether or not the accused actually formed a required specific intent, premeditated, deliberated, or harbored malice aforethought, when a specific intent crime is charged.

(b) As a matter of public policy there shall be no defense of diminished capacity, diminished responsibility, or irresistible impulse in a criminal action or juvenile adjudication hearing.

(c) This section shall not be applicable to an insanity hearing pursuant to Section 1026.

(d) Nothing in this section shall limit a court's discretion, pursuant to the Evidence Code, to exclude psychiatric or psychological evidence on whether the accused had a mental disease, mental defect, or mental disorder at the time of the alleged offense.