(a)The defense of diminished capacity is hereby abolished. In a criminal action, as well as any juvenile court proceeding, evidence concerning an accused person's intoxication, trauma, mental illness, disease, or defect shall not be admissible to show or negate capacity to form the particular purpose, intent, motive, malice aforethought, knowledge, or other mental state required for the commission of the crime charged.
(b)In any criminal proceeding, including any juvenile court proceeding, in which a plea of not guilty by reason of insanity is entered, this defense shall be found by the trier of fact only when the accused person proves by a preponderance of the evidence that he or she was incapable of knowing or understanding the nature and quality of his or her act and of distinguishing right from wrong at the time of the commission of the offense.
(c)Notwithstanding the foregoing, evidence of diminished capacity or of a mental disorder may be considered by the court only at the time of sentencing or other disposition or commitment.
(d)The provisions of this section shall not be amended by the Legislature except by statute passed in each house by rollcall vote entered in the journal, two-thirds of the membership concurring, or by a statute that becomes effective only when approved by the electors.