In Clark v. Arizona, No. 05-5966 (June 28, 2006), the Supreme Court held that Arizona did not violate Due Process when it narrowed the insanity test to inquire only whether the defendant had the capacity to tell right from wrong.
The Arizona legislature amended its insanity statute to drop the portion of the definition of insanity relating to cognitive capacity, that is the ability of the defendant to know what he was doing, leaving only the moral test of whether he could distinguish right from wrong – thereby amending the prior M’Naghten test. Consequently, at Clark’s murder trial, the trial court excluded defense evidence of Clark’s lack of cognitive capacity, evidence that was meant to negate mens rea but would not have been probative of insanity, as now defined.
The Court rejected the claim that Due Process required the full M’Naghten insanity definition. The Court noted that various definitions of insanity have been sustained over the years. Further, as a practical matter, cognitive incapacity will often imply moral incapacity, so the defendant is not significantly prejudiced.
The Court held that excluding evidence of Clark’s mental condition did not violate his Due Process presumption of innocence, and his Due Process right to present evidence showing his innocence. The Court noted that the "presumption of sanity" relieves the government of including as an element of every criminal charge an allegation that the defendant had the capacity to form the mens rea for the crime. The defendant is entitled to present evidence in order to rebut the "presumption of sanity." However the evidence must be relevant and persuasive enough to satisfy the defendant’s burden to establish the defense. Hence, the State can restrict introduction of mental disease evidence which otherwise could be considered for whatever a factfinder might think it was worth on the mens rea issue. Due Process is satisfied if the State has "good reason" to exclude the evidence, e.g. a wish to give the defendant a second, less stringent, avenue for exploring the mental capacity issue. The Court also noted the potential for juror confusion of mental capacity expert testimony, noting that such testimony relies on "judgment" and "judgment is fraught with multiple perils."
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