Clark v. Arizona

Supreme Court of the United States · 2006 · Criminal Law

548 U.S. 735 (2006)

Updated August 1, 2026

Criminal Lawinsanitymens rea evidenceinsanity defenseM'Naghtenright-wrong testmens reamental disease evidence

Facts

Clark shot and killed a uniformed Flagstaff police officer who had stopped him while driving a truck, and Clark did not dispute the shooting or death. He relied on his undisputed paranoid schizophrenia both to assert Arizona's insanity defense and to argue he lacked the specific intent or knowledge required to kill a law enforcement officer. The trial court, citing State v. Mott, would not consider mental-illness evidence to negate mens rea, but did consider insanity evidence on the insanity issue. After hearing lay and expert testimony about Clark's delusions and schizophrenia, the court found him guilty of first-degree murder and found he had not proved insanity.

Issue

Does due process prohibit Arizona from using an insanity test stated only in terms of whether the defendant knew the criminal act was wrong? Does due process also prohibit Arizona from restricting mental-disease and capacity evidence to the insanity defense and not allowing it to negate mens rea directly?

Rule

Due process does not require a State to retain both parts of the traditional M'Naghten formulation; a right-wrong insanity test alone is constitutionally permissible where evidence of cognitive incapacity remains relevant and admissible to show moral incapacity. Due process also permits a State to channel evidence of mental disease and mental capacity to the insanity defense, rather than allow that evidence to be considered directly on mens rea, so long as the State has reasonable grounds for doing so, including preserving its chosen presumption of sanity and avoiding juror confusion and misuse of psychiatric evidence.

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One of 10 multiple-choice questions for this case. Pick an answer to see why.

In Ohio, Devin Mercer is charged with arson after setting fire to a storage shed in Toledo. Ohio's insanity statute excuses a defendant only if, because of mental disease or defect, he did not know his criminal act was wrong; it does not separately mention inability to understand the nature and quality of the act.

Devin argues the statute violates due process because a constitutionally adequate insanity test must include both a cognitive prong and a moral prong. How should a court rule?

Explanation. The majority held that due process does not require a State to retain both parts of the traditional M'Naghten formulation. Because Anglo-American law has long recognized materially different insanity standards, no single formulation is fundamental for due process purposes. A statute framed solely in terms of whether the defendant knew the act was wrong is constitutionally permissible.