Insanity Defense (M'Naghten)

The M'Naghten rule is the oldest and most widely used test for the insanity defense in American criminal law. Originating from the 1843 English case of Daniel M'Naghten, who killed the secretary to the Prime Minister while suffering from paranoid delusions, the test focuses exclusively on the defendant's cognitive capacity at the time of the offense. It does not consider the defendant's ability to control their behavior.

Under M'Naghten, a defendant is legally insane if, at the time of committing the act, they were laboring under such a defect of reason from disease of the mind that they either (1) did not know the nature and quality of the act they were doing, or (2) if they did know it, they did not know that what they were doing was wrong. The first prong addresses a fundamental lack of awareness, such as a person who strangles someone believing they are squeezing a lemon. The second prong addresses moral or legal wrongfulness, such as a person who kills someone believing God has commanded them to do so.

The M'Naghten test has been criticized for its narrow cognitive focus. It does not account for defendants who understand that their actions are wrong but are unable to control their impulses due to mental illness (the so-called "irresistible impulse" scenario). Some jurisdictions have supplemented M'Naghten with an irresistible impulse test, but the basic M'Naghten test standing alone requires only a cognitive deficiency. The burden of proof typically falls on the defendant to establish insanity by a preponderance of the evidence, though some jurisdictions require clear and convincing evidence. A successful insanity defense results in a verdict of not guilty by reason of insanity, usually followed by commitment to a mental institution.