Which legal standard is used for the insanity defense?

The M'Naghten Rule serves as the legal standard for the insanity defense by establishing that a defendant may be excused from criminal liability if, at the time of the criminal act, they were suffering from a mental disease or defect that either prevented them from understanding the nature of their act or from knowing that the act was wrong. This standard focuses on the cognitive ability of the defendant to distinguish right from wrong, which is a critical component when evaluating an insanity plea.

This rule was established in 1843 following the case of Daniel M'Naghten, who was found not guilty by reason of insanity after killing the secretary of the British Prime Minister, believing he was acting under divine instructions. Under this rule, the emphasis is placed on the mental state of the defendant at the time of the offense, making it a historically significant framework in assessing insanity defenses.

Other options represent different standards and principles that are not broadly applied across all jurisdictions. The Durham Rule, for example, focused more on the effect of a mental illness on behavior, while the Substantial Capacity Test and the Model Penal Code Rule offer alternative approaches but do not have the same historical and widespread legal acknowledgement as the M'Naghten standard. This background illustrates the enduring relevance and utility of