Britain's famous insanity test grew from judges answering Parliament's hypothetical questions
The M'Naghten Rules, still a touchstone for the insanity defence, were never meant as a formal definition. In 1843 Parliament put hypothetical questions to a panel of judges after Daniel M'Naghten was acquitted of killing Edward Drummond, a man he had mistaken for Prime Minister Robert Peel. Their answers became law.
The judges' test asks whether, at the time of the act, a disease of the mind left the accused unable to understand what they were doing, or unaware that it was wrong. It sits alongside other standards, such as the Durham rule and the American Law Institute's model code, most of them tied to the old idea of mens rea, the guilty mind. The Latin phrase non compos mentis, not in command of one's mind, expresses the same notion.
The idea is ancient; Greek and Roman law already made room for it. Medieval English law under Edward II set a strikingly low bar, treating someone as insane only if their understanding was no greater than that of a wild beast, meaning a dumb animal rather than a raging one. Juries were at first expected to convict such people and pass the case to the king for a pardon, but from 1500 they could acquit outright. The oldest complete transcript of an insanity trial dates from 1724.
In practice the defence is rarely used in Britain, Ireland and the United States, where partial defences such as diminished capacity come up more often. Psychiatrists and psychologists give evidence about the defendant's state of mind, but the question of responsibility belongs to the jury. Courts also separate competency, meaning whether someone can take part in their trial now, from responsibility at the time of the offence.
Temporary insanity, the claim of a passing loss of reason, first succeeded in 1859 for the New York congressman Daniel Sickles. The US Supreme Court held in 1986 that the insane cannot be executed, and in 2020, in Kahler v. Kansas, that states are not constitutionally required to offer a right-and-wrong insanity defence.
Source: Insanity defense
