M'Naghten rules

The M'Naghten rule(s) (pronounced, and sometimes spelled, McNaughton) is a legal test defining the defence of insanity that was formulated by the House of Lords in 1843. It is the established standard in UK criminal law.^[1]: 5 Versions have been adopted in some US states, currently or formerly,^[2] and other jurisdictions, either as case law or by statute. Its original wording is a proposed jury instruction:

that every man is to be presumed to be sane, and ... that to establish a defence on the ground of insanity, it must be clearly proved that, at the time of the committing of the act, the party accused was labouring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing; or if he did know it, that he did not know he was doing what was wrong.^[3]: 632

The rule was created in reaction to the acquittal in 1843 of Daniel M'Naghten on the charge of murdering Edward Drummond. M'Naghten had shot Drummond after mistakenly identifying him as the British Prime Minister Robert Peel, who was the intended target.^[4] The acquittal of M'Naghten on the basis of insanity, a hitherto unheard-of defence per se in modern form, caused a public uproar, with protests from the establishment and the press, even prompting Queen Victoria to write to Robert Peel calling for a "wider interpretation of the verdict".^[5] The House of Lords, using a medieval right to question judges, asked a panel of judges presided over by Sir Nicolas Conyngham Tindal, Chief Justice of the Common Pleas, a series of hypothetical questions about the defence of insanity. The principles expounded by this panel have come to be known as the "M'Naghten Rules". M'Naghten himself would have been found guilty if they had been applied at his trial.^[6][7]

The rules so formulated as M'Naghten's Case 1843 10 C & F 200,^[8] or variations of them, are a standard test for criminal liability in relation to mentally disordered defendants in various jurisdictions, either in common law or enacted by statute. When the tests set out by the rules are satisfied, the accused may be adjudged "not guilty by reason of insanity" or "guilty but insane" and the sentence may be a mandatory or discretionary, but usually indeterminate, period of treatment in a secure hospital facility, or otherwise at the discretion of the court, depending on the country and the offence charged, instead of a punitive disposal.