M'Naghten Rules

The M'Naghten Rules (pronounced, and sometimes spelled, McNaughton) were the first serious attempt to codify and rationalise the attitude of the criminal law towards mentally incompetent defendants. They arise from a set of questions posed to the House of Lords in M'Naghten's Case(1843) 10 C & F 200 following the attempted assassination of the British Prime Minister, Robert Peel, in 1812 by Daniel M'Naghten. The rules have been a standard test for criminal liability in relation to mentally disordered defendants in common law jurisdictions ever since, with some minor adjustments. When the tests set out by the Rules are satisfied, the accused may be adjudged "not guilty by reason of insanity" 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. The defense is recognized in Australia, Canada, England and Wales, New Zealand, the Republic of Ireland, and most U.S. states with the exception of Montana, Idaho, Utah, and Kansas.
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A defence to criminal law liability developed in England in if at the time of the offence, the accused had a disease of the mind such that he was unable to know that his act was wrong. - (read more on M'Naghten Rules)