M'Naghten Rules

The M'Naghten rules (pronounced, and sometimes spelled, McNaughton) were a reaction to the acquittal of Daniel M'Naghten. They arise from the attempted assassination of the British Prime Minister, Robert Peel, in 1843 by Daniel M'Naghten. In fact, M'Naghten fired a pistol at the back of Peel's secretary, Edward Drummond, who died five days later. The House of Lords 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, even though they have only gained any status by usage in the common law and M'Naghten himself would have been found guilty if they had been applied to his trial. The rules so formulated as M'Naghten's Case 1843 10 C & F 200 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" 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.

The insanity defense is recognized in Norway, Australia, Canada, England and Wales, Hong Kong, New Zealand, the Republic of Ireland, India, and most U.S. states with the exception of Montana, Kansas, Idaho, and Utah. Not all of these jurisdictions still use the M'Naghten Rules.