The M'Naghten rules, pronounced and sometimes spelled as McNaughton, represent a legal test that defines the defence of insanity. First formulated by the House of Lords in 1843, this test has become the established standard in English criminal law and has been adopted in various forms in some US states and other jurisdictions, either through case law or legislation.
The original wording of the M'Naghten rules is expressed as a proposed jury instruction, stating that every individual is presumed to be sane. To establish a defence on the grounds of insanity, it must be clearly proven that, at the time of committing the act, the accused party was suffering from such a defect of reason, arising from a disease of the mind, that they did not know the nature and quality of the act they were doing. Alternatively, if they did know the nature and quality of the act, they must not have known that what they were doing was wrong.
The origin of these rules can be traced back to the case of R v McNaughten . M'Naghten was acquitted of the charge of murdering Edward Drummond, having shot Drummond under the mistaken belief that he was British Prime Minister Robert Peel, the intended target. In response to this acquittal, the House of Lords sought guidance on the defence of insanity from a panel of judges, led by Sir Nicolas Conyngham Tindal, Chief Justice of the Common Pleas. The principles elucidated by this panel are now known as the M'Naghten Rules.
M'Naghten himself would have been found guilty if these rules had been applied at his trial. The rules, as formulated in R v McNaughten  or variations thereof, serve as a standard test for assessing criminal liability in cases involving mentally disordered defendants. When the criteria laid out by the rules are met, the accused may be adjudged not guilty by reason of insanity or guilty but insane. The subsequent disposition may involve mandatory or discretionary (often indeterminate) treatment in a secure hospital facility, or other options at the discretion of the court, depending on the country and the nature of the offence charged.
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