The notion that an individual who suffers from mental disorder may not bear the full weight of responsibility for their actions dates back to ancient times. Although various approaches to a legal test for criminal responsibility date back centuries in the Anglo-American tradition, some major historical trials during the 19th century in Britain culminated in the enunciation of the so-called McNaughten Rules (1843) in which the Law Lords of Great Britain indicated that for an individual to be found Not Responsible for their act or omission, their mental disorder must render them unable to know the nature and quality of the act or that it was wrong.
In the years following, courts on both sides of the Atlantic struggled with the meaning of the individual words of this rule. At the present time in Canada the Criminal Code dictates a test, derived from McNaughten, stating that for an individual to found Not Criminally Responsible, they must have been unable, by virtue of mental disorder, to appreciate the nature and quality of the act or know that it is wrong. Within Canadian jurisprudence, these individual words have been subject to judicial interpretation.