An insanity defence rule in criminal law developed in England in the mid-1800s, available if, at the time of the offence, the accused was insane, had a disease of the mind such that he was unable to know that his act was wrong.
Also spelled “McNaghten" as in "McNaghten's Rules”.
In 1843, Daniel M’Nagthen tried to assassinate British Prime Minister Robert Peel. M'Naghten was acquitted by reason of insanity and the public outcry was so great, that a rule was developed to handle such future claims.
The British Trial Of Lunatics Act of 1883 provided that:
"Where in any indictment or information any act or omission is charged against any person as an offence, and it is given in evidence on the trial of such person for that offence that he was insane, so as not to be responsible, according to law, for his actions at the time when the act was done or omission made, then, if it appears to the jury before whom such person is tried that he did the act or made the omission charged, but was insane as aforesaid at the time when he did or made the same, the jury shall return a special verdict that the accused is not guilty by reason of insanity."
The rule was subsequently adopted by many other common law jurisdictions and was summarized in this 1990 decision of Canada's Supreme Court, Chaulk v the Queen (cited at 3 SCR 1303 and published at canlii.org/en/ca/scc/doc/1990/1990canlii34/1990canlii34.html), in reference to "the M'Naghten Rules on insanity at common law":
"The central proposition in the M'Naghten Rules was that the defence would be available to someone who, because of a defect of reason resulting from disease of the mind, did not know the nature and quality of the act he was doing; or if he did know it, he did not know he was doing what was wrong.
"The first alternative deals with the situation where insanity negatives mental elements in the definitions of offences.
"The second alternative establishes a special exculpatory defence which is based on lack of capacity for normative understanding.
"There is, in effect, an exception to the general rule that ignorance of the law is no excuse. The third way in which insanity could be relevant to criminal culpability is through a cognitive breakdown leading to a mistaken belief in a matter of contextual permission. This situation was covered in the M'Naghten Rules by a ruling that responsibility would be determined as if the facts were as they were believed to be.”