The McNaughton rules refer to the legal test for a defence of insanity. The McNaughton rules act as a legal safeguard preventing severely mentally unwell people from being unjustly punished by the criminal justice system. The McNaughton rules result from the case of Daniel McNaughton, who was found not guilty by reason of insanity after he shot and killed a person who he incorrectly believed to be the Prime Minister.
A person relying on a defence of insanity must show that:
- they had a defect of reason
- caused by a disease of the mind
- meaning that that the person did not know the nature and quality of their acts, or that they did not know that what there were doing was legally wrong.
If these criteria are found to be met by the court, then the defendant would be found not guilty. Psychiatric evidence can be important in assisting the court to come to these decisions.
The McNaughton rules have attracted criticism, including that physical conditions can sometimes impact the functioning of the mind, that psychiatrists often disagree on diseases of the mind and there is no standard way of resolving this, and that persons with defects of reasoning may not be acting as rational moral agents even if they know an act is legally wrong (such as a person who believes they have been given a command to kill by God).
Dr Emma McPhail