The M'Naghten rules, also known as McNaughten or Macnaughton rules are a set of guidelines for an insanity defense formulated by the Judges of the House of Lords in 1843, used in England and Wales until the 1960s:

• Persons acting under the influence of an insane delusion are punishable if they knew at the time of committing the crime that they were acting contrary to law.
• Every man is presumed sane and to have sufficient reason to be held responsible for his crimes.
• A person under a partial delusion is to be considered as if the facts with respect to which the delusion exists were real.
• To establish a defense on the ground of insanity each element of it must be clearly proved that:

(a) at the time of committing the act,

(b) the accused was labouring under such a defect of reason, from disease of the mind, AND (c) as not to know the nature and quality of the act he was doing

OR

(d) if he did know it, AND (e) that he did not know he was doing what was wrong.

UNLESS:

IF (1) the accused was conscious that the act was one that he ought not to do AND (2) if the act was at the same time contrary to the law of the land,

THEN the accused is punishable.

As they were a precedent of England and Wales' court of last resort the rules have been treated in England and Wales as if of statutory force, and were followed in many Commonwealth countries and parts of the US. They were criticized for excusing from criminal responsibility only those whose insanity resulted in lack of knowledge. The fact that an individual might know what they were doing and that it was wrong and yet because of their abnormal mental state might lack the capacity to control the action was not considered. It was argued the lack of capacity must be the key issue over responsibility. The English law was amended in 1957 to include irresistible impulse.

The rules developed out of the trial Rex v. M'Naghten. Daniel M'Naghten was a Scottish woodworker who killed the Prime Minister's secretary Edward Drummond by mistake for Prime Minister Robert Peel, under the delusional belief that he was the victim of a conspiracy and that he was being followed by Catholic spies with the aid of the government. At his trial he stated

"The Tories in my native city have compelled me to do this... They have accused me of crimes of which I am not guilty; they do everything in their power to harass and persecute me; in fact, they wish to murder me."

He was tried and acquitted on the ground of insanity and committed to Bethlem Hospital and then Broadmoor. Nine experts all found him insane.