Insanity defense

The insanity defense, also known as the mental disorder defense, is a defense by excuse in criminal trials arguing that the defendant is not responsible for their actions due to an episodic or persistent psychiatric illness. Exemption from full criminal punishment on such grounds dates back to at least the Code of Hammurabi. Legal definitions of insanity or mental disorder are varied, and include the M'Naghten Rules, the Durham rule, the American Legal Institute definition, and other provisions, often relating to a lack of mens rea ("guilty mind"). In the criminal laws of Australia and Canada, statutory legislation enshrines the M'Naghten Rules, with the terms defence of mental disorder, defence of mental illness or not criminally responsible by reason of mental disorder employed.

In the United Kingdom, Ireland, and the United States, use of the defense is rare; however, since the Criminal Procedure (Insanity and Unfitness to Plead) Act 1991, insanity pleas have steadily increased in the UK. Mitigating factors, including things not eligible for the insanity defense such as intoxication (or, more frequently, diminished capacity), may lead to reduced charges or reduced sentences.