High Quality Content by WIKIPEDIA articles! In criminal trials, the insanity defense is where the defendant claims they are not responsible for their actions due to mental health problems (psychiatric illness or mental handicap). Exemption of the insane from full criminal punishment dates back to at least the Code of Hammurabi. There are different definitions of legal insanity, such as the M`Naghten Rules, the Durham Rule, the American Legal Institute definition, and various miscellaneous provisions (e.g., relating to lack of mens rea).