Insanity defense or the plea of insanity is based on McNaughton’s rules in English law. In the states, four variants of the insanity defense exist, namely: McNaughton, irresistible impulse, substantial capacity, and Durham. Some states do not recognize the insanity defense. There are two requirements for this defense. Firstly, at the time of the crime, the defendant must have been suffering from a ‘mental defect’ or a“defect of reason” or a “disease of the mind,” depending on the jurisdiction. Secondly, due to the mental defect, he/she did not know either the nature and quality of the criminal act or that the act was wrong.