The Insanity Defense
Criminal defendants who use the insanity defense are said to be pleading "not guilty by reason of insanity" (NGRI). It should be noted, however, that in the criminal law "insane" is often very different definition in the courts than the medical definition of "mentally ill" (though there is often some overlap). In Australia and Canada, the insanity defense has been renamed the "mental disorder" defense because of the stigma that the term "insanity" places on the defendant.
The essence of the insanity defense is that the defendant was incapable of understanding the nature of his act, that his mind was so distorted from the social norm at the time of the crime that he was incapable of conceiving common notions of right and wrong, and could not then properly have been said to have mens rea (guilty mind). In cases of liability without fault and strict liability, however, the insanity defense cannot hold because mens rea is not required to establish guilt.
The insanity defense is derrived largely from the 1843 case of Daniel M'Naghten in England. M'Naghten believed that the Tory party was engaged in a conspiracy to persecute and kill him and, deciding to take matters into his own hands, attempted (and failed) to kill British Prime Minister Robert Peel, a Tory. It was decided, after a long trial, that M'Naghten was not guilty by reason of insanity. This subsequently led to the establishment of the M'Naghten Rules, which still deeply influence criminal procedure in the British Commonwealth and the United States.
According the M'Naghten Rules, the insanity defense can be made if "at the time of the commission of the acts constituting the offense, the defendant as a result of a severe mental disease or defect, was unable to appreciate the nature and quality of the wrongfulness of his acts." Intoxication cannot usually qualify as "a severe mental disease or defect," nor can drug use or any form of chemical self-impairment. However, fits of epilepsy, severe mental impairment as a result of diabetes, and any other natural mental impairment either permanent or transient or intermittent can be grounds for the insanity defense. What's most important in a successful insanity defense is that the accused is found to have been legally insane at the time of the offense. If found "not guilty by reason of insanity," the defendant is commonly sent to a mental hospital rather than a prison. In cases of intermittent insanity in the defendant, such as can be legally attributed to diabetes or epilepsy, the defendant may not be sent to a mental hospital.
Attempts were made in recent decades to provide more lenient guidelines for applying the insanity defense, in the Model Penal Code (MPC) and the case of Durham v. the United States, for example. However, when John Hinckley, the man who tried to assassinate President Ronald Reagan in 1981, was found not guilty by reason of insanity according to the more lenient standards then in vogue, public outcry influenced lawmakers to return to the more traditional, stricter guidelines of the M'Naghten Rules. Today, while standards for determining legal insanity differ widely from state to state, most jurisdictions favor stricter interpretations.