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mental disorder of such severity as to render its victim incapable of managing his affairs or of conforming to social standards. Today, the term insanity is used chiefly in criminal law, to denote mental aberrations or defects that may relieve a person from the legal consequences of his or her acts. The case of Daniel McNaughtan, who was found not guilty by reason of insanity after making an assassination attempt on British prime minister Robert Peel (1834), gave rise to the modern insanity defense used in many Western nations today. In the United States, the 1954 case of Durham v. the United States led to the establishment of new rules for testing defendants. Today, psychologists may perform tests to determine whether or not the defendant is mentally stable. Such tests try to ascertain whether or not a defendant can distinguish right from wrong, and whether or not he acted on an "irresistible impulse." John Hinckley's assassination attempt on Ronald Reagan (1981) became another landmark in the history of the insanity defense. The court's initial verdict of "not guilty by reason of insanity" generated public outcry and renewed interest in the verdict of "guilty but mentally ill," which is permissible in some states. This verdict allows defendants deemed mentally ill to be hospitalized but requires them to carry out a reasonable prison sentence as well. In 1983, the Supreme Court ruled it permissable to keep a mentally ill defendant hospitalized for a term longer than the maximum sentence for the crime with which the defendant was charged. Many have contended that the insanity defense is nothing more than a legal loophole, allowing serious criminals to escape imprisonment. In fact, the plea is rarely employed in the United States, and it is estimated that less than 1% of defendants have used it successfully. Recent years have seen the restrictions surrounding insanity defense considerably narrowed, with the sole criteria for a successful plea being the determination of whether or not the defendant knew he was breaking the law.


See R. Simon and D. Aaronson, The Insanity Defense (1988); R. Porter, A Social History of Madness: The World Through the Eyes of the Insane (1989).


Any mental disorder.
In forensic psychiatry, a mental disorder which prevents one from managing one's affairs, impairs one's ability to distinguish right from wrong, or renders one harmful to oneself or others.
Term previously used to indicate mental disorder; no longer used in medical contexts.


1. relatively permanent disorder of the mind; state or condition of being insane
2. Law a defect of reason as a result of mental illness, such that a defendant does not know what he or she is doing or that it is wrong
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