Although definitions of legal insanity differ from state to state, generally a person is considered insane and is not responsible for criminal conduct if, at the time of the offense, as a result of a severe mental disease or defect, he was unable to appreciate the nature and quality or the wrongfulness of his acts.
This reasoning is, because willfull intent is an essential part of most offenses, a person who is insane is not capable of forming such intent. Mental disease or defect does not alone constitute a legal insanity defense. The defendant has the burden of proving the defense of insanity by clear and convincing evidence.
The history of the insanity defense in modern times comes from the 1843 case of Daniel M'Naghten, who tried to assassinate the prime minister of Britain and was found not guilty because he was insane at the time. The public outrage after his acquittal prompted the creation of a strict definition of legal insanity which is known as the M'Naghten Rule.
The M'Naghten Rule basically said a person was not legally insane unless he is "incapable of appreciating his surroundings" because of a powerful mental delusion.
The Durham StandardThe strict M'Naghten standard for the insanity defense was used until the 1950s and the Durham v. United States case. In the Durham case, the court ruled that an person was legally insane if he "would not have committed the criminal act but for the existence of a mental disease or defect."
The Durham standard was a much more lenient guideline for the insanity defense, but it addressed the issue of convicting mentally ill defendants, which was allowed under the M'Naghten Rule. However, the Durham standard drew much criticism because of its expansive definition of legal insanity.
The Model Penal Code, published by the American Law Institute, provided a standard for legal insanity that was a compromise between the strict M'Naghten Rule and the lenient Durham ruling. Under the MPC standard, a defendant is not responsible for criminal conduct "if at the time of such conduct as a result of mental disease or defect he lacks substantial capacity either to appreciate the criminality of his conduct or to conform his conduct to the requirements of the law."
The MPC StandardThis standard brought some flexibility to the insanity defense, by dropping the requirement that a defendant who knows the difference between right and wrong is therefore not legally insane, and by the 1970s all federal circuit courts and many states had adopted the MPC guideline.
The MPC standard was popular until 1981, when John Hinckley was found not guilty by reason of insanity under those guidelines for the attempted assassination of President Ronald Reagan. Again, public outrage at Hinckley's acquittal caused lawmakers to pass legislation that reverted back to the strict M'Naghten standard, and some states attempted to abolish the insanity defense altogether.
Today the standard for proving legal insanity varies widely from state to state, but most jurisdictions have returned to a more strict interpretation of the definition.