A defense of not guilty by reason of insanity is rare in the United States. It is usually based upon evaluations by professionals that the defendant is unable to differentiate between right and wrong. Insanity is a legal, not psychiatric concept of mental illness. At trial, psychiatrists and psychologists may be called upon to submit opinions regarding the defendant’s state of mind at the time the crime took place.
Although mental illness is a prerequisite to a ruling of being found not guilty by reason of insanity, this ruling must meet certain conditions. It must be determined that because of a mental disorder: (i) the defendant must not understand that what he or she did was illegal; (ii) the defendant did not know what he or she was doing; or (iii) the defendant was compelled to commit the crime by an irresistible force.
Generally, a person is considered guilty of a crime if they actually intended to commit that crime. Since mental illness can skew a person’s concept of reality, he or she may not realize the criminal nature of their actions, or they may feel they had no choice but to commit the crime.
Society believes that crimes should be punished. Society also recognizes that people who are mentally ill should receive treatment for their illness. The insanity defense compromises the two in that society believes that people who are mentally incapable of controlling their conduct should not be punished.
The McNaughton rule, derived from the 1843 McNaughton case became the first legal test for insanity. The rule provided for a presumption of sanity unless the defense could prove that, “at the time of committing the act, the accused was laboring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing, or, if he did know it, that he did not know what he was doing was wrong.” Almost half of the United States still recognizes this standard for insanity.
Another standard used to determine sanity was the Durham rule. This rule, based upon a case entitled Durham v. United States, 214 F.2d 862, held that, “an accused is not criminally responsible if his unlawful act was the product of mental disease.” The Durham Rule was rejected in 1972, and in its place the American Law Institute put forth the following: “A person 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 to appreciate the wrongfulness of his conduct or to conform his conduct to the requirements of the law” (MODEL PENAL CODE § 4.01[1]). Approximately half of the states have accepted this Model Penal Code rule for insanity.
Since a guilty by reason of insanity defense is so difficult to obtain, the plea is only used in approximately one percent of all criminal cases. A person may be determined to be insane either at the time the crime was committed, at the time of trial or both. A verdict of guilty by reason of insanity will ordinarily result in the defendant being committed to a mental institution.
Sources:
http://www.howstuffworks.com/question509.htm
http://www.law.cornell.edu/background/insane/insanity.html
http://law.jrank.org/pages/6321/Durham-Rule.html
http://en.wikipedia.org/wiki/Insanity_defense