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Public perceptions are that the insanity defense occurs far more commonly than records indicate. In fact, the insanity defense is used in less than 1% of criminal proceedings and is successful in approximately one-quarter of those cases. Furthermore, defendants who are found insane spend as much, or more, time in state custody than their criminally convicted counterparts. The media may foster the notion that criminals get away with feigning mental defect, only to be released and recidivate. However, the insanity plea is actually based on a long-standing legal tradition and is rarely successfully completed. In fact, approximately 70% of insanity acquittals result from agreements between opposing attorneys, in which the prosecution agrees that society would be better served by placing the defendant in treatment, rather than in prison (Blum, 1992; Bogenberger et al., 1987; Cirincione, 1996; Rogers et al., 1984; Smith and Hall, 1982).
Outside of assisting in sentencing, there are two places for mental disease in the legal system. The first is a defendant's ability to understand the trial process. People who do not understand the nature of the charges or the functioning of the legal system are considered unfit for trial. More germane to this article, if mental illness had a direct effect on a given criminal act, an individual can be found legally insane.
Harvard Law School professor Sheldon Glueck stated in 1927, "Perhaps in no other field of American law is there so much disagreement as to fundamentals and so many contradictory decisions in the same jurisdictions." The concept of insanity has changed over time and locale, but the basic notion of this defense is that some individuals, by reason of mental condition, are not capable of controlling or understanding their own unlawful behavior. However, the presence of a mental illness or other condition does not automatically predict a successful insanity finding.
A great historical pendulum swings between protective and punitive laws. In the normal course of events, laws slowly evolve to allow more and more efforts to treat those mentally ill offenders who can be safely removed from the prison population. Eventually, a psychotic individual commits a highly publicized crime, usually an assassination, and the lawmakers throw up their hands over "doctors dictating the law" and "getting criminals off the hook."
This happened in England in 1841, when the Prime Minister's secretary was killed by a delusional wood-turner from Scotland named Daniel M'Naughten. In this landmark mental health case, the defendant was found not guilty by reason of insanity (NGRI), and the Queen demanded an appellate ruling, resulting in the very strict "M'Naughten Rules" in 1843. These essentially said a person could be found insane if, at the specific time of a crime, they were unable to know the difference between right and wrong. This is known as a "knowledge-only" based test, with no room for uncontrollable impulses.
In the United States, this definition was considered by the courts but was seen as too restrictive. Instead, the American judiciary took a slightly different tack. For example, in Parsons v Alabama (1886), the State Supreme Court opined that a person was NGRI if they had lost "free agency" as a result of mental disease. This would allow an insanity defense if a person knew an act was illegal, but could not control their behavior.
Later, the American Law Institute (ALI) promulgated a Model Penal Code that included a two-pronged provision for insanity. Specifically, an individual 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 [wrongfulness] of his conduct or to conform his conduct to the requirements of the law." It is important to note the or, allowing either knowledge or impulse to be a basis for insanity. By the 1970s, most states had adopted these rules.