Proving that you are not guilty of a crime by reason of insanity is extremely difficult. Only about one percent of criminal defendants even raise the defense, and only a quarter of those are successful. While the legal definitions of insanity vary from state to state, the essential element of insanity defenses is that the defendants lacked the required “criminal intent” to make them legally responsible for their actions.
The most widely used legal definition of insanity is known as the M’Naghten rule, named after a famous English murder case from the 1800s. In a nutshell, the M’Naghten rule requires a defendant to prove either that he did not know what he was doing, or if he did, that he didn’t know what he was doing was wrong. Boiled down even further, a defendant is insane under M’Naghten if he didn’t know right from wrong. Another standard courts will use is called the irresistible impulse test. Under this standard, defendants must prove that they knew their acts were wrong, but couldn’t control themselves.
About one-third of the states use the American Law Institute’s test for insanity, which states that a people aren’t responsible for criminal conduct if, as a result of mental disease or defect, they lack substantial capacity to appreciate the criminality of their conduct or to conform their conduct to the requirements of the law. This is a slightly more lenient standard than M’Naghten, as defendants must only show a “substantial incapacity” that they didn’t know that what they was doing was wrong, rather than having an absolute inability to know the difference.
There is good reason for having the insanity defense available, even if it is rarely used. Since putting people in prison for crime serves in part to deter future crime, it doesn’t make sense to put people in jail who had no control over what they were doing in the first place, or did not intend to do what they did. Most would also argue that treatment of the criminally insane in a mental institution is a more appropriate way to keep the public safe than prison. Rarely do those who are found not guilty by reason of insanity go free, and often their commitment to a treatment facility can last much longer than a prison sentence would have.
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