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Insanity. n. mental illness of such a severe nature that a person cannot distinguish fantasy from reality, cannot conduct her/his affairs due to psychosis, or is subject to uncontrollable impulsive behavior. Insanity is distinguished from low intelligence or mental deficiency due to age or injury.

In states that allow the insanity defense, defendants must prove to the court that they didn't understand what they were doing; failed to know right from wrong; acted on an uncontrollable impulse; or some variety of these factors.

The four versions of the insanity defense are M'Naghten, irresistible impulse, substantial capacity, and Durham.

Overview. The insanity defense refers to a defense that a defendant can plead in a criminal trial. In an insanity defense, the defendant admits the action but asserts a lack of culpability based on mental illness. The insanity defense is classified as an excuse defense, rather than a justification defense.

According to an eight-state study, the insanity defense is used in less than 1% of all court cases and, when used, has only a 26% success rate. Of those cases that were successful, 90% of the defendants had been previously diagnosed with mental illness.

Defendants offer an insanity defense in less than 1% of all felony cases, and are successful only about one-quarter of the time.

However, the insanity defense is rarely used and hardly ever successful. This is generally because of the difficulty in proving legal insanity. Many criminal defendants suffer from mental illness and can produce evidence of this illness such as psychiatric or layperson testimony.

The Criminal Defense of Insanity In reality, however, various criminal studies have established that only about one percent of all felony cases in the United States involve use of the insanity defense. Moreover, even when the defense is asserted, it is successful in only about 30 cases every year.