Insanity Defense Law and Legal Definition
The defense of insanity is asserted by an accused in a criminal prosecution to avoid liability for the commission of a crime. The essence of this defense is that at the time of the crime, the person did not appreciate the nature or quality or wrongfulness of the acts by reason of his/her insanity.The most common variation of the defense of insanity is cognitive insanity. Under the test for cognitive insanity, a defendant must have been so impaired by a mental disease or defect at the time of the act that s/he did not know the nature or quality of the act, or, if the defendant did know the nature or quality of the act, s/he did not know that the act was wrong. In the U.S., a vast majority of states allow criminal defendants to invoke the cognitive insanity defense.
Volitional insanity or irresistible impulse is another form of the insanity defense. A defense of irresistible impulse asserts that the defendant, although able to distinguish right from wrong at the time of the act, suffered from a mental disease or defect that made him or her incapable of controlling her or his actions. Although this defense is common in crimes of vengeance, very few states allow the volitional insanity defense.