A defense asserted by an accused in a criminal prosecution to avoid liability for the commission of a crime because, at the time of the crime, the person did not appreciate the nature or quality or wrongfulness of the acts.

The insanity defense is used by criminal defendants. The most common variation 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 he or she did not know the nature or quality of the act, or, if the defendant did know the nature or quality of the act, he or she did not know that the act was wrong. The vast majority of states allow criminal defendants to invoke the cognitive insanity defense.

Another form of the insanity defense is volitional insanity, or IRRESISTIBLE IMPULSE. A defense of irresistible impulse asserts...