Insanity Defense

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 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. This defense is common in crimes of vengeance. For example, suppose that a child has been brutally assaulted. If an otherwise conscientious and law-abiding mother shoots the perpetrator, the mother may argue that she was so enraged that she became mentally ill and incapable of exerting self-control. Very few states allow the volitional insanity defense.

The insanity defense should not be confused with INCOMPETENCY. Persons who are incompetent to stand trial are held in a mental institution until they are considered capable of participating in the proceedings.

The insanity defense also should be kept separate from issues concerning mental retardation. The U.S. Supreme Court ruled in 2002 in ATKINS V. VIRGINIA, 536 U.S. 304, 122 S. Ct. 2242, 153 L. Ed. 2d 335 (2002) that the execution of mentally retarded criminals constituted "cruel and unusual punishment" and that it was prohibited by EIGHTH AMENDMENT. But if a person is acquitted by reason of insanity, execution is not an option.

The insanity defense reflects the generally accepted notion that persons who cannot appreciate the consequences of their actions should not be punished for criminal acts. Most states regulate the defense with statutes, but a few states allow the courts to craft the rules for its proper use. Generally, the defense is available to a criminal defendant if the judge instructs the jury that it may consider whether the defendant was insane when the crime was committed. The judge may issue this instruction if the defendant has produced sufficient evidence at trial to justify the theory. Sufficient evidence invariably includes EXPERT TESTIMONY by psychologists and psychiatrists.

When invoking insanity as a defense, a defendant is required to notify the prosecution. In some states, sanity is determined by the judge or jury in a separate proceeding following the determination of guilt or innocence at trial. In other states, the defense is either accepted or rejected in the verdict of the judge or jury. Even if evidence of insanity does not win a verdict of not guilty, the sentencing court may consider it as a mitigating factor.