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The Many Legal Definitions of "Insanity"

If you think the law is composed of a set of hard and fast rules, you're insane. Well, maybe you're not insane, it kinda depends on whose rules you're looking at. The legal definition of "insanity" has been tweaked, modified and twisted by judges and lawyers for hundreds of years. After all that analysis, there's little agreement on what "insanity" really is, giving rise to many variations in legal standards from one state to the next.

In general, there are three major variations of the insanity defense, used separately or in a variety of combinations in different jurisdictions:

"But I shot him to kill the evil spirits within!": The M'Naghten Rule

One test for insanity, that has been around for centuries, is called named after the case in which it was first conceived. The M'Naghten Rule provides as follows:

Every man is to be presumed to be sane, and ... that to establish a defense on the ground of insanity, it must be clearly proved that, at the time of the committing of the act, the party accused was laboring under such a defect of reason, from disease of mind, and not to know the nature and quality of the act he was doing; or if he did know it, that he did not know he was doing what was wrong.

Under this standard, an insanity defense will only work if the defendant didn't appreciate the criminality of his conduct. Regardless of whether the defendant could control his impulses, if he knew that his actions were wrong, he would be guilty in a state applying the M'Naughten Rule.

"I know it was wrong to shoot him, but I couldn't stop myself!": The "Irresistible Impulse" Test

It's one thing to know right from wrong, but that doesn't mean you can control yourself.

In response to criticism over the M'Naughten Rule, many states started switching to a different definition of "insanity" -- the Irresistible Impulse Test. Under this standard, even if the defendant could tell right from wrong, if his mental state was such that he "lost the power to choose between right and wrong" and "his free agency was at the time destroyed," he cannot be held legally responsible for his uncontrollable actions.

"Severe Mental Illness": The Federal Test

After John Hinkley was acquitted in the attempted assassination of President Ronald Reagan, Congress took swift action to limit the circumstances under which a defendant could escape conviction by reason of insanity. In federal prosecutions, and in some states that have taken the same approach, jurors are instructed as follows:

It is an affirmative defense to a prosecution under any federal statute that, at the time of the commission of the acts constituting the offense, the defendant as a result of a severe mental disease or defect, was unable to appreciate the nature and quality or the wrongfulness of his acts. Mental disease or defect does not otherwise constitute a defense.

This more narrow definition of insanity greatly reduces the instances in which a defendant may raise her mental capacity as a defense.

Does Insanity Buy Freedom?

Under any of these tests, an accused who is acquitted by reason of insanity isn't likely to taste freedom for quite some time. Unless a judge is convinced that a passing psychosis caused the crime, and that the defendant is "all better now," the accused will probably spend many years or even a lifetime confined to a state-run psychiatric hospital. Unlike a convict who may become eligible for parole, the ironic consequence of a successful insanity plea is that the "sentence" to a psychiatric ward is indefinite and is largely dependent on whether a team of mental health professionals believe that the individual no longer poses a threat to society.

Despite all of the publicity surrounding the insanity defense, it is seldom used and rarely successful. Unless the accused can show a history of extreme mental illness, and a clear causal connection between the illness and the crime, most defendants have to be crazy to claim insanity.