Most states have long recognized a form of the insanity defense, based on the defendant’s mental illness, defect, or inability to understand that the criminal act was wrong. In pleading an insanity defense, the defendant admits the criminal conduct, but asserts a lack of culpability based on mental illness. Many states still model their insanity defense on the old English rule of law (the M’Naghten rule from 1843) in which the defendant asserts he (1) did not know the nature and quality of the act, or (2) did not know that it was wrong.

And it is an affirmative defense to a prosecution under any federal statute (federal law) that, at the time of the offense, the defendant was unable to appreciate the nature and quality of his acts, or the wrongfulness of his acts. See 18 U.S.C. §17.

When a defendant is found not guilty by reason of insanity it does not mean he necessarily goes free. States often have requirements for treatment or institutionalization after such a finding. And some states require, at a minimum, confinement in a treatment institution or facility for the length of time the person would have received if convicted—so a defendant may end up spending more time confined than if he did not raise such a defense.

The law regarding the availability, definitions, and nature of the insanity defense vary from state to state, and are usually located in a state’s statutes.

In New Jersey, the insanity defense is recognized and allows a defendant to assert that they are not culpable for their criminal actions due to mental illness. This defense is based on the principle that the defendant, at the time of the crime, either did not understand the nature and quality of their act or did not know that what they were doing was wrong, aligning with the M'Naghten rule. If a defendant successfully pleads insanity, they are not simply set free; New Jersey law may require the individual to undergo treatment or be institutionalized. The specifics of how long and under what conditions a person may be confined are determined by the court, and in some cases, the period of confinement could be as long as or longer than the sentence that would have been imposed if the defendant had been convicted. The federal statute 18 U.S.C. §17 also provides for an insanity defense, stating that it is an affirmative defense if the defendant was unable to appreciate the nature and quality or the wrongfulness of their acts at the time of the offense. The application of the insanity defense and the subsequent treatment or confinement of the defendant are governed by New Jersey state statutes.