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 Hampshire, the insanity defense is recognized and allows a defendant to assert that they are not culpable for their criminal conduct due to mental illness. New Hampshire follows a variation of the M'Naghten rule, which requires the defendant to prove that at the time of the crime, they either did not understand the nature and quality of their actions or did not understand that their actions were wrong due to a severe mental disease or defect. This is codified in RSA 628:2 of the New Hampshire statutes. If a defendant is successful in their insanity defense and found not guilty by reason of insanity, they are not automatically set free. Instead, they may be committed to a psychiatric institution for treatment. The period of confinement can be equivalent to, or sometimes longer than, the prison sentence that would have been imposed if the defendant had been convicted of the offense. The specifics of the commitment process and the length of confinement are determined by the court and are subject to periodic review.