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 York, the insanity defense is codified under Article 40 of the New York Penal Law. The state follows a version of the M'Naghten rule, which allows a defendant to be found not guilty by reason of mental disease or defect if, at the time of the offense, they lacked substantial capacity to know or appreciate either the nature and consequences of their conduct or that such conduct was wrong. This is considered an affirmative defense, and the burden is on the defense to prove the defendant's mental state by a preponderance of the evidence. If a defendant is found not guilty by reason of insanity, New York law requires that the defendant be evaluated to determine the appropriate treatment or institutionalization. The defendant may be committed to a psychiatric facility, and the duration of confinement can be equivalent to or longer than the sentence that would have been imposed if the defendant had been convicted of the crime. Federal law also provides for an insanity defense under 18 U.S.C. § 17, which similarly allows a defendant to assert they were unable to appreciate the nature and quality or the wrongfulness of their acts due to a mental condition at the time of the offense.