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 California, the insanity defense is codified under California Penal Code Sections 25 and 1026. The state follows a version of the M'Naghten rule, which allows a defendant to be found not guilty by reason of insanity if, at the time of the offense, they were incapable of understanding the nature and quality of their act or unable to distinguish right from wrong due to a mental disease or defect. When a defendant successfully asserts this defense and is found not guilty by reason of insanity, they are not released but are committed to a state mental hospital for treatment. The length of commitment is determined by the court and can be up to the maximum term that could have been imposed if the defendant had been convicted of the offense. The defendant's release from the hospital is contingent upon a finding that they have regained sanity and no longer pose a danger to the health and safety of others. Federal law also provides for an insanity defense under 18 U.S.C. § 17, which similarly allows a defendant to be found not guilty by reason of insanity if they were unable to appreciate the nature and quality or the wrongfulness of their acts at the time of the offense.