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 Arizona, the insanity defense is recognized and is based on the defendant's mental state at the time of the offense. Arizona follows a version of the M'Naghten rule, which requires the defendant to prove that they were suffering from a mental disease or defect at the time of the offense and that they did not know the criminal act was wrong due to this condition. This is codified in Arizona Revised Statutes § 13-502. The insanity defense is considered an affirmative defense, meaning the defendant has the burden of proving insanity by clear and convincing evidence. If a defendant is found not guilty by reason of insanity, Arizona law requires that the individual undergo a mental health evaluation and may be committed to a state hospital or other facility for treatment. The length 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, where a defendant must demonstrate an inability to appreciate the nature and quality or the wrongfulness of their acts due to a mental disease or defect.