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 Montana, the insanity defense is recognized under state law, allowing a defendant to assert that they were not culpable for their actions due to mental illness or an inability to understand the wrongfulness of their actions. This defense is similar to the M'Naghten rule, which requires the defendant to demonstrate that they either did not understand the nature and quality of the act or did not know that the act was wrong. If a defendant in Montana is successful in pleading insanity, they are not automatically set free. Instead, the state has provisions for treatment or institutionalization. Montana law may require that the defendant be confined in a treatment institution for a period that could be equivalent to or longer than the sentence they would have received if convicted. The specifics of the insanity defense, including the criteria for asserting it and the consequences of a successful defense, are detailed in Montana's statutes. Additionally, under federal law, specifically 18 U.S.C. § 17, an affirmative defense is available for federal offenses if the defendant was unable to appreciate the nature and quality or the wrongfulness of their acts at the time of the offense.