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 Colorado, the insanity defense is recognized under state law, allowing a defendant to claim they were not legally responsible for their actions due to a mental disease or defect. Colorado follows a standard similar to the M'Naghten rule, which requires the defendant to prove that, due to a mental condition, they were incapable of distinguishing right from wrong with respect to the act, or were unable to form the culpable mental state required for the offense. If a defendant is found not guilty by reason of insanity, they are not automatically released. Instead, they are typically committed to the Colorado Department of Human Services for treatment and confinement. The length of confinement can be equivalent to, or potentially longer than, the sentence that would have been imposed had the defendant been convicted of the offense. The specifics of the insanity defense in Colorado are detailed in the Colorado Revised Statutes, and the procedures for commitment and treatment following an insanity acquittal are also governed by state law.