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 Wyoming, the insanity defense is recognized and allows a defendant to assert that they are not culpable for their criminal actions due to mental illness. Wyoming follows a version of the M'Naghten rule for the insanity defense, which requires the defendant to prove that, at the time of the crime, they either did not understand the nature and quality of their actions or did not understand that their actions were wrong due to a mental disease or defect. This is an affirmative defense, meaning the burden of proof is on the defendant. If a defendant is found not guilty by reason of insanity, they are not automatically set free. Wyoming law may require the defendant to be committed to a mental health treatment facility for a period that could be equivalent to or longer than the sentence they would have received if convicted. The specifics of how the insanity defense operates and the consequences of a verdict of not guilty by reason of insanity are detailed in Wyoming's statutes. Additionally, under federal law, specifically 18 U.S.C. §17, a similar standard applies for federal offenses, where a defendant must demonstrate an inability to appreciate the nature and quality or the wrongfulness of their acts due to a mental condition.