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 Illinois, the insanity defense is recognized and allows a defendant to claim they are not culpable for their actions due to mental illness, defect, or inability to understand the wrongfulness of their act. 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 is successful with an insanity defense in Illinois, they are not simply released. Instead, they may be required to undergo treatment or be institutionalized. Illinois law may also mandate confinement in a treatment institution for a duration equivalent to the prison sentence the defendant would have received if convicted. The specifics of the insanity defense, including how it is applied and what constitutes legal insanity, are detailed in Illinois state statutes.