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 Kentucky, the insanity defense is recognized and is similar to the M'Naghten rule, which is the traditional standard for insanity in many jurisdictions. Under Kentucky law, a defendant may be found not guilty by reason of insanity if, at the time of the offense, they were suffering from a mental illness so severe that they either did not understand the nature and quality of their act or were unable to distinguish right from wrong with respect to that act. This is codified in Kentucky Revised Statutes (KRS) § 504.020. If a defendant is found not guilty by reason of insanity, they are not automatically set free. Instead, Kentucky law requires that the individual undergo evaluation and may be committed to a psychiatric facility. 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 commitment process and the conditions for release are governed by KRS § 504.060 and related statutes. At the federal level, the insanity defense is also available under 18 U.S.C. § 17, which allows a defendant to assert that they were unable to appreciate the nature and quality or the wrongfulness of their acts due to a mental disease or defect at the time of the offense.