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 Maryland, the insanity defense is recognized and allows a defendant to assert that they are not criminally responsible for their actions due to a mental disorder or disability. Maryland follows a test similar to the M'Naghten Rule, which requires the defendant to prove that, at the time of the offense, they either did not understand the nature and quality of their act or did not understand that the act was wrong due to a mental disorder. This is codified in Maryland's Criminal Law Code § 3-109. If a defendant is successful in their insanity defense, they are not simply released; instead, they are typically committed to a mental health facility for treatment. The period of confinement can be equivalent to, or sometimes longer than, the sentence that would have been imposed had the defendant been convicted of the offense. The specific procedures and outcomes following a verdict of not guilty by reason of insanity are detailed in the Health-General Article of the Maryland Code, particularly §§ 12-111 and 12-112, which outline the commitment and release processes for individuals found not criminally responsible.