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 Pennsylvania, the insanity defense is recognized and allows a defendant to claim they were legally insane at the time of the crime, thus lacking the necessary mental state to be held culpable. Pennsylvania follows a version of the M'Naghten rule, which requires the defendant to prove that, due to a mental defect or disease, they either did not understand the nature and quality of their act, or were unable to know that what they were doing was wrong. This is an affirmative defense, meaning the burden of proof is on the defendant. If a defendant is successful with an insanity defense, they are not simply released. Instead, they may be committed to a mental health institution for treatment. The length of time for treatment or institutionalization can be equivalent to, or sometimes longer than, the prison sentence that would have been imposed if the defendant had been convicted of the crime. Federal law also provides for an insanity defense under 18 U.S.C. §17, which similarly allows a defendant to claim they were unable to appreciate the nature and quality or the wrongfulness of their acts due to a mental condition.