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 Indiana, the insanity defense is recognized and is based on the defendant's mental state at the time of the crime. Indiana follows a variation of the M'Naghten rule, which requires the defendant to prove that, due to a severe mental illness or defect, they were unable to understand the wrongfulness of their actions or conform their conduct to the requirements of the law. This is codified in Indiana Code § 35-41-3-6. When a defendant successfully asserts an insanity defense, they are not automatically released. Instead, they may be committed to a mental health institution for treatment. The period of commitment can be equivalent to or longer than the potential prison sentence had the defendant been convicted. The specifics of the commitment process and the conditions for release are governed by state law, and the burden of proving insanity typically rests on the defense. Additionally, under federal law, specifically 18 U.S.C. § 17, an affirmative defense is available if the defendant was unable to appreciate the nature and quality or the wrongfulness of their acts due to a mental condition at the time of the offense.