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 Tennessee, the insanity defense is recognized and is similar to the M'Naghten rule, which is the traditional standard for insanity in many jurisdictions. Under this rule, a defendant must demonstrate that at the time of the crime, they either did not understand the nature and quality of their actions or did not understand that what they were doing was wrong due to a severe mental disease or defect. This is considered an affirmative defense, meaning the defendant has the burden of proving the condition at trial. If a defendant is successful in asserting an insanity defense, they are not simply released. Instead, Tennessee law requires that the individual be committed to a mental health facility for treatment. The length of time for treatment or confinement 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. The specifics of the insanity defense in Tennessee, including the standards for proving such a defense and the consequences of a successful plea, are codified in the state's statutes.