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 Washington State, the insanity defense is recognized and allows a defendant to claim they were unable to understand the nature of their criminal act or that it was wrong due to mental illness. This defense is similar to the M'Naghten rule, which is the traditional standard for insanity defenses, requiring the defendant to demonstrate a lack of understanding of the act or its wrongfulness at the time of the offense. Under Washington law, if a defendant is found not guilty by reason of insanity, they are not automatically set free. Instead, they may be committed to a mental health institution 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 specifics of the insanity defense, including the process for asserting it and the consequences of a successful defense, are detailed in Washington's state statutes.