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 North Dakota, the insanity defense is recognized and allows a defendant to claim they were not responsible for their actions due to a mental disease or defect at the time of the offense. This defense is based on the principle that the defendant either did not understand the nature and quality of the act or did not know that the act was wrong, similar to the M'Naghten rule. If a defendant successfully proves insanity, they are not found guilty due to lack of culpability. However, this does not mean the defendant is released without any further action. North Dakota law may require the defendant to undergo treatment or be institutionalized. The specific period of confinement or treatment can be equivalent to, or sometimes longer than, the sentence that would have been imposed if the defendant had been convicted of the crime. The exact provisions and procedures for the insanity defense in North Dakota are detailed in the state's statutes, and federal law also provides for an insanity defense under 18 U.S.C. §17.