The Model Penal Code insanity defense is the distinct formulation of the insanity defense found in the Model Penal Code and adopted by 21 states either completely or in a modified format.
The defense is found in MPC §4.01, which states:
- A person is not responsible for criminal conduct if at the time of such conduct as a result of mental disease or defect he lacks substantial capacity either to appreciate the criminality [wrongfulness] of his conduct or to conform his conduct to the requirements of law.
- As used in this Article, the terms "mental disease or defect" do not include an abnormality manifested only by repeated criminal or otherwise anti-social conduct.
The term “appreciate” seeks to retain the cognitive component of the M’Naghten Rule, an earlier form of the insanity defense still used in some states today, while discarding the rigidness of the earlier test’s standard of knowledge. §4.01(1) also allows states adopting the MPC language to choose between criminality, a legalistic concept, and wrongfulness, a more morally-infused concept.
Under §4.01(2), the use of the MPC insanity defense for mental diseases or defects that only manifest through repeated criminality or anti-social conduct is prohibited, which was intended to prevent the use of the defense by repeat criminals, psychopaths, and sociopaths.
[Last updated in July of 2023 by the Wex Definitions Team]