Insanity Defense (Model Penal Code)

The Model Penal Code (MPC) test for insanity, set forth in Section 4.01, represents a broader and more modern approach to the insanity defense than M'Naghten. It was developed by the American Law Institute in the 1950s and 1960s as part of a comprehensive criminal law reform effort. The test has both a cognitive prong and a volitional prong, addressing two distinct ways mental illness can undermine criminal responsibility.

Under the MPC test, a person is not responsible for criminal conduct if, at the time of such conduct, as a result of mental disease or defect, they lacked substantial capacity either to appreciate the criminality (wrongfulness) of their conduct or to conform their conduct to the requirements of law. The word "appreciate" is intentionally broader than M'Naghten's "know," encompassing not just intellectual awareness but emotional and deeper understanding. The phrase "substantial capacity" is also more forgiving than the total incapacity implied by M'Naghten, acknowledging that mental illness often diminishes rather than completely destroys cognitive or volitional capacity.

The MPC test explicitly excludes antisocial personality disorder and psychopathy, providing that the term "mental disease or defect" does not include an abnormality manifested only by repeated criminal or antisocial conduct. After John Hinckley's acquittal for the attempted assassination of President Reagan under a version of the MPC test, Congress and many states retreated from the MPC approach. The federal Insanity Defense Reform Act of 1984 adopted a narrower test closer to M'Naghten, and several states eliminated the volitional prong entirely. Nevertheless, the MPC test remains influential and is still used in a number of jurisdictions.