Model Penal Code (MPC) Insanity Test

The Model Penal Code (MPC) insanity test, found in Section 4.01, provides a broader standard for legal insanity than the M'Naghten rule. It was drafted by the American Law Institute and adopted by a number of federal circuits and state jurisdictions before the public backlash following John Hinckley's acquittal in 1982 led many jurisdictions to retreat to narrower tests.

Under the MPC test, a person is not responsible for criminal conduct if, at the time of the act, as a result of mental disease or defect, the person lacked substantial capacity either (1) to appreciate the criminality (wrongfulness) of their conduct, or (2) to conform their conduct to the requirements of law. The test explicitly excludes antisocial personality disorder, noting that the term "mental disease or defect" does not include abnormality manifested only by repeated criminal or otherwise antisocial conduct.

The MPC test improves on M'Naghten in several important ways. First, it uses "appreciate" rather than "know," recognizing that emotional understanding and cognitive knowledge are different. A person may intellectually "know" their act is wrong but lack the deeper emotional appreciation of its wrongfulness. Second, it includes a volitional prong — the inability to conform conduct to the law — which M'Naghten omits entirely. This covers individuals who understand the wrongfulness of their act but are unable to control their behavior due to mental illness.

Third, the MPC test uses "substantial capacity" rather than requiring total cognitive or volitional incapacity. A person who retains some awareness or some control but lacks substantial capacity is still excused. This is more realistic than M'Naghten's all-or-nothing approach.

The burden of proof varies by jurisdiction. Some require the defendant to prove insanity by a preponderance of the evidence. Others require the prosecution to disprove insanity beyond a reasonable doubt once the defendant raises the issue.

On exams, the MPC test should be compared with M'Naghten. Students should note the key differences: "appreciate" vs. "know," the volitional prong, and the "substantial capacity" threshold.