---
title: "Insanity Defense (Model Penal Code)"
type: Legal Rule
source: https://casebriefly.com/legal-rules/insanity-defense-model-penal-code
---

# Insanity Defense (Model Penal Code)

The MPC insanity test provides that a defendant is not responsible if, due to mental disease or defect, they lacked substantial capacity to appreciate the criminality of their conduct or to conform their conduct to the law.

## Definition

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.

## Elements

- The defendant suffered from a mental disease or defect at the time of the act
- The mental disease or defect caused a lack of substantial capacity
- Cognitive prong: inability to appreciate the criminality (wrongfulness) of their conduct
- Volitional prong: inability to conform their conduct to the requirements of law
- The test excludes abnormalities manifested only by repeated criminal or antisocial conduct

## Key Case

United States v. Brawner, 471 F.2d 969 (D.C. Cir. 1972)

## Landmark Cases

| Name | Citation | Significance |
| --- | --- | --- |
| United States v. Brawner | 471 F.2d 969 (D.C. Cir. 1972) | Adopted the MPC insanity test for the D.C. Circuit, replacing the broader Durham product test |
| United States v. Hinckley | No. 81-306 (D.D.C. 1982) | Hinckley's acquittal under the MPC-influenced test for the attempted assassination of President Reagan led to federal and state reforms narrowing the insanity defense |
| State v. Johnson | 121 R.I. 254 (1979) | Adopted the MPC insanity test, illustrating the broader trend of state adoption before the Hinckley backlash |
| Durham v. United States | 214 F.2d 862 (D.C. Cir. 1954) | Established the now-abandoned product test for insanity, which the MPC test was designed to replace with a more structured approach |

## Exam Tips

- Compare the MPC test to M'Naghten by noting two key differences: (1) 'appreciate' is broader than 'know,' and (2) the MPC includes a volitional prong M'Naghten lacks
- When analyzing the volitional prong, focus on whether the defendant could control their behavior despite knowing it was wrong
- Note the 'substantial capacity' language; the MPC does not require total incapacity, just a significant diminishment
- Be aware of the post-Hinckley reforms; many jurisdictions that previously used the MPC test have since narrowed or abandoned it

## Common Mistakes

- Failing to analyze both prongs separately; the defendant can qualify under either the cognitive prong or the volitional prong
- Assuming the MPC test is used everywhere; after the Hinckley verdict, many jurisdictions reverted to M'Naghten or adopted narrower tests
- Forgetting the psychopathy exclusion; antisocial personality disorder alone does not qualify as a mental disease or defect under the MPC

## Mnemonic Or Memory Aid

"Appreciate and Conform" -- the two prongs of the MPC test: could the defendant appreciate the wrongfulness and conform their behavior?

## Related Rules

- insanity-defense-mnaghten
- intoxication-defense
- mens-rea-mental-states
- duress-defense

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Source: [Insanity Defense (Model Penal Code) — CaseBriefly](https://casebriefly.com/legal-rules/insanity-defense-model-penal-code)