M'Naghten Insanity Defense vs. Model Penal Code Insanity Defense
A detailed comparison of these two criminal law rules, including key differences, exam strategies, and guidance on when to apply each.
Overview
The M'Naghten test and the Model Penal Code (MPC) test are the two primary standards for the insanity defense in American criminal law. Both ask whether the defendant's mental illness should excuse criminal liability, but they define the threshold differently and have different scopes.
The M'Naghten test, originating from the 1843 English case M'Naghten's Case, is a purely cognitive test. It asks whether, at the time of the act, the defendant was laboring under such a defect of reason from disease of the mind that they either did not know the nature and quality of the act, or did not know that what they were doing was wrong. The test focuses exclusively on the defendant's cognitive understanding and does not consider whether the defendant could control their behavior. Some jurisdictions supplement M'Naghten with the "irresistible impulse" test, which adds a volitional component.
The MPC test (Section 4.01) is broader and more nuanced. It provides that a defendant is not responsible for criminal conduct if, 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 MPC test differs from M'Naghten in two important ways: it uses "appreciate" rather than "know" (allowing for emotional as well as intellectual understanding), and it includes a volitional prong (inability to conform conduct) in addition to the cognitive prong. The MPC also uses "substantial capacity" rather than total incapacity, making it more accessible to defendants.
Key Differences
| Aspect | M'Naghten Insanity Defense | Model Penal Code Insanity Defense |
|---|---|---|
| Cognitive standard | Did not 'know' right from wrong (intellectual knowledge only) | 'Appreciate' wrongfulness (includes emotional understanding) |
| Volitional component | None (purely cognitive test); some jurisdictions add irresistible impulse | Yes; inability to 'conform conduct to the requirements of law' |
| Degree of impairment | Total cognitive incapacity required | Lacks 'substantial capacity' (less than total incapacity) |
| Historical origin | M'Naghten's Case (1843, England) | Model Penal Code Section 4.01 (1962, ALI) |
| Modern adoption | Used by a majority of states and the federal system (post-Hinckley reforms) | Used by some states; federal system moved away after Hinckley acquittal |
Exam Tips
On a criminal law exam, always identify which insanity test applies. If the jurisdiction uses M'Naghten, focus on whether the defendant knew the nature of the act and knew it was wrong. If the MPC test applies, argue both prongs: did the defendant appreciate the wrongfulness, and could they conform their conduct? The MPC test is more favorable to defendants because of the volitional prong and the "substantial capacity" threshold. A common exam pattern involves a defendant who knows their act is wrong but cannot control themselves. Under M'Naghten (without irresistible impulse), this defendant is sane. Under the MPC, they may be insane due to the volitional prong. Discuss both standards if the question does not specify.
When to Apply Which
Apply M'Naghten when the jurisdiction follows the traditional cognitive test or when the exam specifies it. Apply the MPC test when the jurisdiction has adopted the substantial capacity standard. In practice, many states use M'Naghten or a variation of it, while the MPC test represents the more progressive approach. If the exam does not specify, discuss both and note how the outcome might differ. Pay special attention to the volitional prong, as this is the most significant doctrinal difference between the two tests.