MPC § 4.01 Substantial-Capacity Test
Definition
The American Law Institute's 1962 insanity formulation: a person lacks criminal responsibility if, as a result of mental disease or defect, they lack substantial capacity either to appreciate the criminality of their conduct or to conform their conduct to the requirements of law.
- Source
- American Law Institute, Model Penal Code
- Year
- 1962
- Section
- MPC section 4.01
- Two limbs
- Cognitive (appreciate) and volitional (conform)
Common questions
How does this test differ from the older M'Naghten rule?+
M'Naghten asks only whether the defendant understood the nature and wrongness of the act, a purely cognitive test, while the MPC test adds a second, volitional limb asking whether the defendant could conform their conduct to the law even if they understood it was wrong.
Why did the volitional limb become controversial?+
Critics argued the volitional prong, essentially an 'irresistible impulse' concept, was too easy to claim and too hard for experts to assess reliably, a concern that gained force after the 1982 Hinckley verdict and led several US jurisdictions to drop the volitional limb from their insanity standard.
Is the MPC test still the majority US standard today?+
It remains the most widely adopted state-level formulation in the United States, though its precise wording and whether both limbs are retained vary by jurisdiction, and it operates alongside stricter cognitive-only tests in some states.
Related terms
- Appreciation vs Knowledge
- The MPC and Canadian Criminal Code use 'appreciate' rather than 'know,' requiring a deeper, emotionally integrated understanding of the nature or wrongfulness...
- BNS § 22
- India's insanity defence provision in the Bharatiya Nyaya Sanhita 2023, replacing IPC § 84: an act is not an offence if done...
- Durham Rule
- The 1954 US Court of Appeals (DC Circuit) product test: an accused is not criminally responsible if the unlawful act was the...
- Guilty but Mentally Ill (GBMI)
- A US verdict alternative introduced in some states from 1975: the defendant is found guilty (and sentenced) but is entitled to psychiatric...
- Hari Singh Gond (2008)
- Indian Supreme Court ruling setting out the four elements for establishing the IPC § 84 (now BNS § 22) defence: mental illness...
- IDRA 1984
- Insanity Defense Reform Act 1984: US federal legislation passed after John Hinckley's acquittal that eliminated the volitional prong, raised the standard to...
- M'Naghten Rules
- The 1843 English House of Lords formulation providing that a defendant is not criminally responsible if, at the time of the offence,...