Also known as:M'Naghten rule · M'Naghten rules · McNaghten rule · M Naghten rule · M'Naghten test · insanity defense standard
Written by attorneys · grounded in primary & secondary sources — see below
A common-law test for criminal insanity that excuses liability when a mental disease causes a defect of reason such that the defendant either did not know the nature and quality of the act or did not know that the act was wrong. The test focuses exclusively on cognitive capacity at the time of the offense. Loss of behavioral control caused by mental illness does not satisfy the standard.
Sources & Authorities
How it applies
Common Examples
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Hallucinations and Retained Awareness
Monica Morgan, a museum curator with chronic psychosis, burned several paintings after drinking heavily. She told police she believed the paintings contained evil spirits and that destroying them fulfilled a moral duty. Monica also calmly acknowledged that colleagues and the public would view the destruction as seriously wrongful. Under the traditional M'Naghten rule the court rejected her insanity defense because she retained the capacity to recognize that society regarded her conduct as wrong.
Delusional Belief About Nature of Act
Miguel Mendoza, a ground technician with severe psychosis, removed critical fasteners from a passenger jet. He believed the jet was a decommissioned museum display that would never fly. Medical experts confirmed the delusion resulted from mental disease. Under the traditional M'Naghten rule the jury could acquit if it found that the disease prevented Miguel from understanding the true nature and quality of disabling an aircraft carrying passengers.
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Common Law
Casebooks
Hornbooks
Course Outlines
Study Supplements
Durham v. United States214 F.2d 862, 874–75 (D.C.Cir.1954)
Common questions
Frequently Asked
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Does an inability to control one's conduct satisfy the traditional M'Naghten test?+
No. The traditional M'Naghten rule examines only cognitive capacity. A defendant who understood the nature and quality of the act and knew it was wrongful remains criminally responsible even if mental illness made the conduct impossible to resist.
Supporting sources
Is a sincere personal belief that the conduct was morally right enough for an insanity acquittal under M'Naghten?+
No. The defendant must lack the capacity to recognize that society regards the conduct as wrong. A personal moral belief that the act was justified does not excuse liability when the defendant still understood society's contrary judgment.
Supporting sources
What must a defendant prove to obtain an acquittal under the traditional M'Naghten rule?+
The defendant must prove a disease of the mind caused a defect of reason that left the defendant unable either to understand the nature and quality of the act or to know that the act was wrongful as society defines wrongfulness.
Supporting sources
214 F.2d 862, 874–75 (D.C.Cir.1954)Criminal Law
…64-68, 109-47 (1933); Leland v. State of Oregon , 1952, 343 U.S. 790, 800, 72 S.Ct. 1002, 96 L.Ed. 1302. 'In five States the M'Naghten Rules have been in substance re-enacted by statute.' Royal Commission Report 409; see, e.g., Sec. 1120 of the New York Penal Law (McKinney Consol. Laws, c.40) which provides that a person is not…