Also known as: diseases of the mind · M'Naghten disease · insanity disease
Written by attorneys · grounded in primary & secondary sources — see below
A mental disorder or intellectual disability that produces a defect of reason depriving the defendant of the capacity to know the nature and quality of an act or that the act is wrongful.
Daniel Diaz, who suffered from longstanding untreated psychosis, entered a cabbage field and shot Mateo while believing Mateo was an animated scarecrow threatening crops. Daniel told officers he thought he was dismantling farm equipment rather than harming a person. Because the psychosis produced a defect of reason that prevented Daniel from grasping the true nature of his conduct, the traditional test entitled him to acquittal.
Danielle Dixon, diagnosed with untreated schizophrenia, slashed several museum paintings while believing the canvases were living entities imprisoning visitors' souls. She told officers she was performing a rescue rather than destroying property. The disease of the mind caused a reasoning defect that left her unable to understand the nature and quality of her actions, satisfying the test for acquittal.
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David Dawson, who had a history of severe psychosis, removed fasteners from a passenger jet while convinced it was a static museum display that would never fly. He calmly explained to crew that he was preserving a decommissioned exhibit. The resulting defect of reason meant he lacked capacity to understand the nature of disabling an operational aircraft, triggering acquittal under the test.
Dorothy Daniels, suffering from severe delusional disorder, shot a customer she believed was a surveillance robot sent to harvest thoughts. She called authorities requesting retrieval of the machine rather than treatment of a human victim. The disease produced a defect of reason that prevented her from recognizing she was shooting a person, meeting the requirements for acquittal.
Devon Drake, who had documented PTSD with delusions, cut fences and released livestock onto a highway while believing the animals were government surveillance drones. He acknowledged knowing the release was legally wrong yet acted on the fixed false belief. The disease of the mind created a reasoning defect that negated capacity to understand the nature of the conduct, supporting acquittal.
Low intelligence can qualify if it produces the required defect of reason that leaves the defendant unable to know the nature and quality of the act or that the act is wrongful. Courts have accepted congenital intellectual deficiency when it satisfies the cognitive incapacity element.
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No. The defendant must also have lost the capacity to recognize that society regards the conduct as wrong. A personal moral conviction does not substitute for the required lack of capacity to appreciate societal standards.
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No. Inability to control behavior or conform conduct to the law, even when caused by mental illness, does not provide a defense. The test focuses exclusively on cognitive capacity to understand the nature and quality of the act or its wrongfulness.
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Schizophrenia is recognized as a disease of the mind, but the defense requires proof that the condition actually caused a defect of reason resulting in the specified incapacity at the time of the act. Mental illness alone does not establish the defense.
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Any mental abnormality qualifies if it produces the required defect of reason and resulting incapacity. Psychopathy and lesser functional aberrations typically do not suffice because they do not destroy the cognitive capacities the test demands.
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…proved that, at the time of the committing of the act, the party accused was laboring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing; or, if he did know it, that he did not know he was doing what was wrong.” 10 Cl. & Fin., at 210, 8 Eng. Rep., at 722. The…