Also known as: M'Naghten · M'Naghten rules · McNaghten · M'Naghten test · M'Naghten insanity defense · McNaughton rules
Written by attorneys · grounded in primary & secondary sources — see below
A test for criminal insanity requiring proof that a disease of the mind caused a defect of reason so that the defendant lacked capacity either to know the nature and quality of the charged act or to know that the act was wrongful as society defines it. Loss of behavioral control alone supplies no defense. A defendant's personal belief that the conduct is morally justified does not establish the defense unless the defendant also lacked capacity to recognize society's contrary judgment.
Maya Malik, who suffered from untreated schizophrenia, drove a delivery van onto a sidewalk because she believed the pedestrians were holograms blocking a government tunnel. At trial she proved that her disease produced a defect in reasoning that left her unable to understand either the physical nature of striking real people or that society would condemn the act as wrongful. The court therefore acquitted her under the M'Naghten test.
Michelle Murphy, diagnosed with chronic psychosis, burned valuable museum paintings because she believed they contained evil spirits that she had a duty to destroy. She calmly told police that she knew her colleagues and the public would view the destruction as seriously wrongful. Because she retained the capacity to recognize society's judgment, the court rejected her insanity claim.
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Marco Marquez, a bus driver with schizophrenia, steered his empty bus into a parked car after months of commanding hallucinations. He admitted he knew the crash was dangerous and illegal yet claimed an overpowering voice left him unable to stop. The court held that his loss of self-control, standing alone, did not meet the M'Naghten standard.
Mohan Malhotra was charged with assault after attacking a coworker while experiencing vivid delusions that the coworker was an algorithm threatening financial markets. He introduced expert testimony of schizophrenia but the prosecution offered lay evidence that he had previously distinguished real colleagues from hallucinations. The jury found the evidence of insanity insufficient and convicted him.
Mina Mehta shot a pedestrian she believed was a holographic projection created by traffic sensors. The jurisdiction applied only the traditional M'Naghten cognitive standard. Because she admitted that striking a real person would be illegal and condemned by society, the court instructed the jury that her psychotic belief did not establish insanity.
Michael Miller removed critical fasteners from an aircraft wing because he delusionally believed the plane was a static museum display. The jurisdiction followed the traditional M'Naghten test rather than a product-of-mental-disease formulation. Because the evidence showed he understood the physical act of loosening fasteners, the court denied his insanity claim.
No. The test focuses exclusively on 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.
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No. The defendant must also have lost the capacity to recognize that society regards the conduct as wrong. A moral disagreement with societal norms is insufficient.
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The defendant must prove a disease of the mind that caused a defect of reason resulting in inability, at the time of the act, either to understand the nature and quality of the conduct or to know that society would regard it as wrongful.
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Yes. The Supreme Court has upheld statutes placing the burden of proving insanity on the defendant under the traditional M'Naghten formulation.
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…have considerable leeway in defining the insanity standard. Arizona’s moral-incapacity formulation is consistent with the M’Naghten test, the most widely accepted insanity test in this country. No evidence indicates that the M’Naghten formulation was a universal or near-universal feature of the Anglo-American legal…