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M'Naghten Rule: Definition & History

Instructor: Kristen Irey
The M'Naghten rule forms the foundation of the modern insanity defense plea. The test was to determine if the defendant in a criminal case knew that his or her actions were wrong and illegal at the time of the act.

M'Naghten Rule: Definition and History

Daniel M Naghten
Daniel M Naghten

The prospect of murder seems like the device of an unsound mind. Sane and normal people do not normally kill and hurt one another, which begs the question: Why aren't all murderers found not guilty by reason of insanity? The answer lies in the difference between clinical mental illness and legal insanity. The cornerstone of legal insanity is the M'Naghten Rule, a test that determines whether or not a defendant is criminally liable for their actions if they suffer from mental illness.

Definition of the M'Naghten Rule

The M'Naghten Rule requires that, should a person who commits a crime be unable to recognize that the crime is morally or legally wrong due to mental disease or mental defect, they should be found not guilty by reason of insanity. This rule, as applied in English and American Law, distinguished criminals based on their capacity to recognize right and wrong to determine legal sanity and not whether they had a mental illness. Consider the example of Ted Bundy who was clearly insane (he was a sociopath) when he raped and murdered women. However, Bundy knew that what he was doing was wrong (he avoided capture and knew his actions were illegal). In this case, the M'Naghten test would find that, while clinically insane, Bundy was legally sane.

History of the M'Naghten Rule

On a chilly London afternoon in 1843, Edward Drummond, the secretary of Prime Minister Robert Peel, was shot to death by Daniel M'Naghten. When apprehended by police, M'Naghten gave paranoid and delusional reasons why he shot Drummond. The court decided, in listening to the evidence, that M'Naghten was not able to understand the nature of the act he was committing. M'Naghten was so delusional, he did not recognize that he had done anything wrong. This formed the basis of the modern insanity defense, or the M'Naghten Rule.

This rule was applied in court cases in many countries over the years following M'Naghten's trial, but many objected to its legal definition rather than its deference to mental health definitions of insanity. This led to changes to the M'Naghten Rule, perhaps most famously as applied in Durham v. United States in 1954, where the court found that the M'Naghten's rule did not address whether the crime was caused by insanity and supplanted the M'Naghten Rule with the Irresistible Impulse test, requiring that prosecutors show the defendant's motivation for committing the act, even if he could recognize it was wrong, was created by an irresistible impulse to act.

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