M’Naghten Rule | Definition

Doc's CJ Glossary by Adam J. McKee


Course: Introduction / Criminal Law

The M’Naghten Rule is a  legal test of insanity that hinges on the defendant’s inability to know right from wrong; originated in an English court case in 1843, making it the first major test for insanity.

The M’Naghten Rule is one of the earliest and most well-known legal tests for insanity. It originates from the case of Daniel M’Naghten, who, in 1843, attempted to assassinate the British Prime Minister but mistakenly killed his secretary instead. M’Naghten was found not guilty by reason of insanity, and the court’s decision in his case established the M’Naghten Rule as a legal test for determining whether a defendant is legally insane.

Under the M’Naghten Rule, a defendant is considered legally insane if, at the time of the crime, they suffered from a mental disease or defect that prevented them from knowing the nature and quality of their actions or from understanding that their actions were wrong. In other words, the defendant must be so mentally ill that they did not know what they were doing, or they did not know that what they were doing was wrong.

While the M’Naghten Rule has been widely adopted in the United States, some states have modified or replaced it with alternative tests. For example, the American Law Institute’s Model Penal Code (MPC) includes a test for insanity that focuses on the defendant’s ability to appreciate the criminality of their conduct or to conform their behavior to the requirements of the law.

Critics of the M’Naghten Rule argue that it is too narrow and does not adequately account for the complex and varied ways in which mental illness can affect a person’s behavior. Some have suggested that a more flexible approach that takes into account the defendant’s ability to control their behavior, as well as their understanding of the consequences of their actions, would be a better test for insanity.

Despite these criticisms, the M’Naghten Rule remains an important and widely used legal test for insanity in many jurisdictions. Courts continue to grapple with the difficult task of determining whether a defendant’s mental state at the time of a crime was sufficient to absolve them of criminal responsibility, and the M’Naghten Rule provides a framework for making that determination.

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Last Modified: 04/10/2023


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