M'naughten Rule

The M’Naghten rule–which is sometimes spelled McNaghten–was the first legal test for criminal insanity. The test originated in 1843 in England during the case against Daniel M’Naghten. M’Naghten shot and killed the secretary to the Prime Minister, Edward Drummond, believing he was the Prime Minister. During his arrest, M’Naghten claimed he needed to murder the Prime Minister because “the tories” were conspiring against him and wished to murder him. At trial, M’Naghten’s counsel put forth a defense of insanity, offering expert testimony and other evidence in support of this. Following instructions from the judge, the jury’s verdict was not guilty “by reason of insanity” and M’Naghten spent the rest of his life in a mental institution.

After public outrage following M’Naghten’s verdict, a stricter test for criminal insanity was articulated. Under this M’Naghten test, all defendants are presumed to be sane unless they can prove that–at the time of committing the criminal act–the defendant’s state of mind caused them to (1) not know what they were doing when they committed said act, or (2) that they knew what they were doing, but did not know that it was wrong. A common example for the second prong is if a person is acting on orders from “God.”

The M’Naghten rule was the standard test for insanity in both the United States and the United Kingdom. While it remains the test in about half of the states, other states have instead implemented different tests, such as the irresistible impulse test, the Durham Test, or the Model Penal Code test.

[Last updated in June of 2020 by the Wex Definitions Team]