M’Naghten Standard

The M’Naghten Standard is a legal test to guide juries and courts in their determination of whether a defendant should be found not guilty by reason of insanity. Although defendants were acquitted for crimes they committed while they were legally insane for centuries before the M’Naghten Standard was established, no uniform legal test was adopted by the courts until the middle of the 19th century following the case of Daniel M’Naghten (Regina v. M’Naghten, 1843).

Daniel M’Naghten was acquitted for killing the private secretary of the Prime Minister of England, Sir Robert Peel. Mr. M’Naghten had a mental illness that was most likely a form of paranoid schizophrenia. His mental illness was characterized by the belief that he was the victim of an international conspiracy. He believed that if he killed the prime minister, it would lead to the second coming of Christ and the salvation of humanity. Rather than shooting the prime minister, though, he mistakenly shot his private secretary, Edward Drummond, who died a few days later. After he shot Mr. Drummond, he attempted to fire a second shot, but he was apprehended and arrested.

Daniel M’Naghten’s father was able to secure a well-financed defense led by a leading barrister of the day. The central issue in M’Naghten’s case was the proper standard for establishing a legal defense of insanity. M’Naghten was apparently only “partly insane,” and purportedly suffered from delusions concerning politics. Apart from his specific paranoid beliefs, he was able to make plans and was able to live in the society. At the trial, his barristers argued that he should be found not guilty by reason of insanity. The jury acquitted Daniel M’Naghten because of his insanity. M’Naghten was committed to Bethlem and, later, Broadmoor Mental Institution, where he died approximately at the age 50 on May 3, 1865, some 20 years following his trial.

As soon as the verdict in the M’Naghten case was announced, the public became alarmed that insane people could kill without fear of punishment. In addition to the concern the public showed, Queen Victoria (who herself had been the target of would-be assassins on several occasions) and members of the House of Lords also made their disapproval of the verdict known.

Just over 2 months after the decision in M’Naghten was made public, 15 common law judges in Great Britain were summoned to the House of Lords to help determine the proper standard for criminal responsibility of the criminally insane. Fourteen of the judges agreed that essentially the same standard employed in M’Naghten was the correct legal standard. The opinion delivered included the language pertaining to the legal test of insanity, known as the M’Naghten Standard:

A person is presumed sane unless it can be clearly proven that, at the time of the committing of the act, the party accused was labouring 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. The mode of putting the latter part of the question to the jury…had generally been, whether the accused at the time of doing the act knew the difference between right and wrong. (Regina v. M’Naghten, 1843, 10 Cl. & Fin. At 203, 8 Eng. Rep. at 720)

A person labouring under a “partial delusion only, and [who] is not in other respects insane…must be considered in the same situation, as to responsibility, as if the facts, in respect to which the delusion exists, were real.” (Regina v. M’Naghten, 1843, 10 Cl. & Fin. At 203, 8 Eng. Rep. at 723)

There are three significant substantive elements of the M’Naghten Standard. First, the decision maker must determine that the defendant was suffering from “a defect of reason, from disease of the mind.” Today, these words are interpreted to mean that the defendant is suffering from a mental disorder. Next, the decision maker must decide whether the evidence shows that the defendant did not “know” the “nature and quality of the act he was doing.” Thus, the defendant must not have understood exactly what he or she did. Finally, the M’Naghten Standard also requires an inquiry to determine whether the defendant knew “what he was doing was wrong.” Therefore, the defendant who understands his or her act, yet does not have the capability of knowing that the act was wrong, may also be acquitted under the M’Naghten test. Because the final two elements require a subjective exploration of the defendant’s thinking, the M’Naghten test is referred to as a “cognitive” test of insanity. It is also known as the “Right/Wrong Test” since it focuses on an enquiry of the defendant’s capacity, at the time of the offense, to be able to know right from wrong.

Almost immediately, the M’Naghten Standard was employed in cases throughout England, the United States, Canada, Australia, and other common law countries. The substantive requirements of the M’Naghten rule are still being used by numerous jurisdictions around the world, including 26 of the United States. Despite its widespread acceptance, the M’Naghten Standard has been strongly criticized, and it has long been abandoned in England, the country in which it originated so long ago. A number of competing tests for legal insanity have been adopted and some jurisdictions have abandoned the defense of insanity.


  1. Moran, R. (1981). Knowing right from wrong: The insanity defense of Daniel McNaughton. New York: Macmillan.
  2. Moran, R. (1985). The modern foundation for the insanity defense of Daniel McNaughton. New York: Macmillan.
  3. Regina v. M’Naghten, 10 Cl. and F. 200, 8 Eng. Rep. at 718 (1843).

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