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: