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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 ...

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