M'Naughten Rule

M'Naughten Rule

n. a traditional "right and wrong" test of legal insanity in criminal prosecutions. Under M'Naughten (its name comes from the trial of a notorious English assassin in the early 1800s), a defendant is legally insane if he/she cannot distinguish between right and wrong in regard to the crime with which he/she is charged. If the judge or the jury finds that the accused could not tell the difference, then there could not be criminal intent. Considering modern psychiatry and psychology, tests for lack of capacity to "think straight" (with lots of high-priced expert testimony) are used in most states either under the American Law Institute's Model Penal Code or the "Durham Rule." (See: insanity, temporary insanity, "twinky" defense)

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In 1840, for instance, when the phrase "mental illness" had not yet entered the vocabulary, the US census acknowledged just a single form of madness, "idiocy/insanity," and failed even to provide a definition of it, relying instead on presumed common sense and the sort of thinking operative in the common law's long-standing M'Naughten Rule (dating from an 1843 political assassination case) which specifies legal insanity as "the inability to know the difference between right and wrong.