The M'Naghten rule (pronounced, and sometimes spelled, McNaughton) is any variant of the 1840s jury instruction in a criminal case when there is a defence of insanity:

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 testi-mony) are used in most states either under the American Law Institute's Model Penal Code or the "Durham Rule." See also: diminished capacity insanity temporary insanity Twinkie defense