Mansfield rule, the doctrine that a Juror's testimony or affidavit about juror misconduct may not be used to challenge the verdict. This Mansfield rule as intended to ensure that jurors are heard through their verdict, not through their post verdict testimony. In practice the rule lessons the possibility that losing parties will seek to penetrate the secrets of the jury room. The rule of first announced in Vaise v. Delaval, 99 Eng Rep 944 (KB 1785), in an opinion by William Murray, first Earl of Mansfield, the Lord Chief Justice of the Court of King's Bench, Black's Law Dictionary, 7th Edn., p. 976.
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