Mansfield rule.The doctrine that a juror’s testimony or affidavit about juror misconduct may not be used to challenge the verdict. • The Mansfield rule is intended to ensure that jurors are heard through their verdict, not through their postverdict testimony. In practice, the rule lessens the possibility that losing parties will seek to penetrate the secrets of the jury room. The rule was first announced in Vaise v. Delaval, 99 Eng. Rep. 944 (K.B. 1785), in an opinion by William Murray, first Earl of Mansfield, the Lord Chief Justice of the Court of King’s Bench. [Cases:

Criminal Law 957; Federal Civil Procedure 2371; Trial 344. C.J.S. Criminal Law §§ 1415–1418; Trial §§ 921–926.]

[Blacks Law 8th]