Federal Rules of Evidence - Rule 606. Competency of Juror as Witness
(a) At the trial.
A member of the jury may not testify as a witness before that jury in the trial of the case in which the juror is sitting. If the juror is called so to testify, the opposing party shall be afforded an opportunity to object out of the presence of the jury.
(b) Inquiry into validity of verdict or indictment.
Upon an inquiry into the validity of a verdict or indictment, a juror may not testify as to any matter or statement occurring during the course of the jury's deliberations or to the effect of anything upon that or any other juror's mind or emotions as influencing the juror to assent to or dissent from the verdict or indictment or concerning the juror's mental processes in connection therewith. But a juror may testify about (1) whether extraneous prejudicial information was improperly brought to the jury's attention, (2) whether any outside influence was improperly brought to bear upon any juror, or (3) whether there was a mistake in entering the verdict onto the verdict form. A juror's affidavit or evidence of any statement by the juror may not be received on a matter about which the juror would be precluded from testifying.
- Tanner v. United States, 483 US 107 (1987) - Jurors are prohibited from testifying about alcohol, marijuana and cocaine ingestion by jurors during trial and deliberations. Alcohol is not an "outside" influence. Rational: protection of jury system, finality, preservation of dignity and mystery of jury process, discourage harassment; encourage full and open discussions among jurors. Dissent: Juror's should be able to testify about events that occurred at times other than deliberations. Alcohol is an outside influence, different in nature from a virus or lack of sleep.