Rightly or wrongly, courts generally will not overturn a verdict based on evidence related to juror deliberations unless the evidence pertains to some type of outside influence on the jury. In fact, most courts will not even allow the evidence to be admitted. This juror anti-impeachment rule, reflected in FRE 606(B), is in place to: (1) provide verdict finality; (2) encourage jurors to freely express themselves during deliberations; and (3) reduce the amount of post-trial hounding jurors receive from attorneys. This rule, as explained in Fields v. Saunders, does have some limitations.
In Fields, African-American plaintiffs brought an unsuccessful medical malpractice case against several doctors. One of the jurors who sat on the case decided to end his term of jury duty by stopping off at a local bar. While at the bar, the juror, without any prompting, offered his opinion of the case to another bar patron who happened to be an attorney. Among other things, the juror told the attorney that
(1) the plaintiffs would have never won the case with him (the juror) serving in the case, (2) he (the juror) was not impartial despite stating in voir dire he could be, and (3) he (the juror) wanted to "play the judicial system" and believed plaintiffs had the burden to prove the defendants intended harm beyond a reasonable doubt before they could recover. The attorney also related that the juror expressed his dislike for African Americans.
These statements by the juror as relayed by the attorney were admitted into evidence and ultimately led the Oklahoma Supreme Court to reverse the trial court (after an evidentiary hearing, the trial judge had denied plaintiff's motion for a new trial) and remand the case for a new trial. In attempting to distinguish Fields from the typical jury impeachment case, the Oklahoma Supreme Court emphasized that the disqualifying bias was disclosed by the juror himself, and not by other jurors repeating statements made by the juror during deliberations. The Oklahoma Supreme Court also emphasized that this case was an anomaly and involved a rogue juror.