Jury tampering is a crime that occurs when people improperly influence jurors. Jurors can also be improperly influenced—sometimes by their own doing—without anyone committing a crime. However it occurs, improper influence on jurors can effectively undo a criminal case and result in a new trial for the defendant.
During a trial, a person may not communicate with a juror about anything related to the substance of the case. No matter how they do it, people who try to influence jurors are guilty of jury tampering. A classic example of tampering is bribing or threatening a juror to decide a case a certain way. More subtle examples are leaving jurors anonymous notes, slipping them photographs, and telling them information that's been excluded at trial.
Courts don't want outside information or opinion about a case to influence jurors; cases are supposed to be decided on the facts as presented at trial, not on potentially unreliable, uninformed, and unchallenged information coming from elsewhere.
During a trial, jurors are instructed not to discuss the case with anyone outside of the courtroom. To break this rule is to commit juror misconduct, which might get the juror dismissed from the jury, but generally isn't a crime.
Improper juror influence can occur without jury tampering—or even jury misconduct. Neither tampering nor misconduct has occurred if the juror follows the judge's instructions and no one tries to assert sway over the juror. But improper influence can nevertheless happen. An example is a juror accidentally overhearing a conversation about evidence the judge ruled inadmissible in the trial.
If a juror has been influenced by outside information as a result of jury tampering, juror misconduct, or simple mistake, then the judge might declare a mistrial and grant the defendant a new trial. But proving that a juror has been illegally influenced can be difficult.
Courts don't like to dig into a jury's verdict. What happens in the deliberation room is supposed to stay there. Thus, the only evidence of juror impropriety that a court will usually consider is an external communication or influence; anything that is internal to the jury, including discussions and thought processes, is generally off-limits. For example, Juror Jane is eligible to testify that someone outside of court told her that the defendant had a violent history; the out-of-court statement is an external influence. However, evidence of what effect this statement had on her vote would not be admissible; her thought process in this regard is internal to the jury.
If a court hears evidence of an improper influence on a juror, it will try to determine whether that influence was likely to actually affect the juror's verdict. Not every instance of an improper influence or jury-tampering leads to a new trial. So, for example, if a juror acknowledges that someone simply told her that the defendant looked guilty, the court probably won't overturn the verdict. But evidence that a juror received cash payments in exchange for finding the defendant guilty will likely spur the court into action.