Witness impeachment, in the law of evidence, is the process of calling into question the credibility of an individual who is testifying in a trial. There are several ways that a witness may properly be impeached and several other ways that, although effective, are prohibited except under special circumstances.
A party may be impeached through introducing evidence of any of the following (remembered via the mnemonic BICCC):
Another form of impeachment by contradiction has a subtle effect on the order in which the attorneys present their evidence. When a defense attorney calls a witness who testifies about what happened, this gives the opposing attorney the opportunity to present evidence contradicting that witness. Had impeachment by contradiction not been allowed by the rules of evidence, the second attorney would have been barred from presenting the contradicting evidence because he already had his one (and only) chance to prove the facts of the case as he claims them to be. But since his opponent put on a witness, this "opens the door" to him to strengthen his case by going again with more proof of what happened: the only legal excuse for this rehash of his claim is that he is impeaching by contradiction his opponent's witness.
Another use of impeachment by contradiction can be explained negatively: while an attorney cannot contradict an opponent's witness on a trivial ("collateral") fact like the color of the hat she testified she was wearing on the day she witnessed the accident, but on more important matters normally excluded by the rules of relevance, contradiction may be allowed. Thus, a witness might not normally be permitted to testify she is a safe driver, and the opponent cannot normally prove she is in general an unsafe driver but should the witness nonetheless happen to testify she is a safe driver (say because no objection was made to the question), her opponent can now contradict her by eliciting on cross-examination that she has been involved in several accidents. Had contradiction impeachment not been permitted, then the unsafe character of the witness would have been barred by the rules of evidence. This impeachment-by-contradiction evidence is admitted solely to impeach: it cannot be used to prove anything about the events being litigated but only to discredit the witness's credibility. The theory is that when a witness can be contradicted, it should be taken into account in determining the reliability of the witness. Hence, the jury is instructed by the judge "do not use [the impeachment evidence] as proof of any facts, but only to consider whether the witness in question should be believed." All experienced courtroom observers agree that jurors will have great difficulty understanding this distinction, known as "limited admissibility" or "admissibility for a limited purpose". Even more unlikely is the prospect that a juror who understands the instruction will be psychologically capable of obeying it. The only practical impact of this limited admissibility is that the evidence cannot be used to prop up a weak case that would otherwise be dismissed by the court for insufficient evidence because it was admitted only for the impeachment of a witness.
Under the common law of England, a party could not impeach his own witness unless one of four special circumstances was met. This was due to the Voucher Rule, which required that the proponent of the witness "vouch" for the truthfulness of the witness. The special circumstances were:
This rule has been eliminated in many jurisdictions. Under the United States' Federal Rules of Evidence, (F.R.E. 607), any party may attack the credibility of any witness.
The general rule is that the proponent of a witness may not attempt to build up the witness's credibility prior to his being impeached. The rationale is that the witness is presumed trustworthy. It also speeds proceedings by not spending time bolstering when the other side may not even impeach the witness.
In order to rehabilitate a witness, the proponent is confined to using the same techniques used by the opponent to impeach the witness. That is, if the opponent impeached via bias, then rehabilitation is limited to negating the claim of bias. If the opponent brought in a rebuttal witness who testified to the character of principal witness as that of a liar, rehabilitation is limited to a character witness who testifies principal witness is a truthful person. Note that this is a different consideration from the ever-present right to cross-examine any witness, including character witnesses.
If the opponent shows that the witness made a prior inconsistent statement and implies that after that statement and prior to trial the witness was "gotten to" or otherwise developed a motive to lie in court, rehabilitation can be attempted by showing that the witness made a prior consistent statement (consistent with the testimony) before the alleged events that gave rise to the alleged motive to lie. The jury is left with two pre-trial statements that are inconsistent with each other, but only one is inconsistent with the testimony, and both were made before the witness was allegedly gotten to, so there might be softening of the accusation that the testimony flows from such as a bribe. Also, there is always a case for allowing a prior consistent statement made at any time before trial to help explain away what is arguably only a seemingly inconsistent statement that is subject to interpretation because, for example, it was lifted out of the context that would explain the statement.