Witness impeachment, in the law of evidence of the United States, is the process of calling into question the credibility of an individual testifying in a trial. The Federal Rules of Evidence contain the rules governing impeachment in US federal courts.
Parties that may impeach
Under the common law of England, a party could not impeach its own witness unless one of four special circumstances was met. The Voucher Rule required the proponent of the witness to "vouch" for the truthfulness of the witness. Here are the special circumstances: If the witness were an adverse party (such as the plaintiff calling the defendant to the stand, or vice versa). If the witness were hostile (such as the witness refusing to co-operate). If the witness were one that the party was required by law to call as a witness. If the witness surprised the party who called him by giving damaging testimony against that party. The rule has been eliminated in many jurisdictions. Under the US Federal Rules of Evidence, Rule 607 permits any party to attack the credibility of any witness.
Methods
In the US, a party has the option of discrediting a witness through impeachment by cross-examining the witness about facts that reflect poorly on the witness's credibility or, in some cases, by introducing extrinsic evidence that reflects negatively on the witness's truthfulness or knowledge.
In Pennsylvania, the procedure for determining whether a testifying defendant may be impeached is known as a Bighum hearing.
Categories
A party may impeach a witness in the US by introducing evidence of any of the following (remembered via the mnemonic BICCC):
Bias
Courts permit parties to cross-examine a witness in order to impeach that witness based on demonstration of bias. Witness bias may be catalyzed by any number of circumstances, ranging from the witness's blood relationship to a party to his financial stake in the outcome of the litigation. Most US jurisdictions require a cross-examiner to lay a foundation before extrinsic evidence can be used to demonstrate bias for impeachment purposes. Although Rule 610 provides that evidence of a witness's "religious beliefs or opinions is not admissible to attack or support the witness's credibility," an inquiry into the witness's religious beliefs or opinions for the purpose of showing interest or bias because of them is not within the rule's prohibition.
If a witness is accused of bias, and there is an opportunity to cross-examine during the current trial, any statements made at a previous trial/hearing and which are consistent with the testimony at the present trial are admissible, not hearsay.
Inconsistent statement
A party may impeach a witness by introducing those of his prior statements that are inconsistent with his current testimony at trial. In a minority of jurisdictions that follow FRE 801, the prior inconsistent statement may be used not only to impeach but also as substantive evidence.
A prior inconsistent statement is admissible as substantive evidence if
the statement was given under penalty of perjury at a trial, hearing, or other proceeding or in a deposition; the witness testifies at the present trial; and the witness is subject to cross-examination about the prior statement. (801(d)(1), 2014, Federal Rules of Evidence by Muller and Kirkpatrick)
A prior inconsistent statement offered solely for impeachment purposes is admissible regardless of whether it satisfies those requirements.
The cross-examining attorney need not disclose or show the contents of a prior inconsistent statement to a witness prior to the moment he is questioned. If the witness's attorney asks to see the prior inconsistent statement, however, the questioning attorney must show or disclose its contents.
Character
The majority of US jurisdictions permit parties to impeach witnesses by demonstrating their "bad" character regarding truthfulness. Under the Federal Rules a party may demonstrate that by reputation or opinion testimony. That is, a witness's credibility cannot be bolstered, only impeached.