Impeachment of Witness

In litigation, impeachment of a witness is the process by which a party undermines the believability of testimony given on the stand. It does not necessarily prove the witness is lying; it gives the trier of fact reasons to discount what was said.

In U.S. federal practice, impeachment is governed primarily by Federal Rules of Evidence 607–609 and 613. Rule 607 allows any party — including the one who called the witness — to attack credibility. The recognized grounds typically include:

• Bias, interest, or motive to lie (e.g., a cooperating witness who received a plea deal).
• Prior inconsistent statements under Rule 613, where counsel confronts the witness with earlier contradictory testimony or writings.
• Character for untruthfulness under Rule 608, through reputation or opinion evidence, and sometimes specific instances on cross-examination.
• Prior criminal convictions under Rule 609, subject to balancing tests and a general 10-year limit on old convictions.
• Defects in capacity — poor eyesight, intoxication at the time of observation, or faulty memory.
• Contradiction by other evidence, including expert testimony.

Common-law systems outside the United States use similar tools. In England and Wales, the Criminal Procedure Act 1865 (often called Lord Denman's Act), sections 3–5, governs proof of prior inconsistent statements and hostile witnesses. Civil-law jurisdictions handle witness reliability differently, generally leaving weight to the judge rather than formal impeachment rules.

Impeachment is distinct from disqualification (preventing someone from testifying at all) and from impeachment of a public official, which is a constitutional removal process. It also differs from striking testimony: an impeached witness's statements usually remain in the record, but the jury may give them less weight.

Skilled cross-examination often relies on the collateral evidence rule: extrinsic evidence is generally inadmissible to impeach on purely collateral matters, though prior inconsistent statements and bias are treated as non-collateral and may be proved by outside evidence.