Before taking the stand to testify in court, all witnesses take an oath swearing to tell the truth, the whole truth, and nothing but the truth. The reason for that oath is simple: the purpose of any trial is to determine the truth. Because it is vitally important for all trial testimony to be true and complete, the question of whether a witness tends to lie is highly relevant to the judge or jury. Thus, either party may challenge a testifying witness’s reputation for truthfulness.
Though witnesses are usually immune from attacks on their character, questions about their character for truthfulness are typically allowed. Nonetheless, even when the witness’s character for truthfulness is being challenged, the challenging attorney generally may not introduce extrinsic evidence to prove or disprove the witness’s answers regarding their own truthfulness. There is, however, one notable exception-Federal Rule of Evidence 609 (“Rule 609”).
