When someone testifies under oath in a hearing, trial, or deposition, the other side will typically challenge the witness through cross-examination. The opposition’s tools include not only the ability to probe the witness’s actual ability to observe certain incidents, or the precision of the witness’s memory. The opponent might also challenge the essential credibility of the witness – that is, the chances that the witness is likely to be telling the truth, aside from what those facts may be. Going after a witness like this is called “impeaching” a witness, and it’s a direct attack on the character of the person testifying, attempting to show the judge or jury, “No matter what this person says, she doesn’t deserve to be believed.”
Lawyers impeach witnesses by using one or more of several approaches, some of which are explained below.
Confronting a witness with that person’s own statements that are at odds with the person’s testimony is a very common way to impeach the witness. But the opposition can’t just introduce the statement without giving the witness a chance to explain. The court may require that the lawyer disclose the statement to the witness during the witness’s trial testimony, including the circumstances under which it was made, and give the witness a chance to admit or deny it.
For example, suppose a witness to a fight testifies in court that the victim threw the first punch. At a bar after the fight, however, the bystander described the scene to several listeners, telling them that the defendant initiated the scuffle. The prosecutor, of course, will want the jury to disregard this witness’s testimony in court, which (if believed), would give the defendant a self-defense argument. So the prosecutor will want to impeach the witness, and will do so by questioning the witness about his statements at the bar. After giving the witness a chance to explain them, the prosecutor will call the bar patrons to testify, and will argue to the jury that because he has described the incident in inconsistent ways, the bystander just can’t be believed.
A traditional and common sense way to impeach a witness is to show that he or she is biased against one of the parties, or has a personal interest in the outcome of the case. The relationship between the parties may be good or bad. For example, imagine a witness who is a business partner with the defendant, who stands to lose money if the defendant loses the case. The plaintiff may introduce evidence of the business dealings between the two people, and show how the outcome of the case will directly affect the witness. For this reason, the lawyer will argue, the witness’s testimony ought not to be believed.
Witnesses who take the stand implicitly tell the jury, “I’m a truthful person, you can believe me.” Opponents can challenge this assertion by introducing evidence to the contrary, calling witnesses who will testify that the person’s reputation for truthfulness is woefully lacking. In most states, such evidence is limited to showing the witness’s lack of veracity, not his bad moral character. In other words, a person may be an adulterer, but he may not be impeached on this fact alone. If he’s a lying adulterer, however, his reputation for lying could come into evidence.
People who have previously broken the law may have such disrespect for the rule of law that they will not respect the oath they take before testifying – so goes the rationale that underpins the ability of the opposition to challenge that person’s credibility by pointing to a past criminal conviction. This ground for impeachment is subject to state rules and, above all, the discretion of the trial judge. Most misdemeanor convictions are deemed too minor to count; and even when the conviction is for a felony, the court will consider its nature and age. For example, someone with multiple and recent convictions for perjury may well have a passing commitment to the truth (and those convictions will be admitted to challenge the witness's credibility). But someone who has a single, old conviction for assault may nonetheless be extremely honest (albeit hot-tempered).
When a lawyer wants to impeach a witness during a trial before a jury, rules of court will often require that the lawyer tell the judge and opposing counsel in advance, alerting them to the statement that the lawyer intends to use. That preview is necessary because the judge has the power to disallow the impeachment if the judge thinks that it’s prejudicial impact on the jury will outweigh its value in calling the credibility of the witness into question. For example, imagine a defense witness in an auto accident case, whom the plaintiff learns has lied on a U.S. citizenship application. Even if the plaintiff can prove that the statement on the application is false, it’s unlikely that the judge will allow this incident of untruthfulness to be shared with the jury. Though it might affect their view of the witness’s credibility, it’s far more likely to prejudice them against the witness, all because of a statement that has nothing to do with what happened at the accident scene.
