A witness is a person who saw or heard an alleged crime take place or someone who has important information about the issues involved in the case. In a criminal trial, the prosecution and the defense can witnesses to testify about what they know about the case.
Jurors alone judge the credibility (believability) of witnesses. Jurors can believe all, part, or none of a witness's testimony. Here are some of the factors jurors use to determine witness credibility and tools lawyers use to undermine witness credibility.
What Is a Credible Witness?
A credible witness is a witness jurors view as reliable and believable. People, including jurors, have to evaluate the accuracy and truthfulness of what other people tell them every day. Determining whether a witness in court is credible is similar to deciding whether a co-worker or acquaintance is credible.
A credible witness doesn’t have to be perfect. Credible witnesses tell the truth about what they saw and heard, and sometimes that means saying, “I don’t know” or “I don’t remember.”
Why Is Witness Credibility So Important?
Sometimes, a witness is the only source of a critical piece of information in a trial. And even when there’s also physical evidence, like fingerprints and DNA evidence, witnesses are the storytellers who can explain the evidence and convince a juror to vote “guilty” or “not guilty.”
A criminal prosecution can be derailed if even one juror decides that the alleged victim isn’t credible. Similarly, if a criminal defendant chooses to put on a defense, like an alibi, and the jury doesn’t believe the defendant’s alibi witness, the defendant is almost certainly going to be found guilty.
Witness Credibility Factors
Throughout a trial, the judge instructs the jury on the law, including how to evaluate witness credibility. Instructions vary from state to state, but typically the judge tells jurors that they alone determine the truthfulness of the testimony of each witness and suggests factors for the jury to consider, including:
- How well could the witness see, hear, or know the things that the witness testified about?
- How well was the witness able to remember and describe what happened?
- What was the witness’s behavior while testifying?
- Was the witness’s testimony consistent or inconsistent with other testimony and evidence, including the witness’s own prior statements?
- How reasonable was the witness’s testimony in light of all of the evidence?
- Did the witness have a motive to lie?
- Did the witness have a conscious or unconscious bias for or against the prosecution or defense?
- Did the witness have a personal or financial interest in the outcome of the case?
- Did the witness’s background, education, or experience affect the believability of the witness’s testimony?
- Did the government offer the witness leniency or immunity for their own possible crimes in return for their testimony?
- Has the witness been convicted of a felony?
- What is the witness’s reputation for honesty and truthfulness?
The judge will instruct the jurors to set aside any bias or prejudice they may have and judge the testimony of each witness by the same standard.
What’s the Difference Between Competence and Credibility in Witnesses?
Witnesses have to be “competent”—legally qualified—to testify. Competency isn't the same as credibility or expertise (more on expert witnesses below). Witnesses are generally competent if they have the capacity to observe, remember, and explain the events or other matters about which they’re testifying. Witnesses also need to understand their obligation to tell the truth under oath.
It’s up to the judge to decide if a witness is competent. Examples of witnesses who might not be competent include very young children and people with:
- untreated mental health conditions
- severe developmental disabilities, and
- conditions that affect memory, like dementia.
Jurors who are considering whether a witness is credible may take into account some of the same factors that a judge uses to assess witness competency, such as age, intelligence, and memory abilities.
Expert Witness Credibility
Expert witnesses testify about matters that come within their knowledge or expertise, rather than about something they saw or heard related to the trial. For instance, a ballistics expert may testify at a criminal trial, or an accident reconstruction expert may testify in a personal injury case.
Just because witnesses are experts doesn’t mean the jury must automatically believe their testimony. In fact, it’s common to have “dueling” experts offering differing opinions on the same question. For example, in a case involving eyewitness identification, the defense may call a cognitive psychologist to testify about how factors, like stress and the presence of a weapon, can lead to mistaken identification. The prosecution will then call its own expert to highlight whatever factors support the credibility of the identification. When that happens, jurors must decide which expert is more believable.
How Lawyers Attack Witness Credibility
Lawyers have various tools to attack the credibility of a witness (known as “impeaching” a witness). There are a few basic methods that can be used to discredit witnesses:
- Cross-examination. After a witness has testified, the lawyer for the other side can cross-examine the witness, asking questions designed to raise doubts about the witness’s credibility.
- Other witnesses. Whenever possible, attorneys will try to call other witnesses whose testimony contradicts or at least calls into question testimony by a witness for the other side.
- Outside evidence. Lawyers may also introduce outside ("extrinsic") evidence that isn’t directly related to the case but is relevant to a witness’s credibility, such as documents showing the witness’s financial interest in the outcome of the case, social media posts showing that the witness is friends with the defendant, or the witness’s criminal record showing prior convictions for felonies or crimes involving dishonesty. State and federal courts have different rules on what kinds of extrinsic evidence may be used to impeach witnesses.
While giving instructions to the jury at the close of the trial, the judge may explain how the jurors should weigh the testimony of certain witnesses in light of other evidence or testimony. For instance, if a witness has violated a witness sequestration order by talking to other witnesses before testifying, the judge may point that out and tell the jury it may consider that fact when deciding whether to believe the witness’s testimony.
Judges typically instruct jurors not to automatically reject testimony just because of inconsistencies or conflicts. People sometimes honestly forget things and two people may witness the same event differently.
Jurors may consider anything that reasonably tends to prove or disprove the truth or accuracy of a witness’s testimony. But jurors may still choose not to believe witnesses who haven’t been impeached or contradicted.
In federal courts and in most states, parties may impeach their own witnesses—for instance, if a witness said something the party didn’t expect.
How to Prove a Witness Isn’t Credible on Cross-Examination
During cross-examination, an attorney tries to undermine or impeach a witness's credibility by showing that the witness is not reliable or that the witness may have misstated something or even lied during the direct examination.
For example, let's say that an eyewitness to a DUI accident said that a traffic light was green in an accident report and at a preliminary hearing and then testified that the light was red at trial. An attorney can ask the witness questions about the previous statements to show inconsistencies in the witness's story.
Attorneys may also ask questions during cross-examination to reveal a witness's bias or prejudice toward a party in the case. If a witness is the spouse (or ex-spouse) of one of the parties, you better believe the jury will hear about the relationship on cross-examination and decide whether it impacts the witness's credibility.
Attorneys may also use cross-examination to show when witnesses have a stake in the outcome of the case, which could influence their testimony. Criminal defendants have an obvious stake in the outcome of their cases, but civil plaintiffs and defendants often have a lot of money on the line.
The attorney who called the witness to the stand can raise objections to questions posed during cross-examination. The judge then rules on the objection. If the objection is sustained, the witness doesn't have to answer the question. If the objection is overruled, the witness does have to answer the question.
When to Speak With a Lawyer
If you’ve been called as a witness, and you’re worried that something you say at the trial might harm you, it’s best not to rely on the advice of the lawyer or prosecutor who has called you to testify. Talk to a lawyer who’s not involved in the case to get independent advice. Your lawyer can help you prepare to testify and advise you on whether you can “take the Fifth.”