Witness testimony can be one of the most compelling types of evidence in a trial, especially in criminal cases But some witnesses are more trustworthy or believable than others. And witnesses sometimes contradict each other. In jury trials, it’s up the jurors to decide whether and to what extent they believe any of the witnesses who testified at the trial. They’ll take into account the credibility of the witnesses when they’re deciding their verdict in the case.

Why Is a Witness’s Reliability Important?

Sometimes, a witness is the only source of a critical piece of information. Even when there’s also physical evidence, witnesses are the storytellers who can explain the evidence and create a coherent narrative that convinces the jury. And juries still place high value on eyewitness testimony, despite all the research on false recollection and cases where defendants were exonerated by DNA evidence after being fingered by eyewitnesses.

What Makes a Witness More or Less Believable?

Juries may consider many different factors when they’re deciding whether they believe witnesses’ testimony, including:

• Do the witnesses have a personal or financial interest in the case? For instance, do they have a relationship with the crime victim or one of the parties (the defendant or prosecution)?
• Did the government offer them leniency or immunity for their own possible crimes in return for their testimony?
• Are the witnesses biased for or against either party?
• Is the testimony clear, consistent, and convincing?
• Does other evidence support or contradict what the witnesses said? This could include a witness’s own previous statements that contradict the testimony at the trial.
• Do the witnesses have a reputation for being honest and trustworthy?
• Was there anything that hindered the witnesses’ ability to see or hear the events they’re testifying about? Do they have visual or hearing impairments? Were they under the influence of drugs or alcohol at the time?
• Do the witnesses’ appearance and demeanor support or undermine their credibility? Do they appear uncertain or sure of their testimony? Juries might be less likely to believe witnesses whose age or mental condition suggests memory problems.

How Jurors’ Preconceptions Affect Whether They Believe Witnesses

Of course, a witness’s credibility can be a subjective question, especially when it comes to appearance and demeanor. Jurors’ own preconceptions and biases can affect whether they believe any particular witness. For instance, many people are more likely to trust police officers to tell the truth; others may have the opposite reaction to law enforcement witnesses. During the process of selecting a jury, lawyers try to identify and weed out prospective jurors who express strong, preconceived opinions about the credibility of people who will be key witnesses in the case. Still, jurors’ subconscious or implicit bias may affect whether they’re persuaded by witnesses from certain ethnic or class backgrounds.

Are Expert Witnesses Necessarily Trustworthy?

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, an expert may testify on DNA evidence at a criminal trial, or an accident reconstruction expert may testify in a personal injury case. However, just because witnesses are experts, that doesn’t mean the jury must automatically believe their testimony. In fact, it’s common to have “dueling” experts for both sides offer differing opinions on the same question. When that happens, jurors will have to decide which expert is more trustworthy.

Impeaching or Discrediting Witnesses

Lawyers can take various steps to attack the credibility of witnesses (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 meant to elicit answers that could 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 the 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. If witnesses haven’t been impeached or contradicted, jurors may still choose not to believe them but shouldn’t completely disregard their testimony.

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.

When to Speak With a Lawyer

When you’ve been called as a witness, and you’re concerned about anything that could harm you if it comes out at the trial, it’s best not to rely on the advice of the lawyer or prosecutor who has called you to testify. You should consider consulting with an attorney who’s not involved in the case and can give you independent advice.