We've all read reports and seen news coverage of a witness declining to answer questions on the grounds that the responses may incriminate him—he "claims the Fifth." A witness in that situation is relying on the Fifth Amendment to the United States Constitution, which provides that no person "shall be compelled in any criminal case to be a witness against himself." If, by answering, the witness could provide evidence that might aid the government in prosecuting him, then he has the right to refuse.
(For another angle to this right, see Immunity From Prosecution. Also see Can one person claim the Fifth Amendment on another's behalf?)
Witnesses can assert the privilege against self-incrimination in civil proceedings as well as criminal ones, despite the seemingly limiting language of the Fifth Amendment. They can assert it in state or federal court, in a wide variety of proceedings (including trials, depositions, administrative law proceedings, and investigatory proceedings like grand jury hearings). But interestingly, if the witness fears exposure to prosecution in a country other than the U.S., the privilege does not apply. (United States v. Balsys, 524 U.S. 666, 672 (1998).)
Not every disclosure can be the subject of a Fifth Amendment assertion—only those that the witness "reasonably believes could be used in a criminal prosecution or could lead to other evidence that might be so used." (Kastigar v. United States, 406 U.S. 441, 444-445 (1972).) The key word here is "reasonably." Responses to questions that would be of no use to prosecutors, or that couldn't contribute to a prosecution because of the passing of the statute of limitations, cannot take refuge within the Fifth. But if the prosecution can make use of the answer itself or any evidence it might lead to, the witness is entitled to claim the privilege against self-incrimination.
Often, witnesses are involved in two cases at the same time: one civil and one criminal. For example, a witness might be called to testify before a governmental body while a criminal case against her is in the investigative stages. The witness's lawyer will likely advise her to invoke the Fifth Amendment. Why is this a good idea?
First, because the potential consequences of a criminal proceeding (including jail or prison) are more dire than those of a civil case, a witness may be able to get a judge to "stay," or postpone, the civil matter until the criminal one is concluded. Even if there's no stay, a witness is usually better off asserting the privilege if truthful answers will tend to incriminate her with respect to the criminal case.
There's a real risk that innocent mistakes or omissions in a civil matter that's held before the completion of a criminal investigation will come back to haunt the defendant. It's even possible that a mistake in a civil proceeding will lead to a prosecution not just for the underlying criminal act, but also for being untruthful, in which case eventual charges might include obstruction of justice or even perjury.
A witness can waive the right to invoke the Fifth by later making statements about the topic in question. For example, if a witness invokes the Fifth, but goes on to selectively answer questions about the same subject matter, a judge might decide that the later answers vitiated the initial waiver. But judges are hesitant to declare the privilege waived because of its importance. For instance, a brief and general statement about one's innocence, following invocation of the Fifth, probably won't constitute a waiver.
Importantly, even if a judge finds that a person waived the privilege, that waiver will extend only to the current proceeding. A witness who answers questions subsequent to invoking the Fifth, who is ordered by a judge to continue answering based on waiver, can reassert the privilege in a later, different proceeding. For example, a defendant who waives the privilege while testifying in one case can assert it when called to testify in another.
Additionally, a witness can begin testifying but invoke the privilege when answers to later questions would be incriminating. If the prosecutor commences by asking benign questions that the witness answers ("What were you wearing that night?"), but moves into questions that go to the heart of the matter ("How many times did you meet with the defendant?"), the witness may claim the privilege.
At first blush, it might seem that whenever answers to questions might incriminate oneself, it makes sense to decline to answer. Many times claiming the Fifth is the best course of action, but there may be instances when it will do more harm than good. For example:
Yet, a witness who could claim the Fifth but is tempted to answer should first discuss the issue thoroughly with an attorney. Suppose a witness in a civil case may be able to supply credible answers that could form the basis for a strong defense against a future criminal proceeding. A prosecutor confronting these convincing, under-oath answers might decide not to file charges. Of course, if the witness fibs, omits something, or gets confused on cross-examination, she may doom herself. That's why the guidance of a knowledgeable attorney is crucial.
The decision as to whether to invoke one's Fifth Amendment right against self-incrimination in a civil or criminal proceeding is very complicated, involving an assessment of both the facts and the relevant law in your jurisdiction. Only careful discussion and preparation with your lawyer will enable you to make a wise decision. Never head into a situation where testimony could end up hurting you without professional advice.