When any witness, including a defendant, testifies, his or her credibility is at issue. The fact-finder (either judge or jury) must decide whether and how much to believe the witness. That's why courts allow into evidence certain kinds of past convictions—to aid the determination of how trustworthy the witness is.
Generally, prosecutors can't use evidence of prior convictions to prove a defendant's guilt or tendency to commit crimes, but they can sometimes use them to question the truthfulness or credibility of the defendant's testimony.
Courts won't admit evidence of any old conviction to impeach a witness. Rather, the crime must typically be either a felony or an offense involving dishonesty, whether a felony or misdemeanor. Some states don't allow judges to admit any information regarding prior convictions unless the defense has first offered evidence to establish the defendant's credibility.
Prosecutors can usually use evidence of actual convictions only. They can't ask the defendant-witness about pending charges or arrests that didn't result in convictions. But they can ask testifying defendants about convictions that are currently pending on appeal or for which sentence hasn't yet been imposed.
Judges will weigh, on the one hand, the value of the evidence of a prior conviction in helping the jury determine the defendant's credibility against, on the other, possible prejudice. Judges must take into account the possibility that jurors will presume the defendant guilty if they learn about his or her past crimes.
In deciding whether to admit evidence of a prior conviction, courts consider the following factors:
Judges usually consider evidence of prior convictions for the same or a similar crime (as the one the defendant now faces) very prejudicial. For this reason, courts in many states won't admit very similar prior convictions if the prosecution can use a different prior conviction to impeach the defendant.
Courts are more likely to admit evidence of crimes involving dishonesty than crimes of violence or those similar to the offense being tried. Federal courts and some state courts automatically allow evidence of prior crimes involving dishonesty without any prior determination of prejudice.
Consider the following examples, which show how judges determine whether to admit prior convictions of testifying defendants:
If you face criminal charges, consult an experienced criminal defense lawyer. Only such an attorney can properly advise you about your chances at trial, represent you at trial, and analyze the pros and cons of you testifying (including the chances of a prior conviction coming into evidence).