Basic to U.S. trial procedure is the principle that a defendant's past convictions, if any, may not be introduced to show that he is probably guilty because he was guilty in the past. Should a defendant choose to testify, however, the prosecutor is permitted to try to impugn his credibility. In most states, prior convictions may then be introduced on the theory that they cast needed light on whether his word can be trusted.
The line is a fine legal one, and it understandably tends to disappear for most jurors, who, try though they will, cannot consider the defendant's veracity in...