When a party wants to show how someone acted out of court, using character evidence for that purpose is generally prohibited. But when a party wants to show that a witness likely lied while testifying, character evidence about the witness’s character trait for truthfulness is allowed. In the first situation, character evidence to show conduct out-of-court, risks of prejudice against the defendant are high, and the vagueness of character evidence is particularly problematic.
When character evidence is offered to show whether a person lied on the witness stand, the risks of prejudice against the defendant are reduced since the witness may not be the defendant. Also, the probative value of the character evidence is likely to be high, since the trait – truthfulness – is clearer than other traits such as violence, and the conduct to which the trait is linked, testifying, may be less complex than much other conduct in the world.