Impeachment by showing bias and motivation

In a criminal case the Confrontation Clause permits cross-examination about a prosecution witness's juvenile record or probation status when the inquiry is necessary to expose bias or motive to lie. The usual evidentiary protections for juvenile records yield to the defendant's right to test credibility on this point.

Bias impeachment shows a specific reason the witness might favor one side in the present case. Character impeachment attacks the witness's general propensity for truthfulness. Bias evidence is always relevant and is not subject to the same limits that govern prior convictions or dishonest acts.

Religious beliefs or opinions may not be used to attack a witness's general credibility. When membership in a religious organization creates a concrete, defendant-specific motive or hostility, however, the conduct showing that motive may be explored as bias evidence.

Bias is never treated as a collateral matter. Courts permit wide cross-examination on any relationship or interest that could cause a witness to shade testimony, subject only to ordinary limits such as undue prejudice or confusion.