United States v. Abel 469 U.S. 45, 105 S.Ct. 465, 83 L.Ed. 450 (1984) Abel and two other guys were arrested on suspicion of bank robbery.  The other two guys pled guilty, but Abel decided to plead innocent. One of the guys, Ehle, agreed to testify against Abel. At a pretrial conference, Abel informed the court that he would attempt to impeach Ehle's testimony with the testimony of a guy named Mills. Mills wasn't involved in the robbery, but had spent time in prison with Ehle. Mills claimed that Ehle told him that he was going to lie about Abel's involvement in the robbery to get favorable treatment. The prosecution countered that if Mills testified, they would introduce evidence to show that Abel, Mills and Ehle were all members of the Aryan Brotherhood. One of the tenants of the Aryan Brotherhood is that always deny the existence of the organization.  The prosecution hoped to get Mills to lie under oath about the organization, and thereby discredit his credibility. At trial, Ehle testified that Abel was involved, and Mills testified that Ehle told him he was going to lie.  When the prosecution attempted to cross-examine Mills with regards to the Aryan Brotherhood, Abel objected. Abel objected to the use of the term 'Aryan Brotherhood' on the grounds that it was unduly prejudicial.  The prosecution agreed to use the term 'secret prison organization' instead. The prosecution brought Ehle back on the stand, where he testified that because they were all in the same 'secret prison organization', it would have been 'suicide' for Ehle to tell Mills that he was going to screw over Abel to get better treatment. In the Aryan Brotherhood, ratting out another member marks you for death. The Trial Court found Abel guilty of bank robbery.  He appealed. The Appellate Court reversed. The Appellate Court found that Ehle's rebuttal testimony was admitted not just to show that Mills had reason to lie to protect Abel because they were in the same club, but also that because Mills belonged to an organization that supported perjury, he must be lying. "The suggestion of perjury, based upon a group tenet, was impermissible...Membership without more has no probative value.  It establishes nothing about the individual's own actions, beliefs, or veracity." The US Supreme Court reversed. The US Supreme Court found that evidence of Mills' and Abel's membership in the prison gang was sufficiently probative of Mills' possible bias towards Abel to warrant admissibility. The Court noted that FRE 401 defines relevant evidence as any evidence having a tendency to make the determination of a fact more or less probable.  And FRE 402 says that any relevant evidence is admissible unless specifically excluded. In this case, Mills' membership in an organization that lies is not proof that he is lying, but it is evidence that he is more likely to lie. Basically, this case showed that bias is never considered a collateral issue, it is always a relevant issue.  Therefore you can always ask a witness about it, and you can introduce extrinsic evidence to impeach a witness who is suspected of being biased. The term bias means any relationship between a party and a witness which may lead the witness to slant, unconsciously or otherwise, the testimony.  Bias may be induced by a witness’ like, dislike, fear, or self-interest.