A witness admits on the stand that they had lied to the court previously. Wouldn''t it be wrong for the judge to rely on that person for credible testimony?

Not necessarily. In a jury trial, jurors determine the credibility of witnesses, with guidance from the Judge in the form of instructions provided after all the evidence has been received. The judge instructs the jurors that they are the sole and exclusive judges of the credibility of each of the witnesses called to testify. If they believe that a witness has testified falsely as to any material matter, they may decide to believe all of that witness'' testimony, only a portion of it, or none of it. The judge also provides guidance on how jurors should assess the witnesses'' credibility. Jurors are told to carefully scrutinize all of the testimony given, and the circumstances under which each witness has testified. They are advised to consider each witness''s intelligence, motive to falsify, state of mind, and appearance and manner while on the witness stand. Jurors also should consider the witness''s ability to observe the matters as to which he or she has testified and consider whether he or she appears to have an accurate memory or recollection of these matters. It is also important for jurors to consider any relation a witness may bear to either side of the case, the manner in which each witness might be affected by a verdict one way or the other, and the extent to which, if at all, each witness is either supported or contradicted by other evidence in the case. Inconsistencies or discrepancies in the testimony of a witness or between the testimony of different witnesses, may or may not cause a juror to disbelieve or discredit such testimony. Two or more persons witnessing an incident or a transaction may simply see or hear it differently. In weighing the effect of a discrepancy, however, jurors should always consider whether the discrepancy results from innocent error or from intentional falsehood. Each juror should make her own judgment or assessment concerning the believability of a witness, and then attach such importance or weight to that testimony, if any, that she feel it deserves. If the witness is an informant, someone who provides evidence against someone else for money, or to escape punishment for his or her own misdeeds or crimes, or for other personal reason or advantage, the jurors are instructed to examine and weigh such testimony with greater care than the testimony of a witness who is not so motivated. In other words, the jurors must determine whether the informant''s testimony has been affected by self-interest, or by the agreement he or she has with the government, or by prejudice against the defendant.