Can a witness be cross-examined on the issue of credibility?

The mental illness or emotional instability of a witness can be relevant on the issue of credibility, and a witness may be cross-examined on that subject, if the illness affects the witness's ability to perceive, recall or describe the events in question. (People v. Herring (1993) 20 Cal.App.4th 1066, 1072; People v. Anderson (2001) 25 Cal.4th 543, 608 (conc. opn. of Kennard, J.).) "The Sixth Amendment to the Constitution guarantees the right of an accused in a criminal prosecution 'to be confronted with the witnesses against him.' This right is secured for defendants in state as well as federal criminal proceedings . . . . [] . . . [] Cross-examination is the principal means by which the believability of a witness and the truth of his testimony are tested. Subject always to the broad discretion of a trial judge to preclude repetitive and unduly harassing interrogation, the cross-examiner is not only permitted to delve into the witness' story to test the

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witness' perceptions and memory, but the cross-examiner has traditionally been allowed to impeach, i.e., discredit, the witness." (Davis v. Alaska (1974) 415 U.S. 308, 315-316.)