In the end, most trials boil down to one issue and only one issue—credibility. Therefore, the adverse party may try to subtly, or not so subtly, impeach or reduce the credibility of an expert witness. This is completely proper. This section is designed to teach, in a very basic way, some of the evidentiary rules dealing with an attorney’s assault against an expert’s credibility.
Expert Witness Bias
Although there is no federal rule addressing bias directly, the United States Supreme Court has held that a witness may be impeached on the ground of bias. Judges usually give attorneys wide latitude to show a witness’s potential bias. If the witness being impeached denies the bias, the judge has discretion to allow extrinsic evidence to show the bias. An expert must expect that evidence that may tend to show her bias can and will be held to be admissible because it is relevant to her credibility. Although bias attacks go to the weight, and not the admissibility, of expert opinions, bias attacks can be very effective as bias is something the jury is likely to understand (unlike many other aspects of expert testimony).
As a practical matter, the best way for expert witnesses to deal with bias attacks is to prevent them by only testifying in cases where an actual or perceived bias does not exist.
Excerpted from the SEAK text; The A-Z Guide to Expert Witnessing