The "Professional Witness"

"An expert who is a "professional witness" is not necessarily biased. Courts most often use this term to describe an expert who "spends substantially all of his time consulting with attorneys and testifying." Independent of any bias such an expert might have, spending all of his time testifying will inevitably erode his scientific skills and knowledge, and he will become less qualified to opine on the subject of his (former) expertise. A court rightly might refuse to admit the testimony of such a witness because "he' is more a professional witness than an expert." Strictly speaking, though, the court would not be rejecting the witness's testimony itself; the court instead would be deciding that the proposed witness is not qualified as an expert at all.

No issue of bias necessarily exists here, at least on the part of the witness. It is true that if a "professional witness" is one who lacks real scientific competence, he may be more likely to testify for the party whose position is less scientifically accepted, because that party will find it more difficult to hire competent experts. That does not show that the witness is biased, though, or that his testimony is false. It instead might be that "professional witnesses" can be found with a wide range of views, so that the only bias in their use is in the universal selection by parties of witnesses that will support their positions." - From Conflicts of Interest in Scientific Expert Testimony by Mark R. Patterson

Enjoy.