Common law, based on canon law, disqualified a person from testifying on account of bias or interest in the outcome of the case. It was one of several grounds for disqualification--the others being insanity, infancy, infidelity and infamy. Today, by and large, the rules on competency have been converted into rules of credibility, whereby nearly every proposed witness is allowed to testify, but their credibility may be attacked on cross-examination. Quite often, cross-examination ends up being a smearing process, and, as a result, many experts are reluctant to testify or they charge large fees.
In the second quarter of the 19th century, U.S. physicians became increasingly involved in medicolegal matters. After 1840, physicians exercised growing influence in the disposition of estates. They established a major role for themselves in the adjudication of wills where they were offered fees much higher than those offered in other medicolegal proceedings. The power of physicians to overturn wills through retroactive rulings of testamentary incompetence eventually became so great that several states restricted what they regarded as an "epidemic of contests of wills." Today, psychiatrists mainly serve as expert witnesses in personal injury (including malpractice) and criminal cases (Mohr, 1993).
Unlike lay or fact witnesses, experts are compensated for their testimony--experts like to say they are paid for their work on the case, not for their testimony. The fees paid to an expert must be "reasonable." Moreover, the fee may not be contingent upon the outcome of the case, so as to avoid inducement to testify falsely. In fact, legislation in some states makes testifying on a contingency fee basis a misdemeanor. Furthermore, experts retained on a contingency basis violate ethical guidelines, and any member of the profession learning of it has an obligation to report them to the board of medicine. Some argue that in many cases, the ban on expert contingency fees deprives litigants of the services of a competent expert. As for consulting firms involved in finding expert witnesses for litigants, a number of courts have ruled that paying such firms on a contingency fee basis is also improper (First National Bank of Springfield v Malpractice Research, Inc., 1997).
It would be an unconstitutional taking of property to require an expert to provide opinion testimony without compensation--nonetheless, jurors tend to look upon expert witnesses as "hired guns."
The courts have made rulings to counter this viewpoint. At one time, as the Illinois Supreme Court indicated in 1909, expert witnesses could not be questioned with regard to the number of occasions on which they had previously testified for a given category of party (e.g., plaintiffs or defendants) (McMahon v Chicago City Railway Co., 1909). Several other decisions of the Illinois Supreme Court indicated that experts should not be questioned regarding compensation received for testifying in cases unrelated to the parties or their attorneys.
In 1988, the Illinois Supreme Court revised these decisions restricting questioning about prior testimony or fees (Trower v Jones, 1988). As a consequence, the qualifications of the expert may now be challenged. In federal courts, testifying experts must now submit a list of all prior cases within the past four years in which they provided testimony and their current compensation (Federal Rules of Civil Procedure, undated). They must also provide a signed written statement summarizing their opinion and the basis for that opinion along with their curriculum vitae. The report is admissible as evidence.