The key problem with the use of expert witnesses is that the jury is asked to determine the credibility of the expert testimony presented. In the typical case, plaintiff and defendant both have experts whose testimony conflicts. The conflict may be subtle, or simple and direct: the plaintiff’s experts in the breast implant litigation say that silicone causes autoimmune disease and that women with implants have a high incidence of autoimmune problems. The defense says it does not and that women with implants have no more problems with autoimmune diseases than does a control group. The jury, of course, has been selected to ensure that no jury members have any special knowledge of medical epidemiology or autoimmune disease. They must evaluate highly technical, conflicting testimony to determine billions of dollars in liability.

Jurors cannot evaluate the scientific merits of such testimony. They have no alternative but to judge the testimony of expert witnesses based on the personal credibility of the witness. Positive factors such as academic degrees, specialty board certification, and publications influence credibility. So do factors such as physical appearance, race, gender, command of English, and personality. For an expert witness, the foremost qualifications are effective presentation and teaching ability. The expert must educate the jury in the technical matter at hand, just as he or she might educate an undergraduate physiology class. The objective is to convince the jurors that they understand the technical issues. Once there is a perception of understanding, the expert can convince them that they are making an independent decision that his or her testimony is correct, rather than just agreeing with him or her.

The risks of this system are that scientifically or medically unsound evidence, presented by credible-appearing witnesses, can supplant proper evidence. [Huber PW. Galileo’s Revenge: Junk Science in the Courtroom. New York: Basic Books; 1991.] There are legal rules that establish the standards that judges are to use in determining whether an expert should be allowed to testify and the extent of that testimony. Judges are charged with ensuring that this does not happen by excluding the testimony of experts who are not properly qualified, or whose testimony goes against known scientific principles. [Federal Rules of Evidence Rule 104, 28 U.S.C.A.] Judges, however, are not any more scientifically knowledgeable than most jurors, and are subject to the same misunderstandings as jurors.