[In the last edition, we saw opinions in the poskim regarding the testimony of one who was paid for the testimony by one of the litigants. Now we will address an issue that is common in contemporary litigation: a professional’s ‘testimony’ on a matter in which he has particular expertise.]

In general there are two possible elements to expert testimony. One, which we discussed last time, is when the person testifies factually about that which he observed. His unique role is in the fact that he spent time and used expertise to receive the information. However, experts are often brought, not to present facts, but to give their professional opinion on facts presented by others. Here they are needed not just to not lie but to be trustworthy of giving an objective appraisal, which is much more difficult to ensure.
The distinction between facts and appraisals finds expression in a related area of halacha. A witness may testify about a non-relative with whom he has a significant relationship unless he has a more direct personal interest in the case. However, the relationship is enough to disqualify one from serving as a dayan. The S’ma (33:1) explains the distinction as follows. Regarding testimony, we are not concerned that he will lie. Regarding judgment, that "which depends on logic and thought … changes due to friendship and enmity, even without evil intention."
The type of expert testimony we are discussing involves "logic and thought" similar to that of a judge. For this reason we often find a huge gulf between the witnesses brought by the different sides to the dispute. Therefore, beit din does not treat expert witnesses brought by a litigant as proof. [Editor’s note – There is another distinction. It is difficult to prove that an appraisal, where there is a range of legitimate viewpoints, was purposely fallacious. Furthermore, on a matter where one does not have to see anything in order to testify, it is possible for a litigant to ‘shop around’ until he finds the witness whose (even honest) opinion is close to his.]
The proper way for beit din to act in cases where appraisals are needed in fields that are beyond the dayanim’s expertise is for beit din to choose the expert. We have precedent in halacha for such a concept, in regard to deciding the value of property which will be used as payment for a debt, where three appraisers are called upon to decide (Choshen Mishpat 103). Because there is an element of opinion there, specifically three opinions are elicited. Practically, since it is usually prohibitively expensive to involve three professionals, beit din usually uses one expert witness. [Editor’s note- beit din sometimes encourages the sides to decide among themselves on an expert who is acceptable to both, as long as they do not speak to him independently.] To help balance the matter, the sides are given the opportunity to question the expert and raise questions about his opinion. If the sides want, each can hire his own experts. While they will not be trusted as objective, they can help beit din consider the matter from different viewpoints.