Who is an Expert?

The distinction between expert witnesses and lay witnesses, as defined by the Federal Rules of Evidence, is likely far more complicated than most people realize. Despite their typical presentation on television, experts are not always doctors, scientists, or professors. As Rule 702 states, an expert can testify based on the expert’s own scientific, technical or other specialized knowledge. Given the last clause, an expert is not confined to a specific set of professional qualifications. For example, an expert can be a baseball player who has played and/or coached at a high level and is able to use his knowledge of the game to clarify a material fact of the case for the jury. As demonstrated by Marisa Tomei’s character in the 1992 movie My Cousin Vinny, an expert can even be an ordinary person with no credentials, whose knowledge is perhaps obtained from a specialized hobby such as restoring old cars, as long as the person can prove that he or she has the knowledge required to testify to the particular subject matter and can assist the jury.

Of course, on the flip-side, Rule 701 confines the testimony of a lay witness to that witness’s own perception of a fact or set of facts at issue and the witness will not be allowed to use scientific, technical or specialized knowledge to speculate on these facts. Put more simply, lay witnesses can only report something that they have perceived by using their five senses, whereas expert witnesses typically do not perceive facts firsthand, but instead either offer an informed opinion as to a material element of the case, or help to clarify certain technical elements of the case for the jury.

By the way, what constitutes an appropriate basis for an opinion is the object of much focus by the appellate courts, state and federal. An opinion must be based upon reliable principles and methods. See Guide to Massachusetts Evidence, Section 702; See also Commonwealth v. Lanigan, 419 Mass. 15, 24-27 (1994).