Ever wonder what distinguishes a “rebuttal” expert and from an improperly, untimely disclosed expert?

The issue typically arises like this. One party discloses an expert by their expert disclosure deadline. The opposing party then highlights the deficiencies in the first expert’s testimony typically through some combination of an opposing expert report and a related motion (Daubert motion or summary judgment motion). Before the trial court takes action with respect to the allegedly deficient expert, the party that disclosed the deficient expert makes a further expert disclosure – characterized as a “rebuttal” expert.

Keeping in mind that ultimately the question of whether an expert disclosure will be permitted, regardless of how denominated, is a discretionary question for the trial judge, the “rule” that determines whether such supplemental or rebuttal disclosures are permissible is fairly straightforward. “Rebuttal” testimony cannot be offered in a party’s case-in-chief, and rebuttal testimony cannot be used simply to bolster the testimony of the expert whose testimony is offered during the case-in-chief. Thus, for example, a plaintiff whose original expert indeed is deficient ordinarily cannot avoid the result of a deficient expert disclosure through the use of a “rebuttal” expert. The same would be true for a defending party who required expert testimony to establish an affirmative defense. Rather, rebuttal testimony may only respond to evidence offered by the defendant, and the scope of rebuttal testimony is limited to that which is directed to rebut new evidence or new theories proffered in the opponent’s case-in-chief.

Thus a party that discover serious deficiencies with their disclosed experts should seriously consider promptly seeking leave to make a late disclosure or supplement their original expert’s opinion rather than simply attempting to rely on a “rebuttal” expert.