Expert Psychological Testimony, Forms of

Expert evidence comes to court in a variety of forms and, in particular, at a couple of levels of generality. It often involves general research findings that, although relevant to a particular case, also transcend that case. General research findings will sometimes be used in the establishment of applicable law and decided by judges but, more typically, will be considered a component of the fact-finding duties of triers of fact. A large part of expert evidence, however, involves factual issues that are specific to particular cases, though they may be claimed instances of more general phenomena. Courts and scholars have proposed various models to account for the several ways in which expert evidence is manifested in court. The three most influential of these models are considered here.

To be admitted, expert opinion must be relevant to a material fact in dispute. This basic requirement ties proffered expertise to the substantive law of the case. Hence, for example, if applicable law requires that a substance be proven to have caused the plaintiff's injury, expert proof regarding causation will be relevant; and if this proof is reliable and valid, it will usually be admitted. But proving simple causation of this sort can be a complicated matter. The plaintiff must first prove that the substance in question sometimes does cause the injury and, moreover, that it did cause the injury in this case. As this example illustrates, the facts in dispute in a legal case can appear at a couple of levels of generality. This fact has great relevance both to how and whether experts testify and the procedural legal response to proffers of different forms of expert opinion.

The recognition that facts arrive in court in different forms has spawned several scholarly attempts to impose some schematic theme on them. Three frameworks, in particular, have received considerable attention and are discussed below. These are (1) Kenneth Culp Davis's distinction between legislative facts and adjudicative facts; (2) John T. Monahan and Laurens Walker's threepart division of social authority, social frameworks, and social facts; and (3) post-Daubert courts' differentiation between general causation and specific causation.