Experts, Mental States, and Acts

In my book Proving the Unprovable, I discussed at length the considerations that might govern the admissibility of expert psychiatric and psychological testimony in criminal and quasi-criminal cases. This evidence law symposium gives me the opportunity to elaborate on one particular thesis of that book—that the definition of expertise in the criminal justice system, derived in the federal courts and in most states from Daubert v. Merrell Dow Pharmaceuticals Co., should vary depending on whether the issue involved is past mental state or past conduct. I will also briefly discuss whether that proposition should be limited to expert testimony introduced by defendants in criminal trials, or whether instead it should be expanded to use of experts by the prosecution and by civil litigants. Of course, assessing the admissibility of expert testimony involves more than the reliability inquiry associated with Daubert. Such evidence must also be material (logically related to the substantive issue in question), helpful (in the sense of incrementally improving the factfinder’s understanding of the issue), and not substantially likely to be prejudicial or misleading (because, for instance, the evidence is more likely to lead the factfinder to consider immaterial issues). In the following discussion I generally ignore these other admissibility criteria and focus on the reliability factor addressed in Daubert (or what I have called the probative value inquiry), as it applies to expert testimony on past mental states and past acts.