Group to Individual (G2i) Inference in Scientific Expert Testimony
A fundamental divide exists between what scientists do as scientists and what courts often ask them to do as expert witnesses. Whereas scientists almost invariably inquire into phenomena at the group level, trial courts typically need to resolve cases at the individual level. In short, scientists generalize while courts particularize. A basic challenge for trial courts that rely on scientific experts, therefore, concerns determining whether and how scientific knowledge derived from studying groups can be helpful in the individual cases before them (what this Article refers to as “G2i”). To aid in dealing with this challenge, this Article proposes a distinction between two types of expert evidence: framework evidence that describes general scientific propositions and diagnostic evidence that applies the general propositions to individual cases. It then examines the evidentiary implications of that distinction. Most importantly, admissibility standards for expert testimony should differ depending on whether experts are proffering framework or diagnostic evidence. Judicial analysis of “fit,” expert qualifications, testability, error rates, peer review, general acceptance, helpfulness, and other traditional admissibility criteria for expert evidence will often vary, sometimes significantly, based on this distinction. The Article provides general guidelines about the best practices judges should follow in sorting through these considerations. These guidelines will permit courts to manage G2i inferences in a more informed and coherent way than they do currently.