Differential Etiology: Inferring Specific Causation in the Law from Group Data in Science

In every toxic-tort case, the plaintiff must prove that the defendant exposed the plaintiff to something that caused an injury. The causal proof is in two parts: proof of general causation and proof of specific causation. General causation addresses the question of whether the exposure can cause injury in anyone. In the area of toxic torts, the evidence available to answer this question comes in the form of group-based studies in the fields of toxicology, epidemiology, and genetics—studies that search for the effects of causes.

Proof of general causation, however, is not enough. Because court cases focus on the individual, the plaintiff must prove specific causation. The plaintiff must show by a preponderance of the evidence that the exposure—and not something else—caused the plaintiff’s harm. This typically is done through expert “differential etiology” testimony. This testimony is not focused on the search for the effects of causes but rather for the causes of effects.

Unfortunately, there is no body of science to which experts can turn when addressing this issue. Ultimately, much of the evidence that can be brought to bear on this causal question is the same group-level data employed to prove general causation. Consequently, the expert testimony often feels jerry-rigged, an improvisation designed to get through a tough patch. Court opinions that rule on the admissibility of this testimony rarely offer any systematic guidance about what is and is not an adequate differential etiology.

Too often lawyers and courts have treated differential etiology as a matter of logical deduction and not as what it actually is: an inferential process combining statistical reasoning with a conceptual model of the causal interrelationships underlying observed data. This Article seeks to bring clarity to the differential-etiology determination by offering a scientifically oriented exposition of differential etiology. It provides a taxonomy of specific-causation situations and a systematic discussion of the factors that courts should consider in making a specific-causation determination. Our goal is to assist lawyers and judges in reasoning from group data to individual cases, what is referred to in the literature as the G2i problem.