JOSEPH SANDERS [*]
JULIE MACHAL-FULKS [**]
In E.I. du Pont de Nemours & Co., Inc. v. Robinson,  the Texas Supreme Court adopted an interpretation of the rules concerning the admissibility of expert testimony nearly identical to that adopted two years earlier by the United States Supreme Court in Daubert v. Merrell Dow Pharmaceuticals, Inc.  The court affirmed the trial judge's exclusion of Dr. Carl Whitcomb, the plaintiff's only causation expert, who was prepared to testify that the defendant's contaminated fungicide damaged the plaintiff's pecan orchard. In a vigorous dissent, Justice Cornyn noted that the expert's testimony was based in part on a series of first-hand observations of the orchard.  He noted that the excluded testimony "is roughly analogous to that which may be offered by a physician, who may testify based on nothing more than a personal examination, the patient's history, and correspondence with other physicians."  Physicians are frequently called upon to offer opinions identifying an injury's cause based both on a physic al examination of a patient and the exclusion of other causes of the patient's condition. When this type of testimony is presented by physicians, it frequently goes by the name of "differential diagnosis," although some courts have more appropriately called it "differential etiology." 
Justice Cornyn may have believed that the expert's testimony in Robinson must be admissible because it was so similar to the typical testimony of many medical doctors. Such testimony had been employed in tort cases for many years without criticism from courts or commentators.  Perhaps, however, Justice Cornyn foresaw the opposite implication: If Dr. Whitcomb's testimony was inadmissible, it might call into question the differential diagnosis testimony of many physicians. Had Judge Cornyn investigated the law review literature more thoroughly, he would have discovered that such questioning had already begun in the late 1980s and early 1990s.  A number of factors seem to have played a role in this change, among them the increasing use of scientific experts in court, the rise of toxic tort actions, and renewed interest in the criteria used to judge the admissibility of expert testimony. As the 1990s progressed, courts were presented with more admissibility challenges to differential diagnosis testimony. T here is now a considerable body of case law on point. 
Most would agree that the result of these challenges is a body of evidence law that creates more barriers to the admissibility of this evidence.  However, there is no complete consensus on the requirements for admitting such testimony. On the contrary, the case law is unsettled in some regards.  This lack of agreement is not surprising, because differential diagnosis testimony attempts to address some very difficult causal questions, especially when offered in toxic tort cases.
In this article, we use the differential diagnosis opinions to explore a pair of interrelationships. First, we are interested in the relationship between admissibility and causation. In this regard, it is important to understand that in many toxic tort cases the center of gravity on causal questions has shifted to an earlier point in the trial. No longer solely a question for the jury, causation is resolved in an in limine hearing before a jury is even empaneled.  The central point is that adjective law  and substantive law do not exist in isolation from each other. Our goal is to shed light on how admissibility decisions shape causal questions and, in turn, how causal principles affect admissibility decisions regarding differential diagnosis. Second, we are interested in the relationship between law and science. Specifically, we argue that the Daubert decision has caused courts to be more "scientific" in assessing the admissibility of such testimony. In Part II, we present the basic causal framework employed by most courts in toxic tort cases. …