The Admissibility of Differential Diagnosis Testimony to Prove Causation in Toxic Tort Cases: The Interplay of Adjective and Substantive Law
Sanders, Joseph, Machal-Fulks, Julie, Law and Contemporary Problems
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. Part III sketches out the admissibility rules developed in Daubert and its progeny. Part IV examines the differential diagnosis opinions in the context of both the causation analysis of Part II and the admissibility rules presented in Part III. Part V attempts to explain differences in the opinions based on causal and non-causal factors. It argues that a key to understanding the developing case law in this area is to appreciate the degree to which courts have adopted the interpretive conventions of science in assessing admissibility.
As we shall see, while some cases simply seem to be wrongly decided, many cases could go either way. These cases offer us an opportunity to examine present judicial views as to how tight a causal chain plaintiffs must present to state a prima facie case in this evolving area of law. The final part offers a few thoughts on the proper balance between causal clarity and the role of the jury in tort cases.
CAUSAL ISSUES IN TOXIC TORT CASES
Cause-in-fact in toxic tort cases is usually thought of as two separate issues: general causation and specific causation.  General causation asks whether exposure to a substance causes harm to anyone. Specific causation asks whether exposure to a substance caused a particular plaintiff's injury. Under traditional tort theory, a successful plaintiff must prevail by a preponderance of the evidence on both issues.  The plaintiff must not only show that, more likely than not, the substance causes the injury in question, but also that, more likely than not, the plaintiff's specific injury was caused by the substance. 
It would be a mistake to argue that the causal issues in toxic tort cases are fundamentally different from those presented in other tort cases. However, toxic tort cases do differ in degree in several significant ways.  First, often there is causal ambiguity. The level of exposure to a substance or a drug--the dose rate--is often uncertain. Evidence of a relationship between the substance and the injury is often uncertain.  The timing between exposure and disease may be suspect.  These difficulties may create both admissibility and sufficiency questions. Second, there is a fundamental problem of multiple causation. Asbestos, the subject of the first great toxic tort case,  is atypical because it causes "signature" diseases. Asbestosis and mesothelioma are diseases so strongly related to asbestos exposure that there is little doubt that a person with these illnesses who has been exposed to asbestos contracted them because of the exposure.  Other substances, however, do not cause unique inj uries, and substances that do cause signature diseases may also cause others.  If, for example, an individual is exposed to asbestos and develops lung cancer, one cannot be certain that the exposure caused the disease. It could be caused by something else, such as cigarette smoking.  Third, and related to the problem of multiple causation, there is often limited evidence of specific causation, that is, evidence that the substance caused the injury to this particular plaintiff.  Plaintiffs may find it difficult to prove that a particular injury was the result of the defendant's substance or another cause.
Courts have provided some solutions to these causal difficulties. The most noteworthy is the use of general causation evidence, such as epidemiological studies, to prove or disprove specific causation.  However, this solution is viable only where there is good evidence on general causation.  Courts have been less willing to entertain alternatives permitting proportionate recoveries based on the probability that a specific injury was caused by a given exposure, or collectivized, risk-based claims in mass exposure cases.  All of these proposed solutions tacitly recognize the difficulty of proving specific causation, and some explicitly attempt to relieve the plaintiff of the burden of proof on this element. Because courts have generally refused to relieve the plaintiff from proving specific causation, differential diagnosis evidence is often a crucial component of the plaintiff's case.  Without some evidence that the substance in question caused the specific injury to the specific plaintiff, co urts are likely to grant the defendant summary judgment. To get this evidence to the jury, the plaintiff must first pass through the Daubert gate. 
DAUBERT, KUMHO, AND THE ADMISSIBILITY OF EXPERT EVIDENCE
The question of when to trust an expert is as old as expert testimony itself.  Prior to Daubert, the most important admissibility opinion was Frye v. United States.  In that case, the defendant, accused of murder, offered the results of a precursor to the lie detector test to prove his innocence.  The court refused to admit the testimony based on the test results until the technique was "sufficiently established to have gained general acceptance in the particular field in which it belongs."  After the adoption of the Federal Rules of Evidence, a number of federal circuit courts abandoned the Frye test.  Some circuits, however, continued to use Frye after the adoption of the Rules. The Ninth Circuit was among this latter group.  In Daubert v. Merrell Dow Pharmaceuticals, Inc.,  Jason Daubert and Eric Schuler sued the defendant, the manufacturer of Bendectin, a morning sickness drug. They claimed that the drug, taken by their mothers during pregnancy, caused the plaintiffs' limb red uction birth defects.  The trial judge granted the defendant summary judgment after concluding that the strongest inference a jury could draw from the evidence was that "Bendectin could possibly have caused plaintiffs' injuries," a finding insufficient to avoid granting the defendant's motion.  On appeal, the Ninth Circuit affirmed.  It held that the plaintiff's expert testimony was inadmissible because its underlying methodology diverged substantially from the procedures and techniques generally accepted in the field. 
The Supreme Court granted certiorari, primarily to announce Frye's demise.  The Court held that, although the Federal Rules of Evidence do not incorporate the Frye test, they do require more than relevance.  Rule 702 requires reliability as well as relevance; testimony that is relevant but unreliable is inadmissible.  This raises the question: What constitutes reliability? In Daubert, where all of the experts purported to be scientists,  the Court turned to science for an answer. Reliable opinions are those reached using the "methods and procedures of science."  The court added that "[i]n a case involving scientific evidence, evidentiary reliability will be based upon scientific validity." 
Though the Daubert court did not outline a systematic presentation of what scientists mean when they inquire about validity, it did offer four factors courts might consider when making a reliability/validity assessment: (1) Whether the expert's theory or technique is falsifiable and has been tested;  (2) the reliability of a procedure and its potential rate of error;  (3) whether the theory or technique has been subjected to peer review  and the results have been published;  and (4) in a partial resurrection of the Frye test, whether the expert's methods and reasoning enjoy general acceptance in a relevant scientific community.  In addition, the Court noted that Rule 702 requires that expert evidence "assist the trier of fact to understand the evidence or to determine a fact in issue."  Justice Blackmun said that "[t]his condition goes primarily to relevance.... The consideration has been aptly described by Judge Becker as one of 'fit.' 'Fit' is not always obvious, and scientific vali dity for one purpose is not necessarily scientific validity for other, unrelated purposes."  The "fit" requirement involves an assessment of whether the expert's chain of reasoning contains an inferential gap that is too wide. 
The Court expressly limited this holding to scientific evidence.  In addition, the opinion clearly stated that the four reliability factors--testability, error rate, peer review and publication, and general acceptance--are not exclusive.  These positions present two related questions to post-Daubert courts: Does Daubert's reliability requirement apply at all to non-scientific evidence and, if it does, what role do the Daubert factors play in these cases?
Many courts concluded that Daubert did apply to non-scientific testimony, but often they could not agree about the proper role of the Daubert factors. Moore v. Ashland Chemical, Inc.  provides an instructive example. In Moore, the plaintiff became ill after he was forced to clean up a spill of solvents inside the back of a truck.  The trial judge excluded the causation testimony of one of his experts, a specialist in pulmonary, environmental, and internal medicine, which stated that one hour of exposure caused the plaintiff to contract reactive airways dysfunction syndrome.  The plaintiff appealed, and the Fifth Circuit reversed, ruling that the exclusion was in error.  It held that, while the Daubert standard applied to all expert evidence, the Daubert factors are "hard" science methods or techniques that should apply only to experts who profess to base their testimony on "hard" science knowledge.  These criteria should not be used to judge the admissibility of a clinical physician's exp ert testimony.  Rather, such testimony should be judged by the principles and methodology of the field of clinical medicine.  After an en banc review, the circuit reversed and reinstated the judgement for the defendant.  The en banc opinion turned primarily on a "fit" analysis. However, it also supported the use of Daubert factors when assessing the admissibility of clinical medical testimony. 
The proper role of the Daubert factors has arisen in other areas where the expert's testimony rested on professional "experience." For example, the issue presented itself with respect to forensic testimony in the criminal context  and products liability design defect testimony.  The Supreme Court finally intervened in this debate in Kumho Tire Co. v. Carmichael.  In July 1993, eight members of the Carmichael family were involved in a serious automobile accident when the right rear tire of their minivan blew out.  After the accident, the plaintiffs' expert examined the tire and concluded that the failure was not the result of any abuse.  He therefore concluded that the failure was caused by a defect in either the tire's design or its manufacture. Prior to his testimony, however, the expert became ill and transferred the case to his employee, Dennis Carlson, who reviewed the file and confirmed the initial conclusion.  Carlson did not personally examine the tire before rendering his opin ion; he first inspected the tire approximately one hour before his deposition. The defendant moved to exclude Carlson's testimony for failing to satisfy Daubert.  The trial judge agreed, finding that "none of the four admissibility criteria outlined by the Daubert court are satisfied in this case."  Because the expert testimony was the plaintiffs' only evidence of defect, the district judge then granted the defendant summary judgment.  The plaintiffs appealed, arguing that the district court should not have applied Daubert's reliability framework because Carlson was not a "scientific" expert. 
On appeal, the Eleventh Circuit undertook a de novo review of the trial court's decision to apply Daubert.  The court concluded that Daubert applies only to scientific testimony, that Carlson's testimony was non-scientific, and that the district court erred as a matter of law in applying the Daubert criteria.  The Eleventh Circuit declared itself prepared to affirm a well-reasoned trial court decision to exclude Carison's testimony on reliability grounds if, upon remand, the trial court did so without invoking the Daubert criteria.  However, in another part of the opinion, the appellate court said that the question in this case is whether Carlson's testimony is based on his application of scientific principles or theories (which we should submit to a Daubert analysis) or on his utilization of personal experience and skill with failed tires (which we would usually expect a district court to allow a jury to evaluate).  This sentence suggests that the court believed that a more lenient admissibi lity standard was appropriate for non-science experts.
The trial court, however, never had a second chance to evaluate the testimony. The Supreme Court granted certiorari, reversed the Eleventh Circuit, and held that excluding Carlson's testimony was not an abuse of discretion.  As to the role of the Daubert factors, the Court adopted a flexible position:
We also conclude that a trial court may consider one or more of the more specific factors that Daubert mentioned when doing so will help determine that testimony's reliability. But, as the Court stated in Daubert, the test of reliability is …
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Publication information: Article title: The Admissibility of Differential Diagnosis Testimony to Prove Causation in Toxic Tort Cases: The Interplay of Adjective and Substantive Law. Contributors: Sanders, Joseph - Author, Machal-Fulks, Julie - Author. Journal title: Law and Contemporary Problems. Volume: 64. Issue: 4 Publication date: Autumn 2001. Page number: 107. © 2009 Duke University, School of Law. COPYRIGHT 2001 Gale Group.
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