The Scientific Basis of Causality in Toxic Tort Cases
Conclusion
The requisite evidence in a toxic tort case regarding causation consists of scientific knowledge, which is the collective result of individual scientific experiments as memorialized in peer-reviewed scientific journals and other appropriate written repositories, and appropriate inductive generalizations and deductive inferences. In contrast to what is true of individual scientific studies, the degree of certainty of an inductive inference based on the studies cannot be stated in mathematical terms because it is a judgment. Thus, the inference that “cigarette smoking causes cancer” cannot be stated with numerical precision, although the results of particular experiments involving animals or human subjects exposed to cigarette smoke under particular conditions can and must be quantified to constitute part of the corpus of scientific knowledge.
The expert must possess training and experience sufficient to enable him to analyze and explain the laboratory and epidemiological studies pertinent to the effects of the toxic agent on cells, animals, and human beings. The training ordinarily expected of such an expert includes a Doctor of Philosophy degree in science, because it is evidence that the witness received the highest level of formal training in scientific methodology and reasoning. Scientific expertise is best evidenced by performance of experiments involving the toxic agent and subsequent publication of the results in the scientific literature. If the witness earned a Ph.D. in science and performed many experiments and published many scientific articles involving the biological effects of the toxic agent involved in the case, the court would have a firm basis to regard the witness as qualified to offer opinions in the case.
The amount of the toxic agent present in particular scientific studies in relation to the amount actually received by plaintiff is always a fundamental issue regarding whether the toxic agent caused plaintiff’s disease. Thus, the expert’s training and experience must also include knowledge of the laws or principles that govern the dosimetry of the toxic agent.
Unlike the physician or the technical expert, who is permitted to testify on the basis of professional status and anecdotal evidence, the toxic tort expert must testify on the basis of scientific knowledge. Consequently, there cannot be a scientific expert in the absence of scientific knowledge. The customary process for evaluating the validity of scientific knowledge is peer review, whereby anonymous peers judge the merits of scientific experiments or studies and the results are offered for publication. The peer-review process does not attempt to establish whether the results are the product of good science or bad science. Peer review is not a litmus test for truth, validity, or general acceptance. Rather, it is the standard assessment procedure that precedes the addition of a particular study to the corpus of scientific knowledge. The expert in a toxic tort case should ordinarily rely on peer-reviewed reports to perform analyses and reach conclusions.
Peer review is an indication of the acceptability of a scientific report with regard to its intrinsic validity. A further question extends to the extrinsic validity of a scientific study designed, conducted, or otherwise influenced by one of the parties in the litigation, or by someone in privity with these parties. An expert witness who chooses to rely upon particular scientific reports, therefore, has a responsibility to make a reasonable effort to establish their extrinsic validity. The court, opposing counsel, and the jury should also consider the extrinsic validity of the reports relied upon by the expert when determining whether the expert’s testimony is admissible, or how it should be weighed.
Blue-ribbon committees should enjoy no presumption regarding the qualifications or validity of their work product because both the membership and work product of such committees are invariably shaped by the appointing authority. The committee’s decisions represent a consensus of the persons recognized as experts by the appointing authority, but those decisions bear no necessary relation to a consensus of all qualified experts. Consequently, the reports of blue-ribbon committees should not be admissible as evidence of the truth of the propositions they recite.
Scientific studies potentially available for consideration by the toxic tort expert are test-tube, animal, and epidemiological studies, and each has particular strengths and weaknesses. The scientifically and ethically preferable data is that obtained from animal studies. Although the expert must always consider the limitations of animal or test-tube studies with regard to inferring causal relationships in human subjects, it is unquestionably correct, under the proper circumstances, to use such data to form causal inferences.
The expert in a toxic tort case must rationalize an assertion that the plaintiff’s dosage of the toxin and the plaintiff’s disease were causally related, and not merely associated with each other. The expert cannot rely on personal observation as the basis of a cause-and-effect relationship because the onset of human disease in a particular person is simply not amenable to direct observation. Since an expert cannot rationalize a cause-and-effect relationship based on direct observation, the causal conclusion must be derived from scientific evidence, namely, an appropriate and reliable corpus of scientific data that permits the expert to infer what happened within the plaintiff’s body as a consequence of exposure to the toxic agent.
The logic of scientific reasoning constrains the order in which the expert must approach a decision whether the plaintiff’s exposure to the toxic agent and his subsequent disease were causally related or merely associated. The expert must first consider the pertinent laboratory and epidemiological studies, and determine whether the toxic agent caused the effects reported in the studies. Thus, the seminal question is whether the toxic agent can cause the plaintiff’s disease, given the available information that characterizes the effects the agent is capable of causing. The expert’s opinion must be based on the strength of the scientific studies. No causal assertion in science is certain, and only some causal assertions can be expressed to a numerical degree of certainty with numerical precision. The can cause conclusion of the expert is not the result of a particular experiment, and therefore must be stated using qualitative terms such as “possible,” “likely,” or “nearly certain.”
If the expert sustains the burden of showing that “X can cause Y,” the question then arises whether the amount of toxic agent the plaintiff experienced probably caused his disease. The expert must show that the plaintiff’s exposure to the toxic agent was greater than that ordinarily received by persons who do not develop the plaintiff’s disease. Assuming the plaintiff was not exposed to any other agent that can also cause the plaintiff’s disease, and that the plaintiff’s exposure to the toxic agent occurred at high levels of the toxic agent, an expert could justifiably conclude that, although the possible causative role of unknown factors cannot be eliminated, the existence of only one known sufficient cause makes it likely that the single known risk factor present was the actual cause of the plaintiff’s disease.
The court has a gatekeeping role regarding the admission of scientific evidence. Before the expert’s conclusions may be presented to the trier of fact, the court must be satisfied that it is reasonably possible that the conclusions are true. In making its determination, the court’s focus must be on the expert’s decision making process. Was the conclusion based on controlled observations of nature published in the peer-reviewed scientific literature? Were the applicable principles of scientific inference properly applied to the scientific data? If the court answers both questions affirmatively, it is reasonably possible that the testimony is true, and therefore is reliable.
The scientific studies and reports that form the basis of the expert’s opinion are exceptions to the hearsay rule as they relate to scientific data, but not as they relate to opinion. The scientific expert may not invoke the opinion of another scientist or a blue-ribbon committee, either in support of or in lieu of the expert’s own analysis. The representations of opinions are not acceptable as a substantial part of the basis for an expert to testify for or against the truth of the proposition that “X can cause Y.”
An expert in toxic tort cases carries a heavy burden. In the face of cross-examination and direct testimony by opposing witnesses, the expert must choose valid scientific data on which to rely, and form his principal inductive and deductive opinions thereon in a lucid and credible manner so as to be understood and believed by the lay trier of fact. It is unlikely that the trier of fact will be swayed by an expert who testifies in a manner that is inconsistent with the scientific facts when the adverse party is represented by able counsel.