Courts sometimes deal with public health problems where the cause of harm to one individual or a group of individuals cannot be established. In such cases, epidemiology is used to help define a relationship which links the harm and the cause. In mass tort cases, epidemiologic studies are used either to refute or to support claims involving an increased risk of disease from exposure to a toxic substance. Consequently, how to use epidemiology when deciding mass tort cases is becoming an increasingly important question in public health law. Courts use epidemiological evidence to decide cases where a causal connection can be established between the exposure and the outcome. In addition, courts use epidemiology for events that either have no “eyewitness or disproportionately involve certain types of products for which ‘traditional’ forms of evidence of causation are lacking.”
Recently, epidemiology has become a familiar form of proof in mass torts litigation, and those who are considered epidemiologists are often sought as expert witnesses in these cases. However, the necessary evidentiary requirement of epidemiology studies occasionally does not coincide well with the basic principles of causation in tort law. For example, even when presented with overwhelming epidemiological evidence, juries have sometimes returned a verdict for plaintiffs, which indicates that some juries are not convinced by epidemiological evidence.
There are two imperative questions here: (1) how does epidemiology affect mass tort litigation; and (2) what relative weight should the courts give to epidemiological evidence? These questions are particularly significant in the area of causation. In order to establish causation, the plaintiff must demonstrate that it is “more probable than not” that the harm being complained of would not have occurred had the defendant followed the appropriate standard of care. From prior case law, courts have derived rules for causation, namely the “but for” test and the “substantial factor test.” Under the first test, the defendant’s conduct is deemed to be a cause of the harm “if the [harm] would not have occurred but for that conduct.” However, under the second test, the defendant’s conduct is a cause of the harm if that conduct was a substantial factor in producing the harm.
The plaintiff bears the burden of proving causation, which is generally an issue of fact. The plaintiff must introduce support indicating a reasonable basis for the conclusion that the defendant’s conduct was “more likely than not . . . a cause in fact” of the outcome. However, courts do not require the plaintiff to establish the case beyond a reasonable doubt. The plaintiff need not entirely negate the possibility that something other than the defendant’s conduct caused the harm. It is sufficient for the plaintiff to introduce evidence from which a reasonable person may conclude that it is more probable than not that the defendant caused the event. The preceding standard is generally known as the preponderance of the evidence standard, which means that it must be greater than fifty percent. Unlike traditional tort law, which follows the preponderance of the evidence standard, epidemiology relies on statistical significance and is not necessarily based on the greater half of the evidence. In public health litigation, for example, statistical evidence based on aggregate data is sometimes introduced to show that the defendants created a statistically significant increase in the likelihood that the harm would occur.
The distinctions between the standards of proof employed in epidemiology and in law inform the central thesis of this paper. This analysis began by describing the role of epidemiology in mass torts and public health litigation. It later argues that because mass torts cover such a wide area, there are several problems related to epidemiology in litigation, particularly scientific uncertainty and inconsistent factual claims.
Part II discusses recent cases where epidemiological evidence was raised and debated, distinguishing between vaccine-related and non-vaccine-related cases. Courts have differentiated vaccine-related cases from non-vaccine-related cases, principally because Congress enacted a vaccine act designed to compensate victims. In both vaccine and non-vaccine related cases, the legal concepts of specific and general causation are extensively used.
Part III examines the two legal concepts of general and specific causation in epidemiology and how courts have tried to balance the epidemiological causation standard with general torts principles. Part IV analyzes how epidemiological evidence differs from other evidence in terms of the tensions it raises for the legal system, and argues that despite these tensions, courts still hold that causation must be shown by epidemiological evidence. Part V discusses the policy implications of what gets used in court and argues that reliance on human studies, as the best evidence, may be misplaced since one cannot freely experiment on human beings. This section also considers whether epidemiologists should get involved in policy issues, discussing two divergent schools of thought.
The paper concludes by suggesting that although the presence of epidemiological evidence does not necessarily end the inquiry; where the evidence is available, it should be used only if the evidence meets a heightened standard. The heightened standard argued for in this paper is a screening standard for admission that considers not only a doubling of the risk by the exposure, but also jury instructions that clearly inform the jury of the strengths and weaknesses of epidemiological studies. The paper also calls for the American College of Epidemiology and the Council for State and Territorial Epidemiologists (“CSTE”) to develop model guidelines for the use of epidemiological evidence in the courtroom. These guidelines could mirror the public health law bench books developed 31for some states to refer to during public health emergencies.
23 St. Thomas L. Rev. 157