conventional disputes of all kinds, most of which have little or no “technical” content. In general, a trial is an educational and inferential process in which evidence is heard, weighed against other evidence, and applied to the circumstances of the case to determine a verdict.

Admitting scientific and other expert evidence that is unfamiliar to the average juror presents a fundamental shift in this paradigm, stated one participant. Further, noted the participant, there is a sharp difference between “conventional” evidence, to which non-experts on a jury may add personal life experiences that aid them in deliberations, and more specialized scientific evidence that “can only be deferred to.” One danger seen by this participant is that judges, knowing that jurors may defer to evidence or opinions they do not understand, may be excessively rigorous in excluding expert evidence in order to “protect” jurors.


As much as courts would like to winnow out unsound evidence, however, the task is not an easy one. As one lawyer emphasized, “all evidence is incomplete, and it’s always going to be.” To require absolute reliability in evidence is to expect an elusive and “non-existent purity in science” where virtually all scientific data might fail the most rigorous tests of reliability and relevance in some degree. This may be inherently unfair to whoever has the burden of proof. In toxic tort cases, the plaintiff has the burden of proof, and a court may question the fairness of asking a plaintiff to suffer because of scientific uncertainty. The lawyer added that virtually all scientific data might fail the most rigorous tests of “reliability” and “relevance” in some way. From a scientific standpoint, it should be apparent that a legal structure that requires the defendant to prove a negative would also be problematic.

Other participants discussed the understandable desire on the part of courts for certainty and “bright lines” by which to decide on the admissibility of evidence. They said it is unfair to require a higher standard of accuracy from expert witnesses in the courtroom than exists in the world of science. Other participants noted that it is unfair to admit evidence that would not stand up to the scrutiny of the scientific community.


Courts may lack sufficient knowledge about a given chemical, disease agent, or other issue. The presence or absence of a valid scientific study may by itself determine a verdict in the courtroom. One scientist described a “decision tree” by which to explore the factors that motivate groups to fund certain studies and not others.

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