148
Breyer, J., concurring
uncertain, or where testimony about general risk levels in human beings or animals is offered to prove individual causation. Yet, as amici have pointed out, judges are not scientists and do not have the scientific training that can facilitate the making of such decisions. See, e. g., Brief for Trial Lawyers for Public Justice as Amicus Curiae 15; Brief for New England Journal of Medicine et al. as Amici Curiae 2 ("Judges . . . are generally not trained scientists").
Of course, neither the difficulty of the task nor any comparative lack of expertise can excuse the judge from exercising the "gatekeeper" duties that the Federal Rules of Evidence impose—determining, for example, whether particular expert testimony is reliable and "will assist the trier of fact," Fed. Rule Evid. 702, or whether the "probative value" of testimony is substantially outweighed by risks of prejudice, confusion or waste of time, Fed. Rule Evid. 403. To the contrary, when law and science intersect, those duties often must be exercised with special care.
Today's toxic tort case provides an example. The plaintiff in today's case says that a chemical substance caused, or promoted, his lung cancer. His concern, and that of others, about the causes of cancer is understandable, for cancer kills over one in five Americans. See U. S. Dept. of Health and Human Services, National Center for Health Statistics, Health, United States 1996-97 and Injury Chartbook 117 (1997) (23.3% of all deaths in 1995). Moreover, scientific evidence implicates some chemicals as potential causes of some cancers. See, e. g., U. S. Dept. of Health and Human Services, Public Health Service, National Toxicology Program, 1 Seventh Annual Report on Carcinogens, pp. v-vi (1994). Yet modern life, including good health as well as economic well-being, depends upon the use of artificial or manufactured substances, such as chemicals. And it may, therefore, prove particularly important to see that judges fulfill their Daubert gatekeeping function, so that they help assure that the powerful engine of tort liability, which can generate
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