Technological risk and small probabilities

Abstract

Many scientists, businessmen, and government regulators believe that the criteria for acceptable societal risk are too stringent. Those who subscribe to this belief often accept the view which I call “the probability-threshold position.” Proponents of this stance maintain that society ought to ignore very small risks, i.e., those causing an average annual probability of fatality of less than 10⁻⁶.

After examining the three major views in the risk-evaluation debate, viz., the probability-threshold position, the zero-risk position, and the weighted-risk position, I focus on the arguments for the first of these views, since it is the position which currently undergirds most public policy (especially in the U.S.) regarding acceptable risk. After analyzing Arrow's argument from decision theory, Comar's and Gibson's argument from ontology, and Starr's and Whipple's argument from epistemology, I conclude that these defenses of the probability-threshold position err in a variety of ways. Most commonly, they fail because they tacitly accept the assumption that magnitude of probability, alone, provides a sufficient condition for judging the acceptability of a given risk. In the light of these errors, I suggest that it might be more desirable for risk assessors, decision theorists, and policymakers to weight various risk-cost-benefit parameters according to alternative ethical criteria, rather than to evaluate risks solely in terms of mathematical considerations.