Bayesian Approaches to the Precautionary Principle

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A. The Precautionary Principle

In recent years, variations of the Precautionary Principle have been adopted in international environmental agreements and by national regulatory agencies. (1) Despite the apparent increase in its application, the Precautionary Principle remains ill-defined. Generally, it espouses the belief that under conditions of substantial scientific uncertainty environmental regulations should err on the side of caution in order to prevent harm. (2) The Rio Declaration on Environment and Development states that "In order to protect the environment, the precautionary approach shall be widely applied by States according to their capabilities. Where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation." (3) Advocates argue that the Precautionary Principle merely reinforces common sense notions of environmental stewardship. (4) However, opponents view it as a fundamentally unscientific rule of decision that exploits the public's fear of the unfamiliar and promotes radical environmental agendas or protectionist trade policies disguised as environmental regulations. (5)

Advocates further argue that the application of the Precautionary Principle is justified by science's demonstrated fallibility in anticipating environmental harms such as asbestosis and ozone depletion. (6) Additionally some potential environmental hazards cannot be quantified with certainty by existing scientific methods. Thus the Precautionary Principle would allow such harms to be regulated even if conclusive proof of harm has yet to be established. (7) In this sense, the Precautionary Principle may be viewed as a burden-shifting device that places the responsibility of demonstrating a product's or process's safety on those who would introduce it, rather than a demonstration of harm on those who would regulate it. (8) In this vein, some commentators view the Precautionary Principle as a particular policy application of a more general "safety principle," in which people are allowed to weigh harms that they find particularly dreadful more heavily than would be the case under traditional cost-benefit methods. (9) However, environmental regulations entail real costs, both in terms of foregone social benefits and--in a world of limited regulatory resources--foregone environmental protections. (10) Thus, because of these unintended negative consequences, the Precautionary Principle is vulnerable to criticisms directed against those who would take it too far. (11)

The distinction between uncertainty and ambiguity, or what will be referred to in this paper as true uncertainty is important to understanding the scope of the Precautionary Principle. (12) Uncertainty generally refers to situations in which a harm is probabilistic in nature, but for which a probability distribution is known or may be assigned. (13) True uncertainty refers to situations in which even the probability of harm is not known. (14) It is this latter situation with which advocates of the Precautionary Principle are primarily concerned. (15) An example of an uncertain harm might be the probability of a product malfunctioning, especially if that product is subject to testing. Several trials may be performed, to determine the frequency with which a malfunction will occur, which in turn may be used to formulate a probability distribution. Truly uncertain harms often arise when controlled testing is impossible and there is no experience from which to construct a probability distribution. Unfortunately, this is precisely the type of uncertainty--true uncertainty--which is characteristic of many environmental problems. It would be highly impractical, for example, to try to construct a controlled experiment for global warming.

B. …


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