In practice, the Precautionary Principle is widely thought to provide concrete guidance. How can this be? I suggest that the principle becomes operational if and only if those who apply it wear blinders – only, that is, if they focus on some aspects of the regulatory situation but downplay or disregard others. But this suggestion simply raises an additional question: What accounts for the particular blinders that underlie applications of the Precautionary Principle? When people's attention is selective, why is it selective in the way that it is? I believe that much of the answer lies in an understanding of behavioral economics and cognitive psychology. Five points are especially pertinent:

• the availability heuristic, making some risks seem especially likely to come to fruition whether or not they actually are;

• probability neglect, leading people to focus on the worst case, even if it is highly improbable;

• loss aversion, making people dislike losses from the status quo;

• a belief in the benevolence of nature, making man-made decisions and processes seem especially suspect;

• system neglect, understood as an inability to see that risks are part of systems, and that interventions into those system can create risks of their own.

Politicians and interest groups exploit the underlying mechanisms, driving public attention in one or another direction. And taken together, these mechanisms show the sense in which the relevant blinders are not arbitrary or coincidental. They have an unmistakable structure.