Abstract: The precautionary principle has been highly influential in legal systems all over the world. In its strongest and most distinctive forms, the principle imposes a burden of proof on those who create potential risks, and it requires regulation of activities even if it cannot be shown that those activities are likely to produce significant harms. Taken in this strong form, the precautionary principle should be rejected, not because it leads in bad directions, but because it leads in no directions at all. The principle is literally paralyzing - forbidding inaction, stringent regulation, and everything in between. The reason is that in the relevant cases, every step, including inaction, creates a risk to health, the environment, or both. This point raises a further puzzle. Why is the precautionary principle widely seen to offer real guidance? The answer lies in identifiable cognitive mechanisms emphasized by behavioral economists. In many cases, loss aversion plays a large role, accompanied by a false belief that nature is benign. Sometimes the availability heuristic is at work. Probability neglect plays a role as well. Most often, those who use the precautionary principle fall victim to what might be called "system neglect," which involves a failure to attend to the systemic effects of regulation. Examples are given from numerous areas, involving arsenic regulation, global warming and the Kyoto Protocol, nuclear power, pharmaceutical regulation, cloning, pesticide regulation, and genetic modification of food. The salutary moral and political goals of the precautionary principle should be promoted through other, more effective methods.