Abstract: When strong emotions are involved, people tend to focus on the badness of the outcome, rather than on the probability that the outcome will occur. The resulting “probability neglect” helps to explain excessive reactions to low-probability risks of catastrophe. Terrorists show a working knowledge of probability neglect, producing public fear that might greatly exceed the discounted harm. As a result of probability neglect, people often are far more concerned about the risks of terrorism than about statistically larger risks that they confront in ordinary life. In the context of terrorism and analogous risks, the legal system frequently responds to probability neglect, resulting in regulation that might be unjustified or even counterproductive. But public fear is itself a cost, and it is associated with many other costs, in the form of “ripple effects” produced by fear. As a normative matter, government should reduce even unjustified fear, if the benefits of the response can be shown to outweigh the costs.