Certain ethical views hold that we should pay more attention, even exclusive attention, to the worst-case scenario. Prominent examples include Rawls's Difference Principle and the Precautionary Principle. These views can be anchored in formal principles of decision theory, in two different ways. On the one hand, they can rely on ambiguity-aversion: the idea that we cannot assign sharp probabilities to various scenarios, and that if we cannot assign sharp probabilities, we should decide pessimistically, as if the probabilities are unfavorable. On (...) the other hand, they can rely on risk-avoidance: the idea that we should pay more attention to worse scenarios, even when we can assign sharp probabilities. I distinguish these two foundations. I also show how they can be modified to support versions of these views that pay more but not exclusive attention to worst-case scenarios. Finally, I argue that risk-avoidance provides a superior foundation than ambiguity-aversion for the Difference Principle and the Precautionary Principle; in particular, it correctly identifies which ethical facts should matter to those who champion these principles. (shrink)