Abstract: Evolutionary explanations for behavioral findings are often both fascinating and plausible. But even so, they do not establish that people are acting rationally, that they are not making mistakes, or that their decisions are promoting their welfare. For example, present bias, optimistic overconfidence, and use of the availability heuristic can produce terrible mistakes and serious welfare losses, and this is so even if they have evolutionary foundations. There might well be evolutionary explanations for certain kinds of in-group favoritism, and also for certain male attitudes and actions toward women, and also for human mistreatment of and cruelty toward nonhuman animals. But those explanations would not justify anything at all. It is not clear that in Darwinia (a nation in which departures from perfect rationality have an evolutionary explanation), policymakers should behave very differently from in Durkheimia (a nation in which departures from perfect rationality have a cultural explanation).