 A brief survey of some of the traits that are likely biological adaptations in a number of species—such as a tendency to kill ‘step-children’ or to philander—ought to convince us that even if similar tendencies are likewise adaptations in human beings, present in us for similar evolutionary reasons, this would be no argument at all that we ought to engage in such behaviors. Clearly the burden would be on the prescriptive evolutionary ethicist to explain why a trait’s having been good at propagating the alleles coding for it tens or hundreds of thousands of years ago should be thought to have anything at all with the rational or moral justification of our embracing it. And the prospects for meeting this burden have never seemed strong: it is hard to see how such evolutionary facts can possibly have normative authority or force for a rational agent (McDowell 1995). Regardless of why one has a given trait, the question for a rational agent is always: is it right for me to exercise it, or should I instead renounce and resist it as far as I am able? (Korsgaard  2006).[9]