According to Westermarck’s widely accepted explanation of the incest taboo, cultural prohibitions on sibling sex are rooted in an evolved biological disposition to feel sexual aversion toward our childhood coresidents. Bernard Williams posed the “representation problem” for Westermarck’s theory: the content of the hypothesized instinct is different from the content of the incest taboo —thus the former cannot be causally responsible for the latter. Arthur Wolf posed the related “moralization problem”: the instinct concerns personal behavior whereas the prohibition concerns everyone. (...) This paper reviews possible ways of defending Westermarck’s theory from the representation and moralization problems, and concludes that the theory is untenable. A recent study purports to support Westermarck’s account by showing that unrelated children raised in the same peer groups on kibbutzim feel sexual aversion toward each other and morally oppose third-party intra-peer-group sex, but this study has been misinterpreted. I argue that the representation and moralization problems are general problems that could potentially undermine many popular evolutionary explanations of social/moral norms. The cultural evolution of morality is not tightly constrained by our biological endowment in the way some philosophers and evolutionary psychologists believe. (shrink)