Scholars suggest that evolutionary psychology may provide a foundation for assumptions regarding human values. I explore this suggestion by developing two arguments regarding the permissiveness of norms regulating male and female sexual activity. The first relies on the standard rational choice assumption that people value resources, and the second relies on an assumption suggested by evolutionary psychology that actors value seeing their children successfully reach adulthood. These two assumptions produce contrasting predictions regarding sex norms. I describe the implications of these predictions for explaining cross-cultural variation and present evidence that supports the evolutionary psychology-based predictions in this context. I also suggest implications of the two approaches for explaining norms cross-nationally and within the United States. The article provides support for the utility of evolutionary psychology in developing assumptions about values.