During the past century, social scientists have documented many cross-cultural sex differences in personality and behavior, quite a few of which now appear to be found in all human societies. However, contrary to most scientists’ expectations, these so-called universal sex differences have been shown to be more pronounced in Western industrial societies than in most non-Western developing societies. This chapter briefly reviews the evidence bearing on these findings and offers a biologically based theory that could help shed light on why cross-cultural sex differences exist. The following hypothesis is offered: The expression of many genes influencing sexually dimorphic traits is more likely among descendants of couples who are least closely related to one another. If so, societies in which out-marriage is normative (i.e., Western industrial countries) will exhibit a stronger expression of genes for sexually dimorphic traits compared to societies in which consanguineal marriages are common.