The nurture of nature: why physical and psychological differences between the sexes are greater in healthier, wealthier societies.

Men and women differ morphologically, physiologically, cognitively, psychologically and behaviourally, and many of these differences are getting larger. We propose a synthesis of two disparate yet related mechanisms in combination with sexual selection theory to explain this divergence of the sexes. In large part through sexual selection, males and females have evolved many trait differences, some subtle, others stark. In stressful environments (e.g. frequent nutritional shortfalls, disease risk), those differences are attenuated because in the sex where the trait is greater (such as height in men, or various forms of memory in women) the effects of those stressors are more apparent. Societies exposed to these stressors have more restrictive social mores and harsher criminal punishments for norm violations, restricting the behavioural expression of individuals and hence sex-based preferences (e.g. working with people vs. things). It follows that reduction in ecological threats (e.g. pestilence) and out-group social threats (e.g. warfare) leads to a lessening of in-group social restrictions resulting in more self-referential cognitions, emotions and motivations, and through this a fuller expression of individual and thus sex differences in various psychological domains. In this scenario, increases in bodily sex differences, such as height, are predicted to be associated with increases in psychological and behavioural sex differences (e.g. facets of personality). Our model integrates evolutionary and biological processes with social customs and mores-that is, combines nature and nurture-into a cohesive framework to explain historical and cross-cultural variation in the magnitude of many sex differences.