24. What Are Couples Made of?

Compared to the evolutionary psychology of mating, the evolutionary demography of unions is little developed. We know quite a bit about why and how people have sex, much less about why and how they have spouses. Yet couples continue to be a central building block of families, the biosocial tie within which most adults live, most sex takes place, and through which most children are made and raised. Arguably, sexual selection in humans happens through long-term pair bonds rather than short-term relationships.

Evolutionary theory approaches unions as reproductive contracts: a precarious balance of conflict and compromise between individual reproductive and sexual strategies. Sexual strategies are predicted to vary especially with age, gender and resources, but also with ecological and social conditions such as increasing gender equality and lower fertility.

This chapter discusses the formation of unions in high-income, increasingly gender equal societies from the intersection of family demography and evolutionary studies. How is selection of spouses affected by having more highly educated women in the population? Why does contemporary family formation often involve a stage of cohabitation before marriage? I argue that sexual strategies theory could move beyond the division into short-term versus long-term pair bonds, and suggest that cohabitation represents one mid-term form of temporal and psychological commitment to a romantic partner.