Humans are complexly social and exceptionally cooperative. Human behavioral ecology has been successful by explaining small-scale dyadic cooperative interactions like food sharing using standard theoretical perspectives of kin selection and reciprocity. Such models have been enhanced with additional ideas including mutualism, partner choice, costly signaling, competitive altruism, and biological market theory. Concepts such as stake, interdependence, and need-based support have provided additional predictive power. While these tools have been effective at explaining face-to-face cooperative relationships, their inability to explain the large, complex social groupings typical of human societies has led researchers to look to new models of multilevel and cultural group selection theory for solutions. Culture plays a key role in structuring human sociality and facilitating group selection by reducing behavioral variation within groups while increasing variation among groups. Gene-culture coevolution can favor psychological propensities like conformist bias that assort people into symbolically marked groups of trusted partners with shared identities and normative ways of organizing cooperative life. Societies whose cumulative culture maintain institutions that encourage magnanimity and coordination between members achieve collective action benefits unobtainable otherwise and outcompete other groups whose institutions fail to organize their members in ways that lead to cooperative outcomes at scale.