Human society is organized in structured social networks upon which large-scale cooperation among genetically unrelated individuals is favored and persists. Such large-scale cooperation is crucial for the success of the human species but also one of the most puzzling challenges. Recent work in social and behavioral neuroscience has linked human cooperation to oxytocin, an evolutionarily ancient and structurally preserved hypothalamic neuropeptide. This review aims to elucidate how oxytocin promotes nonkin cooperation in social networks by reviewing its effects at three distinct levels: individual cooperation, the formation of interpersonal relationships, and the establishment of heterogeneous network structures. We propose oxytocin as a proximate mechanism for fostering large-scale cooperation in human societies. Specifically, oxytocin plays an important role in facilitating network-wide cooperation in human societies by 1) increasing individual cooperation, mitigating noncooperation motives, and facilitating the enforcement of cooperative norms; 2) fostering interpersonal bonding and synchronization; and 3) facilitating the formation of heterogeneous network structures.
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