All this means that that adolescent dispersal results in well-known and emotionally bonded kin living in nearby social groups, and as a result, the nature of relationships between members of different social groups in humans is unique among the primates. When members of two chimpanzee communities meet, there is open hostility. Males do not necessarily know their half-sisters, who might not have shared any significant childhood co-association with them. And male chimpanzees have no reason to cooperate with males from another community, who might occasionally mate with their sisters but provide no investment to their sister’s offspring. Chimpanzee males from other communities are competitors and enemies to be killed or to be avoided.
Humans recognize emigrant siblings because they are likely to have shared long juvenile periods together with the same parents. Human males recognize that their sister or daughter is pair bonded to a long-term mate who invests significantly in provisioning and caring for their sister’s or daughter’s offspring. Thus, when a human male meets a male from another “community,” he must immediately consider that the new male might potentially be: 1) his daughter’s mate, 2) his sister’s mate, 3) his daughter or sister’s son, or 4) a close male relative of his daughter’s or sister’s mate. All those individuals have strong shared genetic interests with males in neighboring groups, and they are expected to invest in children who are closely related to the males in neighboring groups. The result is that friendly interactions and visiting are far more likely with the human reproductive and mating system. Indeed, researchers have shown empirically, using data from more than 30 hunter-gatherer societies, that intergroup visiting and residential transfers are extremely common.