Groups of fire ants, chimpanzees, meerkats and other animals engage in lethal conflicts. But we humans are especially good at it, killing ‘outsiders’ on a scale that altered the course of our evolution. Pre-historic burials of large numbers of men and women with smashed skulls, broken forearms and stone points embedded in their bones — as well as ethnographic studies of recent hunters and gatherers— strongly suggest that warfare was a leading cause of death in many ancestral populations. Yet at the same time, humans are unusually cooperative, collaborating with non-kin, for example in hunting and sharing food, on a scale unknown in other animals.
Paradoxically, the grisly evidence of our warlike past may help explain our distinctly cooperative nature.
This distasteful idea is based on the evolution of what my co-authors and I have termed ‘parochial altruism’. Altruism is conferring benefits on others at a cost to oneself; parochialism is favouring ethnic, racial or other insiders over outsiders. Both are commonly observed human behaviours that are well documented in experiments. For example, people from the Wolimbka and nearby Ngenika groups, in the Western Highlands of Papua New Guinea, have no recent history of violence. Yet when asked to divide a pot of money between themselves and another, they give more and keep less for themselves if the other is a member of their own group rather than an outsider.
But parochial altruism is puzzling from an evolutionary perspective because both altruism and parochialism reduce fitness or material well-being compared with what a person would gain were he or she to eschew these behaviours.