Researchers have long been puzzled by large societies in which strangers routinely engage in voluntary acts of kindness, respect and mutual benefit even though there is often an individual cost involved.

While evolutionary forces associated with kinship and reciprocity can explain such cooperative behavior among other primates, these forces do not easily explain similar behavior in large, unrelated groups, like those that most humans live in.

The study also found the extent to which a society uses punishment to enforce norms increases and decreases with the number of people in the society.

"It is likely that small and large communities regulate cooperation - mutual defense, conservation, etc. - in different ways, because different mechanisms of monitoring and enforcement of norms work better at different scales of society," explained McElreath, an associate professor of anthropology at UC Davis.

"A small town in Kansas, for example, can likely rely upon reputation and the fact that everyone knows everyone else, while the residents of New York City need some mechanism, like punishment, that can work in the absence of reliable reputations," he said.

"Study co-author Richard McElreath says the cooperative nature of each society is at least partly dependent upon historical forces. (Credit: Karin Higgins/UC Davis photo)"

Source: University of California