Circle of friends.  A statistical analysis of social groups reveals a common structure shared by cultures around the world.

Too Many Friends?

You can pick your friends, or so the saying goes. But how many of these friends will be close, and how many will be mere acquaintances? New research suggests you have very little say in the matter: The sizes of your social circles may be hardwired into your brain.

Anthropologists have long known that people from cultures the world over--from hunter-gatherers in the Amazon to pastoralists in Africa--share similar social patterns. For example, individuals tend to organize their relationships as a hierarchy of "rings": People in the innermost ring-- known as the support clique--are the ones that come to the rescue in times of crisis. The next ring out is the sympathy group, consisting of people with "special ties" who are seen less regularly. Additional rings are composed of less intense relationships with more people. According to one theory, the reason for such similarities across cultures is that evolution has hard-wired social networks into our brains. But getting beyond anecdotal support for the theory has been difficult because anthropologists traditionally focus on one culture at a time.

To make a global comparison, a team led by Robin Dunbar, a bioanthropologist at the University of Liverpool, U.K., and Didier Sornette, a physicist at CNRS, Nice, France, pooled 61 anthropological studies of social networks from around the world. The size of social rings in each culture initially appeared random, but when the researchers turned to a branch of mathematics that deals with fractal systems (in which patterns hold at very different scales), they discovered a surprising pattern: Each level of a social ring contains three times more people than the next inner ring.

The discovery of this pattern creates more questions than answers, admits Dunbar, whose team published its results online last week in the Proceedings of the Royal Society of London. But the researchers conclude "there must be an evolutionary significance to the scaling ratio."

Allen Johnson, an anthropologistat the University of California, Los Angeles, finds the study convincing. While the idea that the brain is hard-wired for social networking has been around a long time, he says it is controversial because it seems too "reductionistic" for such a simple rule to determine our complex social lives.

Related sites
The paper
Dunbar's site
Sornette's site