Robin Dunbar
How Many Friends Does One Person Need? Dunbar’s Number and Other Evolutionary Quirks
Harvard University Press, 2010. 312 pp.
excerpt from a review from Los Angeles Review of Books:
Our big brains — in particular our species’ inordinately large neocortex — evolved, Dunbar argues, in lockstep with our ability to manage increasingly large social groups: to read motives, to keep track of who is doing what with whom, of who is a reliable sharer, who a likely freeloader, and so on. Many evolutionary biologists have made this point over the years, of course. Where Dunbar is unique is in having assigned a definite number to what constitutes a stable human group or community. The “Dunbar’s number” of his title is (drum roll…) 150. Extrapolating from the estimated size of Neolithic villages, of Amish and other communities, of companies in most armies, and other such data, Dunbar argues that this number is, more or less, the limit of stable social networks because it represents the limit, more or less, of our cognitive capacities.

The number is highly debatable, but it turns out that, Facebook aside, the average person has about 150 friends — people he or she might actually recognize and be recognized by at a random airport, 150 people he or she might feel comfortable borrowing five dollars from. As for how many friends we have evolved to “need” in a more intimate sense, that is a different matter. According to Dunbar, most of us have, on average, about 3-5 intimate friends whom we speak to at least weekly, and about 10-15 more friends whose deaths would greatly distress us. These circles can include kin; indeed, the more extended family we keep in close touch with, the fewer friends we are likely to have — precisely because our neocortices can only manage so many relationships. What is perhaps most intriguing is the degree to which the inner circles change over time; close friends can drop through the circles of intimacy if we do not spend time with them, and even out of the 150, especially when someone new captures our attention. By contrast, kin have enough staying power that we can visit and expect to be housed by a cousin we have never met or a great-aunt after decades of neglect. In short, while friendships “decay” if not actively cultivated, kin relationships do not. Or so Dunbar claims.