Facebook sociologist tests Dunbar number
March 1st, 2009

Social networks are as old as humans. Are social media, a more recent invention, extending ancient limits on individual ability to maintain large networks? Anthropologist Robin Dunbar, now at Oxford, hypothesizes that the leap in brain growth in our species is directly and co-causally connected with sociality — the ability to identify others in our group, remember the history of our social interactions with identified individuals, reciprocate cooperation and punish non-cooperation. Dunbar hypothesized that the upper limit on the number of other people that any human can maintain as a stable network is around 150. The “Dunbar number” used to be famous only among social scientists, particularly primatologists and anthropologists, but now the recent and apparently overpopulating profession of social media theorists have gotten into the act. Peter Marsden at Harvard is among those who think that most humans maintain a much smaller “core” network. Enter Facebook. Andreas Kluth reports in The Economist that Cameron Marlow, the “in-house sociologist” at Facebook ran social network analyses on Facebook users and found that “the average number of “friends” in a facebook network is 120, consistent with Dr. Dunbar’s hypothesis, and that women tend to have somewhat more than men.” Some people have networks of 500, Marlow also noted that the average number of people with whom individuals have frequent interactions is small and stable.

Thus an average man—one with 120 friends—generally responds to the postings of only seven of those friends by leaving comments on the posting individual’s photos, status messages or “wall”. An average woman is slightly more sociable, responding to ten. When it comes to two-way communication such as e-mails or chats, the average man interacts with only four people and the average woman with six. Among those Facebook users with 500 friends, these numbers are somewhat higher, but not hugely so. Men leave comments for 17 friends, women for 26. Men communicate with ten, women with 16.

What mainly goes up, therefore, is not the core network but the number of casual contacts that people track more passively. This corroborates Dr Marsden’s ideas about core networks, since even those Facebook users with the most friends communicate only with a relatively small number of them.

Put differently, people who are members of online social networks are not so much “networking” as they are “broadcasting their lives to an outer tier of acquaintances who aren’t necessarily inside the Dunbar circle,” says Lee Rainie, the director of the Pew Internet & American Life Project, a polling organisation. Humans may be advertising themselves more efficiently. But they still have the same small circles of intimacy as ever.

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