Today is August 15th, and so for today’s Eponymy in August, the number 150, the consensus estimated value for Dunbar’s Number, is the star of the show.
Robin Dunbar is an anthropologist and evolutionary psychologist, who has taught at Bristol, Cambridge, University College London, Liverpool, and Oxford. His specialty is primate psychology and social behaviors, having done his PhD on the social organization of a species of monkey.
Dunbar’s Number is mathematically derived by observing that the size of primate societies (across species) is directly correlated to the size of the neocortex in each species’ brain structure. The neocortex is responsible for social connections, so its size limits how many stable relationships can be carried out by one individual. Based on the size of the neocortex in humans, the expected value of the upper limit on the size of a human society is 148, conventionally rounded to 150 (but with a high statistical variance).
Thus communities with a high incentive to remain together (hunter gatherer tribes, academic departments, army deployments, etc.) will top out at 150 or maybe 200 people before they split off into factions. Without physical proximity and a need for togetherness, even smaller numbers may become unstable as well.
Dunbar’s work has an impact on what we know about social media, as well. His more recent work highlighted how a power law with a base of 3 dictates the levels of our Facebook friendships. On average we have five intimate friends, 15 best friends, 50 good friends, 150 friends, 500 acquaintances and 1,500 people we recognize on sight, all of whom *could be* Facebook friends, but it starts to get difficult to manage your friend streams. Conversely, in 2010 the social network Path tried to encourage people to only connect with their real friends by enforcing a 150-friend limit.