These findings jibe with the research of Robin Dunbar, a professor of evolutionary psychology at Oxford. He has theorized that “group size” of both humans and nonhuman primates — the number of people (or, say, chimpanzees) one can maintain social cohesion with — correlates to “relative neocortical volume,” or the ratio of the neocortex to the rest of the brain.
The oft-cited “Dunbar’s number” is an average of 150 casual friends for humans (really, a range of 100 to 200). These are the people who might come to your wedding or funeral.
Within this roster, there are embedded layers of intimacy that grow smaller by a factor of three: 50 of these make the next cut to buddies, about 15 are good friends, around five confidants form our circle of trust, and finally we have an average of 1.5 people we deem our closest relationships. (Conversely, we can keep track of roughly 500 acquaintances and 1,500 faces we can match to names.)