Many years ago Mr. Dunbar famously noticed that there is a tight correlation between the size of a primate’s brain and the size of the social group its species generally forms. On this basis human beings should live in groups of around 150. The neat thing about this prediction was the way it seemed to fit the number of good friends most people have, as measured by the length of address books, the size of hunter-gatherer bands, the population of neolithic villages and the strength of army units. In recent years, Facebook has also seemed to confirm the hunch, with rosters of friends often settling around the Dunbar number.
Now Mr. Dunbar, who teaches at Oxford, has taken the argument a step further in work yet to be published, by correlating the size of a specific part of an individual’s brain with the size of that individual’s social network. He and his colleagues asked volunteers to list the initials of every person they had had social contact or communication with over the previous week, before stepping into a magnetic resonance scanner to measure the volume of their “orbitomedial prefrontal cortex.” Sure enough, the size of this lobe of the brain correlates well with the size of a person’s circle of friends. (It remains to be seen, of course, which causes which.)
— Matt Ridley, “How Many Friends Can Your Brain Hold?”