“There’s an interesting pattern that may explain it. If you measure the size of the cerebral cortex—the brain’s “machine of understanding”—in different primate species relative to the rest of their brain and plot it against the number of group members typical for each of those species, the two numbers fall on a straight line: the more members, the... See more

“Dunbar’s number” is a theoretical cognitive limit on the number of stable social relationships humans can maintain at one time. According to Robin Dunbar, a British anthropologist, humans have the cognitive capacity to keep track of somewhere around 150 close personal connections. Beyond this limited circle, we start treating people less like

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Dunbar actually doesn’t say that we devote “grooming time” to the whole social group of 150. Rather he says that the 150 is made up from welding together much smaller “primary networks”: coalitions, friendship groups. Intensive grooming (language, for humans) is reserved for close friends. Our intimate group is very small, averaging just five.