In 1992 the evolutionary anthropologist Robin Dunbar studied the relationship between brain size and group size in numerous primate species. He found that the species with the largest brains were able to maintain the largest cohesive social groups while those with the smallest brains were able to maintain only the smallest cohesive social groups. His mathematical model predicted that, given the size of the human brain, human groups would retain their cohesiveness up to approximately the size of only 150 individuals. The number 150, which came to be called “Dunbar’s number,” turns out to be
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