In his 1992 article, Dunbar listed measures of brain volume and average social group size for 38 species of lemurs, monkeys, and apes. He showed that if you plotted one measure of brain volume (size of neocortex) vs. social group size, the plot seemed to lie along a straight line—the bigger the brain, the larger the group. So Dunbar proposed a novel idea: the size of a species’ brain determines the optimal size of their social groups. Maintaining relationships, argued Dunbar, requires brain power. More relationships require more neurons. Extrapolating his straight line from primate brains to
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