what does correlate with brain size? The answer, Dunbar argues, is group size. If you look at any species of primate—at every variety of monkey and ape—the larger their neocortex is, the larger the average size of the groups they live with. Dunbar’s argument is that brains evolve, they get bigger, in order to handle the complexities of larger social groups. If you belong to a group of five people, Dunbar points out, you have to keep track of ten separate relationships: your relationships with the four others in your circle and the six other two-way relationships between the others. That’s what
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