The Social Brain Hypothesis

What it is

Across non-human primates, group size covaries with the relative size of the neocortex. The inference is that a larger social world needs more computation behind it.

The hypothesis and the number extrapolated from it are separable, and this site keeps them apart.

In effect

Dunbar's fuller 1993 argument is the half that gets dropped. Primates hold groups together by grooming, grooming time rises linearly with group size, and a human-sized group could not be maintained that way without intolerable time demands. He proposed language as the efficient substitute, citing the observation that roughly sixty per cent of human conversation time is spent on relationships and personal experiences.

What it does not say

It does not license a precise human figure. Lindenfors, Wartel and Lind showed the extrapolation cannot produce one, with estimates ranging across methods and intervals wide enough to be useless.

It also does not settle why brains got large. Competing accounts exist and this publication has not audited them, which is why the page is marked contested.

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