Dunbar’s number: why my theory that humans can only maintain 150 friendships has withstood 30 years of scrutiny

By Robin Dunbar, Professor of Evolutionary Psychology, Department of Experimental Psycology, University of Oxford

Exactly 30 years ago, I was pondering a graph of primate group sizes plotted against the size of their brains: the larger the brain, the larger the group size. I was curious to know what group size this relationship might predict for humans.

The number my calculations gave was 150. Since this seemed low, I hurried off to the library to look for data on natural human group sizes. Hunter-gatherers live in multilevel societies, with groupings of individuals forming a hierarchically layered structure – families within bands, bands within communities, communities within tribes.

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