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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.

The community level of organisation turned out to be almost exactly 150. Thus was born the “social brain hypothesis” and “Dunbar’s number”, the former referring to the relationship between group size and brain size in primates and the latter referring to the natural group size of about 150 for humans.

Dunbar’s number has attracted a great deal of attention over the years. For example, it has been used in the design of social media platforms, as well as being the basis of “secret handshake” online security algorithms and bot-detection software.

Read the full article on The Conversation website, written by Emeritus Professor Robin Dunbar, Department of Experimental Psychology

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