The Dunbar Number

Evolutionary anthropologist Robin Dunbar has come up with the number of meaningful relationships a human can maintain: approximately 150.  In the spring, he spoke to The Guardian in London. 

How did you come up with this concept?

I was working on the arcane question of why primates spend so much time grooming one another, and I tested another hypothesis – which says the reason why primates have big brains is because they live in complex social worlds. Because grooming is social, all these things ought to map together, so I started plotting brain size and group size and grooming time against one another. You get a nice set of relationships.

It was about 3am, and I thought, hmm, what happens if you plug humans into this? And you get this number of 150. This looked implausibly small, given that we all live in cities now, but it turned out that this was the size of a typical community in hunter-gatherer societies. And the average village size in the Domesday Book  is 150 [people].

It's the same when we have much better data – in the 18th century, for example, thanks to parish registers. County by county, the average size of a village is again 150. Except in Kent, where it was 100. I've no idea why.

Has this number evolved at all?

The Dunbar number probably dates back to the appearance of anatomically modern humans 250,000 years ago. If you go back in time, by estimating brain size, you can see community size declining steadily.