Robin Dunbar explains Dunbar's Number to Aleks Krotoski guardian.co.uk

Not many people have a number named after them, but Robin Dunbar lays claim to the Dunbar Number. Confusingly, no precise value has been attached to this figure, but a commonly cited approximation is 150 – and this is the number of people with whom we can maintain a meaningful relationship, whether in a hunter-gatherer society or on Facebook.

The director of the Institute of Cognitive and Evolutionary Anthropology at Oxford University is also the author of How Many Friends Does One Person Need? (Faber). It's no surprise he's an engaging companion.

What is evolutionary anthroplogy?

Evolutionary anthropology is the generic study of how we came to be modern humans – how our bodies came to be the shape they are, how our minds came to be the way they are.

So how much of our social behaviour is rooted in our biology?

All of it! [But] by that you must be clear that you don't mean genetically rooted – so that we have no choice about the way we behave, we're programmed in the way that an an amoeba is.

If you look at any bird or mammal, never mind things as smart as primates, never mind things as doubly smart as humans… any bird or mammal has its biological inheritance, as it were, which gives the rules of how to play the game of life. But how those rules get played out on a day-to-day basis depends on how the the animal assesses the particular circumstances – it has a lot of flexibility in how it should behave, it just has some guidelines provided by evolution, and some constraints. If you don't have wings, you can't fly… [But there's still] lots of scope for social, environmental, demographic circumstances… and grim economics.

What does your work tell us about the way we interact socially?

The way in which our social world is constructed is part and parcel of our biological inheritance. Together with apes and monkeys, we're members of the primate family – and within the primates there is a general relationship between the size of the brain and the size of the social group. We fit in a pattern. There are social circles beyond it and layers within – but there is a natural grouping of 150.

This is the number of people you can have a relationship with involving trust and obligation – there's some personal history, not just names and faces.

And this is is the Dunbar number! 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.

Why did we evolve as a social species?

Simply, it's the key evolutionary strategy of primates. Group living and explicitly communal solutions to the problem of survival out there on the plains or in the forests… that's a primate adaptation, and they evolved that very early on.

Most species of birds and animals aren't as intensely social. Sociality for most species hovers around pair-bonds, that's as complicated as it gets. The species with big brains are the ones who mate monogamously… The lesson is that there is something computationally very demanding about maintaining close relationships over a very long period of time – as we all know!

How can we grow the Dunbar number?

We're caught in a bind: community sizes were designed for hunter-gatherer- type societies where people weren't living on top of one another. Your 150 were scattered over a wide are, but everybody shared the same 150. This made for a very densely interconnected community, and this means the community polices itself. You don't need lawyers and policemen. If you step out of line, granny will wag her finger at you.

Our problem now is the sheer density of folk – our networks aren't compact. You have clumps of friends scattered around the world who don't know one another: now you don't have an interwoven network. It leads to a less well integrated society. How to re-create that old sense of community in these new circumstances? That's an engineering problem. How do we work around it?

The alternative solution, of course, is that we could evolve bigger brains. But they'd have to be much bigger, and it takes a long time.

What about the role of the web in this?

Can we manage to have meaningful relationships with more than just the old numbers? Yes, I can find out what you had for breakfast from your tweet, but can I really get to know you better? These digital developments help us keep in touch, when in the past a relationship might just have died; but in the end, we actually have to get together to make a relationship work.

In the end, we rely heavily on touch and we still haven't figured out how to do virtual touch. Maybe once we can do that we will have cracked a big nut.

Words are slippery, a touch is worth a 1,000 words any day.