The perfect number for a human social group is 150. The challenge is maintaining a real sense of community
Evolution is, at best, a Heath Robinson affair: we accumulate novel adaptations that seemed like good ideas at the time but that, millennia later, turn out to be hostages to fortune. Our long evolutionary history since we last shared a common ancestor with the chimpanzees has left us with many such scars of evolution.
Walking upright produced many benefits, not least the capacity to invade novel habitats, eventually making possible the long series of migrations out of Africa that resulted in our peopling every habitable corner of the planet. But bipedalism put added strain on the spine, and so back problems remain the most common cause of lost days at work. Bipedalism had other unexpected side-effects. It necessitated the reshaping of the pelvis to provide a stable platform on which to balance the trunk. Our pelvis became more rounded and bowl-shaped, causing the bones that form the birth canal to close in. For several million years, this wasn't too much of a problem: our still-ape-sized brain could slip relatively easily through during birth, even if the fit was a bit tighter. The problem emerged only around one million years ago when a dramatic change in the climate began to favour a rapid increase in brain size. Brain size began an inexorable increase, rising to become four times larger than an ape's. To put the matter bluntly, the problem was one of squeezing an increasingly large peg through an unforgivingly small hole.
Our solution to this problem was to give birth to increasingly premature babies and allow the rest of their development to happen outside the womb. In mammals as a whole, the length of pregnancy is determined by how long is needed to produce a brain that is developed enough to allow the baby to survive on its own after birth. If we humans were to follow this general mammalian pattern, our babies would need a 21-month-long pregnancy. If this eye-watering prospect fills you with alarm, imagine the dilemma faced by your ancestors: keep brain size and gestation as it was, at the cost of not being able to contend so effectively with the then rapidly changing environment, or opt for a bit of lateral thinking. Perhaps fortunately for us, our ancestors managed to pull the rabbit out of the hat and produce increasingly premature babies so as to allow us to evolve ever-larger brains. The outcome was a reduction in gestation from 21 months to our current nine.
But there is no such thing as a free lunch in evolution, so this came at a price. Ape and monkey babies can stagger around on their own within a matter of days, sometimes hours, of being born. Human babies, by comparison, are lumps of flesh and bone that have trouble doing anything for themselves, including regulating their own temperature. Only once they've had that extra 12 months of development can they begin to match what ape babies can do at birth. Which explains why the medics panic over premature babies – those born after less than eight months gestation. Since human babies born at term are already on the edge of survival, a baby born after even less time is really pushing the envelope.
As haphazard as evolution sometimes seems to be, we didn't opt for big brains on a whim. So why did we bother? The short answer is that, among monkeys and apes at least, the size of your brain determines the size of the social group you can cope with. And group size in turn allows you to solve more effectively some of the key problems of survival that every species faces. For most primates, this is the risk of being caught by a predator. Monkeys and apes use living in groups as their primary solution to this: there is an implicit social contract to stick together and defeat the common enemy. But communal solutions of this kind mean that animals have to balance in a rather sophisticated way the benefits of unabashed selfishness (grab what you can at everyone else's expense) with the benefits that derive from cooperation as a group (which necessarily means being a bit more sensitive to others' needs and interests).
The psychological demands of living in large groups mean that, in primates, species-typical group size correlates rather closely with the species' brain size. On the primate model, our oversized brain would predict a group size of around 150, the number now known as Dunbar's Number. We find it in the typical community size of hunter-gatherer societies, in the average village size in county after county in the Domesday book, as well as in 18th-century England; it is the average parish size among the Hutterites and the Amish (fundamentalist Christians who live a communal life in the Dakotas and Pennsylvania, respectively). It is also the average personal network size – the number of people with whom you have a personalised relationship, one that is reciprocal (I'd be willing to help you out, and I know that you'd help me) as well as having a history (we both know how we came to know each other).
The Hutterites illustrate rather clearly just what's involved. They deliberately split their communities once they exceed 150 individuals because, they maintain, you cannot run a community of more than 150 people by peer pressure alone: instead, you need a police force.
We see the same principle at work in the management philosophy of the Gore-Tex company, known for its breathable, waterproof fabrics. Instead of expanding factory size as its business grew, the late "Bill" Gore kept this factory size to 150 and simply built a new, completely self-contained factory next door. The result is a work community where everyone knows everyone else, and there is no need for formal line-management systems or name badges; everyone is committed to each other and to the communal vision. Has this been the secret to its unusual success as a business?
Perhaps the best example, however, remains the military. All modern armies have a similar organisational structure, mostly developed over the last 300 years by trial and error on the battlefield. The core to this is the company – typically around 120-180 in size – almost exactly Dunbar's Number. As anyone who has been in the army will tell you, company is family, far more so than battalion or regiment.
Although wild claims have been made about the number of friends people have on Facebook, the vast majority of us have only 120-130. Yes, you can have 500 or 1,000 friends if you want to sign people up, but this seems to have more to do with competition than with real friendship.
On average, we have five intimate friends, 15 good friends (including the five intimate ones), 50 friends and 150 acquaintances. While it is not altogether clear why our relationships are constrained in this way, one possibility is time. A relationship's quality seems to depend on how much time we devote to it, and since time is limited, we necessarily have to distribute what time we do have for social engagement unevenly. We focus most of it on our inner core of five intimates. Alternatively, it might just be a memory problem: we have a job keeping track of who's doing what, and can only really keep serious tabs on the inner core of five.
But there is one more serious problem lurking behind all this. In traditional small-scale societies, everyone shares the same 150 friends. This was true even in Europe until well into the 20th century, and probably still is true today of isolated rural communities. You might well fall out with them from time to time, but, like the Hutterites, you are bound together by mutual obligation and densely interwoven relationships. And of these, shared kinship was perhaps the most pervasive and important: offend Jim down the road, and you bring granny down on your back because Jim is her second-cousin-once-removed, and she's got her own sister, Jim's grandmother, on to her about it.
In the modern world of economic mobility, this simple balance has upset: we grow up here, go to university there, and move on to several elsewheres in a succession of job moves. The consequence is that our social networks become fragmented and distributed: we end up with small pockets of friends scattered around the country, most of whom don't know each other and, perhaps more importantly, don't know the family part of our networks. You can offend Jim, and almost no one will care. And if they do, you can afford to move on and leave that whole subset of friends behind. Networks are no longer self-policing.
Because modern geographical communities no longer have the social coherence they had up until the 1950s, it is perhaps inevitable that people become less willing to remonstrate with miscreants because others are unlikely to back them up. Bearing these factors in mind, is it any wonder that some inner-city communities fall victim to gang violence? Our real problem for the future is how to overcome this social fragmentation by recreating a sense of community in our increasingly urbanised and mobile world.
Robin Dunbar is professor of evolutionary anthropology at Oxford University
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