How Many Friends Does One Person Need? (3 page)

The fact that the genomic imprinting is this particular way around is intriguing. In most primate species, the key to a female’s reproductive success is the support she elicits from the sisterhood. For females to make their social relationships work, they need to be able to negotiate their way through a complex social world. Analysis of more than three decades of family histories from the population of baboons in Kenya’s Amboseli National Park has shown that the females who are socially most successful also have the largest number of surviving offspring at the end of their lifetime.

But for males, the issue is much less about social skills than about willingness to keep slugging it out in a fight. Now, any sensible individual who gets involved in a fight will quickly realise that discretion is invariably the better part of valour and retire gracefully to live (and maybe fight) another day. But in the mating game, those who retire from the fray don’t get the girl. So a mechanism that stops males thinking too much and lets the red mist take over usually works better. There may be a risk of injury or even death, but in a winner-takes-all game there
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is no point in being second. A small neocortex and a big limbic system is just what you want. If you have to fight for a living, best to bite first and think afterwards.

In effect, the females have won the battle over who controls the neocortex because social skills are more valuable to them, whereas males have won the battle over who controls the limbic system because it pays not to think too much about what you are doing if you get into a fight. The evolutionary battle of the sexes ends up being about control over the bits of the brain, though it is still something of a mystery as to how this is brought about. On second thoughts, I’m not so sure that I like the drift of this conversation... Perhaps we’ll change the subject.

Did you know that our eyes are actually part of our brain? They are an outgrowth of the brain that developed a sensitivity to light, came to the surface and, in doing so, allow us to see what’s going on out there in the external world in a way that touch and smell cannot. As those who go blind through old age or accident know only too well, our life is ruled by vision – and especially the wonders of colour vision.

So, let me for a moment speak confidentially to the men. Have you, I wonder, become exasperated by your wife’s fussing that the colours of her outfit clash when they seem perfectly fine to you? Well, she may be right: it seems that about a third of women see the world in four basic colours, whereas men only have the standard three (red, blue and green). These tetrachromatic (four-
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colour) women have an extra shade of green or an extra shade of red. Heaven forfend – some even have all five colours. It seems that some women really do see a very different world from the rest of us.

According to the standard story that they told us in school biology classes, we have two kinds of vision cells in the retina (the light-sensitive layer at the back of our eyeballs): rods give us the black-and-white vision that we use at night, and the cones give us colour that we use by day. The conventional wisdom is that there are three kinds of cones, each sensitive to a slightly different wavelength of light. These are red, blue and green, just as they are in the screen of your TV. We perceive the colours of the rainbow by the way the intensities of these three colours mix.

Now, the genes for two of these colours (the red–green dimension) are on the X chromosome, and those for blue are elsewhere, on chromosome seven. And this explains why men – but only very rarely women – are sometimes colour-blind and why this is usually red-blindness and almost never blue-blindness. Men only have one X chromosome (inherited from their mother), and if that chromosome is a bit dodgy, they don’t have a back-up for any of the genes that are on it. Since women have two X chromosomes (one inherited from each parent), they always have a back-up in case of emergencies.

And this provides us with a very simple explanation for the four- (or five-) colour effect. Slight mutations of the genes that code for the colour-sensitive pigments in the retina can mean that different people see slightly different shades of red or green. For men, whatever shade you get from your single X chromosome is what you get: that’s
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how you see the world. But women can end up with two slightly different shades of red or green on their two X chromosomes. If both X chromosomes become active during the development of the eyes, these women can have cones that code for both pigment sensitivities, and so end up with an extra colour dimension, in some cases even two extra ones – blue, red, shifted red, green and shifted green, five colours in all.

Now, here’s where the tricky bit comes in. All this would be fine, because it would just mean that women live in a richer colour world than men, and who cares about that?

But Mark Changizi and his colleagues at the California Institute of Technology in Pasadena now have an uncom-fortable twist on this. Sex differences in colour sensitivity of this kind are far from unknown in primates: one particularly well-known one is the fact that, among the New World monkeys, females are trichromats (they have three-colour vision) but males see only two colours. Changizi and his colleagues noticed that sex difference in colour sensitivity in primates correlates with the amount of bare facial skin that a species has. Species which have large areas of bare skin that can change colour as a result of increased or decreased blood flow are precisely those that have full three-colour vision. They make the obvious connection: is the fact that humans are a ‘naked ape’ related to our good colour vision?

And here is where the salt gets rubbed into the wound. Perhaps women’s sensitivity to colour (and especially reds) has something to do with their apparently mysterious capacity to know exactly when your protestations about where you have been all evening are, shall we say, just a little liberal with the truth. In short, do women
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know when men are lying because they can pick up much finer shades of blushing than their partners think they are giving away? How unkind can evolution possibly be?
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The big social revolution of the last few years has not been some great political event, but the way our social world has been redefined by social networking sites like Facebook, MySpace and Bebo. Darwin and his contemporaries could not have conceived of such things, even in their wildest dreams. For a privileged few like Darwin himself, the geographical scatter of their friends might have been greatly enlarged by the new-fangled penny post and a lot of letter-writing. But, in general, the reach of most people’s social worlds was pretty much confined to those they encountered in person. It seems that the social networking sites have broken through the constraints of time and geography that limited people’s social world in Darwin’s day.

One of the curious by-products of this technological revolution has been a perverse kind of competition about the number of friends you have on your personal site. Some of these claims have been, to say the least, exaggerated, with the number of registered friends running into the tens of thousands in some cases. However, even a cur-sory glance around this odd little electronic world quickly tells us two things. First, the distribution of the number
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of friends is highly skewed: most people have a pretty average number of ‘friends’ on their list, with only a handful having numbers above two hundred. Second, there is an issue about what really counts as a friend. Those who have very large numbers – that’s to say, larger than about two hundred – invariably know little or nothing about most of the individuals on their list.

The opening words of Dylan Thomas’s
Under Milk Wood
introduce us to the small, rather dubiously named Welsh fishing community of Llareggub (for those who don’t know, try reading it backwards) whose intertwined relationships wind through his drama like the ribbons on the maypole at the end of the dance. Each individual has his or her place in the social fabric of that small inward-looking world. Each has secrets that would tear that world asunder if they ever came out. In this, we are simply asserting our primate heritage – a heritage of deep social complexity involving personal relationships that, by the standards of more sensible mammals and birds, are unusually tangled and interdependent. And that primate heritage begins with the fact that monkeys and apes have much bigger brains for body size than any other group of animals.

So why do primates have such big brains? There are two general kinds of theories. The more traditional view is that they need big brains to help them to find their way about the world and solve problems in their daily search for food. The alternative view is that the complex social world in which primates live has provided the impetus for
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the evolution of large brains. The main version of this social intelligence theory, once known as the Machiavellian intelligence hypothesis, has the merit of identifying the thing that sets primates apart from all other animals – the complexity of their social relationships.

Primate societies seem to differ from those of other animals in two key respects. The first is the dependence on intense social bonds between individuals, which gives primate groups a highly structured appearance. Primates cannot join and leave these groups as easily as animals in the relatively unstructured herds of migrating antelope or the swarms of many insects. Other species may have groups that are highly structured in this way – elephants and prairie dogs are two obvious examples – but these animals differ from primates in a second respect. This is that primates use their knowledge about the social world in which they live to form more complex alliances with each other than do other animals.

This social intelligence hypothesis is supported by a strong correlation between the size of the group, and hence complexity of the social world, and the relative size of the neocortex – the outer surface layer of the brain that is mainly responsible for conscious thinking – in various species of nonhuman primates. This result seems to reflect a limitation on the number (and/or quality) of relationships that an animal of a given species can keep track of simultaneously. Just as a computer’s ability to handle complex tasks is limited by the size of its memory and proces-sor, so the brain’s ability to manipulate information about the constantly changing social domain may be limited by the size of its neocortex.

In evolutionary terms, the correlation between group
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size and neocortex size suggests that it was the need to live in larger groups that drove the evolution of large brains in primates. There are several reasons why particular species might want to live in larger groups, not least protection against predators. And it is conspicuous that the primates which both live in the largest groups and have the biggest neocortices are species such as baboons, macaques and chimpanzees, which spend most of their time on the ground and live either in relatively open habitats such as savannah woodlands or on the forest edge, where they are exposed to much higher risk from predators than most forest-dwelling species.

This relationship between neocortex and group size in the nonhuman primates raises an obvious question. What size of group would we predict for humans, given our unusually large neocortex? Extrapolating from the relationship for monkeys and apes gives a group size of about 150 –the limit on the number of social relationships that humans can have, a figure that is now graced by the title Dunbar’s Number
.
But is there any evidence to suggest that groups of this size actually occur in humans?

On the face of it, things do not look promising. After all, in the modern world, we live in cities and nation states that contain tens of millions of individuals. However, we have to be a little more subtle: the relationship for nonhuman primates is concerned with the number of individuals with whom an animal can maintain a coherent face-to-face relationship. It is quite obvious that those of us living in, for example, London do not have personal
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relationships with every one of the other ten million who live there with us. Indeed, the vast majority of these people are born, live and die without ever knowing each other’s names, let alone meeting. The existence of such large groupings is certainly something we have to explain, but they are something quite different from the natural groupings we see in primates.

One place we might look for evidence of ‘natural’ human group sizes is among pre-industrial societies, and in particular among hunter-gatherers. Most hunter-gatherers live in complex societies that operate at a number of levels. The smallest groupings occur at temporary night camps and have between thirty and fifty individuals. These are relatively unstable, however, with individuals or families constantly joining and leaving as they move between different foraging areas or water holes. The largest grouping is normally the tribe itself, usually a linguistic grouping that defines itself rather strictly in terms of its cultural identity. Tribal groupings typically number between five hundred and 2,500 men, women and children. These two layers of traditional societies are widely recognised in anthropology. In between these two layers, however, is a third group often discussed, but seldom enu-merated. Sometimes it takes the form of ‘clans’ that have ritual significance, such as the periodic celebration of coming-of-age ceremonies. Sometimes, the clan is based on common ownership of a hunting area or a set of water holes.

For the twenty-odd tribal societies where census data are available, these clan groups turn out to have a mean size of 153. The sizes of all but one of the village- and clan-like groupings for these societies fall between one
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hundred and 230, which is within the range of variation that, statistically, we would expect from the prediction of 150. In contrast, the mean sizes of overnight camps and tribal groupings all fall outside these statistical limits.