How many people? Here, estimates differ. One of the great scientific insights into this question came from British anthropologist Robin Dunbar, who, in a 1992 scientific paper, tied our capacity to know people to the volume of our neocortex, the ‘new brain’ that sits directly behind our foreheads. This part of the brain has developed relatively rapidly – from Chimpanzee-sized to human-sized in about a million years – and it seems that the vastly larger volume of our neocortex has something to do with our social capabilities. On a graph, Dunbar showed the relative size of the neocortex in the three well-known hominids: gorillas, chimpanzees and humans. The number of individuals in a troupe (or a tribe) among humans, correlates perfectly to the size of the neocortex. In other words, the more brains you have in the front of your head, the more people you can know well enough to share with.
In the wild, Gorillas have troupe sizes that average around twenty, while Chimpanzees, with larger brains, can manage close to thirty-five. Human beings, with their freakishly big forebrains, can do a lot better. According to Dunbar’s calculation, we should be able to juggle nearly 150 individuals. This figure has become known as Dunbar’s Number, although Dunbar himself has never stated a precise value.
Tribes vary widely in size, but seem unable to maintain internal cohesion when they grow much larger than Dunbar’s Number. The tribe is well-connected: everyone has known everyone else from birth. Every member of the tribe needs to have a deep knowledge of every other member, because that deep knowledge is a resource that each individual draws upon. You might not know everything about a particular plant, or shellfish, or weather pattern, but someone else will – and you will know that they know it. Tribes are hyperconnected, and exploit their hyperconnectivity to make themselves more successful.
As a tribe grows, it pushes up against the natural limits imposed by brain capacity. As the population passes Dunbar’s Number, it becomes impossible to know everyone else. That would be a very strange feeling: in the midst of people you know as well as you know yourself, there are a few people you can not claim to know at all. Because you don’t know them, messages are misinterpreted, sharing abortive, and that feeling of discomfort tends to amplify. These strangers become more strange over time, and you project all of your fears and insecurities onto them. Eventually this stress becomes too much, and the tribe spontaneously fissions into two smaller tribes — tribes where everyone knows one another well. Then the process begins all over again.
Although we now prefer the conveniences of civilization to the tight and familiar life of the tribe, we still carry our tribes around within us. We may not need those intimate models of one another, once so key to our survival in small groups, but we nonetheless fill our heads with those closest to us. We can’t avoid this; it’s the way our minds work. We have roughly 150 cubbyholes waiting to be filled, and by the time we reach adulthood, we’ve filled them. These slots are not static. As people enter and leave our lives, we make room for them, or let them fade out of our internal existence. We all know the dissonance when we greet an old friend. We can still feel the traces of that friend’s one-time residence inside our head, the residue of relationship.