FRIBOURG.- Given the relationship between neocortex size and social group size in primates, what does this tell us about humans? We are primates just like all the others. Indeed, the most recent evidence from molecular biology suggests that, on the basis of similarities in generic material, chimpanzees and humans are more closely related to each other than either is to the gorilla, their next closest relatives among the primates. Since this relationship between brain size and group size seems to fit chimps neatly, we should expect the same of humans too.



So what size of group should we predict for humans? Humans have a neocortex ratio of 4:1, and if we plug this value into the graph […], we can read off a predicted group size for humans. The answer turns out to be groups of about 150.

Now, one’s first reaction to this is disbelief. After all, humans live in cities like Tokyo and London, New York and Calcutta, places where 10 million people or more live crowded together. How can a figure as small as 150 possibly be correct?

But remember what kind of group the relationship in the graph was based on. Primates live in small groups where everyone knows everyone else, at least by sight even if they don’t know them from personal interaction. Not all the people who live in giant conurbations are social intimate. The vast majority of the people in Tokyo and New York are born, live their lives and die without even being aware of each other. […]





Sociologists have long recognised that individuals have a limited network of acquaintances. Even in a modest-sized town, an individual will know only a tiny proportion of those around him by name or face; he will know even fewer of these well enough to consider them genuine members of his social circle. Attempting to estimate the size of this circle of friends is not easy. However, one quite successful way of doing so involves what are known as ‘small world’ experiments. The name derives from the discovery that sending a message to any random individual anywhere in the world through a chain of personal contacts typically requires only six intervening steps. If 150 people know 150 other people, then six steps would allow you to reach 1506, which is approximately 10 million million people. […]



[These natural groupings of around 150 people] are a consequence of the fact that the human brain cannot sustain more than a certain number of relationships of a given strength at any one time. The figure of 150 seems to represent the maximum number of individuals with whom we can have a genuinely social relationship, the kind of relationship that goes with knowing who they are and how they relate to us. Putting it in another way, it’s the number of people you would not feel embarrassed about joining uninvited for a drink if you happen to bump into them in a bar.

Robin Dunbar, Grooming, Gossip and the Evolution of Language, Cambridge: Harvard University Press, 1998, p. 69, 73, 77