What caused human brains – and those of other apes – to grow so large? One theory is that it resulted from the complexity of our environment – the day to day problems that our ancestors would have encountered in foraging and survival: Where are the fruit trees? Which ones did I pick from yesterday?
Another idea – the social brain hypothesis – is that the complexity of our social groups require a big brain to keep track of, especially when the group is large. A bigger group means more relationships to remember.
Doing either of these things well could potentially lead to a survival advantage, but which actually triggered the evolution of our unique brain size? Dunbar (1992) put the two theories to the test by comparing both foraging area and group size with neocortex ratio – the proportion of brain neocortex to other brain areas (considered a better measure of overall ‘braininess’ than absolute brain size).
A clear correlation was found – a bigger group was associated with a larger neocortex ratio, supporting the social brain hypothesis. In contrast there was no clear relationship between brain size and environmental complexity. It would appear that primates with larger social groups need larger brains in order to keep track of relationships. That is not to say, as Dunbar comments, that those large brains couldn’t then be useful for dealing with environmental problems – but he does not feel it was the evolutionary driving force (Dunbar, 1998).
Since the early work with other primates, Dunbar began to question whether the same principle would apply to human social groups. To fit with the ratio that had been established, a group size of around 150 would be predicted (Dunbar, 1998). When we look at historical settlements, groups of around 120-150 members abound (Dunbar, 1993), and this is also the size of tribes in most hunter-gatherer tribes - the closest model we have of the lifestyle of our early ancestors.
In a neat application of the concept to the modern world, Hill and Dunbar (2003) looked at the size of networks to which people send Christmas cards. 153.5 was the mean total population of the households receiving cards from any individual - only a fraction over the predicted maximum number, and in practice, senders of cards would probably not know every member of a recipient household equally well.
More recently, other researchers have started to look at new technology and social media, to ask whether technological developments have expanded the size of our natural social network. It appears that they haven’t done so, at least not significantly – despite the large number of contacts people often establish on Twitter, the number they regularly communicate with remains under 200 (Gonçalves et al., 2011).
The key idea from the social brain hypothesis is that evolution has determined a cognitive limit on what we can do; just as we have other mental limits such as short-term memory capacity, the brain is simply not capable of maintaining a greater number of close social relationships.
What does this number mean in practice? An obvious question to ask is how close a relationship has to be to count within the number. Do colleagues and extended family count? Just how do we define who is in our 150 and who is not? According to Dunbar, a simple way to look at it is as “the number of people you would not feel embarrassed about joining uninvited for a drink if you happened to bump into them in a bar” (Bennett, 2013). If it truly is a biological limit, it could imply that we should avoid trying to continually extend our circle of friends/acquaintances, and instead foster stronger relationships with those that we are already close to.
Dunbar’s number does seem to be applicable to a large number of situations, from historic communities to hunter gatherer tribes, from the size of army units to Twitter engagement.
However not everyone is convinced. A correlation between brain size and social group size does not prove that there was an evolutionary cause-and-effect. And de Ruiter et al. (2011) have argued that although the neocortex plays an important role in social functioning, its size does not directly determine social skills.
It could also be argued that Dunbar has cherry-picked examples that fit the theory post-hoc, and that had the number from the correlation calculation been different (say 250), he would have been able to find examples of human communities to fit. Nevertheless, the idea that we do have a natural group size seems to fit the evidence of other species, and it makes a lot of sense to suggest that we can't push that limit beyond a certain point without impacting on the quality of the relationships.
Bennett, D. (2012). The Dunbar Number, From the Guru of Social Networks. Retrieved 20/11/2014 from: http://www.businessweek.com/articles/2013-01-10/the-dunbar-number-from-the-guru-of-social-networks
Dunbar, R.I.M. (1992). Neocortex size as a constraint on group size in primates. Journal of Human Evolution, 22, 469-493.
Dunbar, R.I.M. (1993). Coevolution of neocortical size, group size and language in humans. Behavioural and Brain Sciences, 16, 681-735.
Dunbar, R.I.M. (1998). The social brain hypothesis. Brain, 9(10), 178-190.
Gonçalves, B., Perra, N. and Vespignani, A. (2011). Modeling Users’ Activity on Twitter Networks: Validation of Dunbar’s Number. PLoS ONE, 6(8): e22656. doi:10.1371/journal.pone.0022656
Hill, R.A. and Dunbar, R.I.M (2003). Social network size in humans. Human Nature, 14(1), 53-72.
de Ruiter, J., Weston, G. and Lyon, S.M. (2011). Dunbar’s Number: Group Size and Brain Physiology in Humans Reexamined. American Anthropologist, 113(4), 557–568.