Homo Sapiens: The Social Animal

Be it Facebook, Twitter, a phone call, or simply chatting to a friend at the pub, or a family member at home, humans often communicate with one another for long periods of time. So what is it about this social exchange of information that is so appealing for us Homo Sapiens? The philosopher Aristotle may have already put it best 2300 years ago: Humans are by nature a social animal. Or, to update his pithy remark into modern scientific parlance: Human cognitive faculties evolved to engage in social relationships. As such, social information exchange is as old as – and may well be older – than our species. And, as we will see, this social evolution may well be hardwired into the very fabric of our brains.

The Social Brain Hypothesis

Modern anatomical humans – that is you, me and every other person alive on the planet today – evolved out of archaic humans during the Middle Palaeolithic, about 200,000 years ago. An interesting feature of our modern human brains, as well as the brains of our closest living primate relatives such as the chimpanzee, is that they are disproportionally large in relation to body size, when compared to the brain-body ratio of other vertebrates.

Why is this so? There have been numerous hypotheses put forward to explain this ballooning primate brain, many of which have been based on ecological measures.1 For example, perhaps an increased brain size may improve the ability to solve complex problems (such as tool construction) or assist in foraging over a large area. However, these proposals don’t match up with what we observe in the animal kingdom. Crows are amazing problem solvers despite their small brain size and squirrels, who bury their food source in numerous places for later consumption, don’t show the skewed brain to body ratio that would be expected if overall brain size was simply used for the spatial memory needed in foraging. (However, squirrels do show disproportionately large hippocampi in relation to brain size – an area implicated in both spatial and memory processes – when compared to similar non-foraging species).2

What is unique about primates however is that the vast majority are extremely social, engaging in large and sometimes widely dispersed troops. Moreover, the part of the brain that most contributes to this increased brain to body ratio we have been discussing is known as the neocortex. Literally translated this term mean ‘new-bark’ and like the bark of a tree the neocortex is a thin outermost layer of brain tissue which is about 2mm to 4mm thick. The ‘new’ part refers to the fact that this portion of the brain is phylogenetically the youngest (which is a fancy way of saying it is the most recent part to have evolved) and it is implicated in many higher order functions, from sensory perception, to reasoning abilities, to conscious thought.

Interestingly, there is a phenomenon related to the neocortex of primates which was first reported in the early 1990s by the Oxford anthropologist Robin Dunbar. If you examine the ratio of neocortex volume to overall brain volume in primate species a compelling trend occurs. The greater the ratio between neocortex and overall brain volume, the greater the social group size of the primate species. For example, in howler monkeys the average group size is quite small at 8 individuals and the neocortex size accounts for 65% of total brain volume. But as group size increases so does this percentage:

Proboscis monkeys: group size: 14, neocortex ratio 67%.
Capuchin monkeys: group size 18, neocortex ratio 70 %,
Macaques: group size 40, neocortex ratio 72%
Baboons: group size 51, neocortex ratio 74%
Chimpanzees: group size 54, neocortex ratio 76%.  3

So what about humans, I hear you ask? Well, extrapolating from this trend, Dunbar has inferred that based on the size of the human neocortex, the predicted group size for the upright ape should be 150 individuals.4 This has been christened Dunbar’s Number and it shows up again and again in the annals of human history. For example, the average group size for hunter-gatherer communities is 150, while the average size of a Neolithic village (6500 BCE) was between 150-200. Christmas card networks average 154 persons, while the average number of friends on Facebook is around the 150 mark. And on and on it goes from the size of Amish communities to population of medieval villages as recorded in the Doomsday Book. 150 seems to be the group size for humans in which we are able to know each of the members of the group personally and maintain the relationship successfully.

Dunbar also argues that within this larger group (be it baboons, chimpanzees or humans) there exists a smaller, more emotionally connected group, which is known as a grooming coalition. In non-human apes and monkeys these smaller coalitions (which usually consist of around 5 individuals) collect together and groom – clean – one other, often for much longer than is needed for hygienic purposes. This has been shown to release endorphins, neurochemicals which are involved in the brains pleasure response. Such grooming behaviour is believed to facilitate emotional bonding between small groups of individuals.5

From these results Dunbar and colleagues have, over the last 25 years, developed what has come to be known as The Social Brain Hypothesis. The thrust of The Social Brain Hypothesis is as follows: Because maintaining coherent groups is cognitively demanding, neocortex volume (and thus computational power) has evolved to match the cognitive demands of the species’ optimal group size. While at the same time the hypothesis claims many of our core cognitive and emotional faculties are likely to have arisen to solve the reoccurring problems that our ancestors would have encountered in group life.

Recent evidence in support of the The Social Brain Hypothesis comes from the field of human neuroimaging. This technique uses magnetic resonance imaging (MRI) to measure the volume of key areas of the brain. Multiple studies over the last several years have investigated the relationship between the participants’ social group size, brain volume of specific areas and their ability to metalize. Metalizing, in this context, is the ability to keep track of the propositional attitudes (believes, desires, fears, etc.) of a network of people (e.g. John hopes Jenny, who loves Jeff, will believe Jeremy when he tells her he thinks Jack is lying to her about Jeff and Jill).

What these studies have converged upon is a positive three-way correlation between an individual’s social group size, their performance on metalizing tasks and the volume of several key areas of the brain – the basolateral amygdala and prefrontal cortex among other areas, many of which have previously been implicated in a range of social and metalizing functions.6 These are powerful findings because it shows that predictions of The Social Brain Hypothesis hold not only between primate species, but also with-in species.

Co-operators and Defectors: How Stable Social Groups Emerge

But why should all this be so? Why should our being more social correlate with an increased volume of the brain region involved in so many of our higher cognitive processes? Well, being social is hard work. Our environment is full of complicated, dangerous occurrences, but nothing more so than when a certain moving, living, reasoning part of it decides to take a swing at us.

Sociality thus presupposes a level of mutual cooperation in order to have peaceful cohabitation. Such cooperation and cohabitation is very useful, with safety against predators often being cited as one of the major benefits of group living7 (one baboon against a leopard may not stand a chance, but 20 or more certainly do). Group living also provides benefits in activities such as food procurement and child rearing. Thus, those who cooperate will be more likely to survive and reproduce, ensuring their genes are passed into the next generation.

However, group living is also complicated by the fact that there will always exist the incentive for members to defect, gaining an advantage through the exploitation of other members within his or her own group. This, in the end, benefits the passing of this defector’s genes into the next generation at the cost of other group members.

The evolutionary biologist Robert Trivers has argued that as a result of this spiralling tug-of-war between cooperating and defecting, complex social and cognitive counter measures evolved to limit the incentive to defect on the one hand, and to be – or at least pretend to be – a good co-operator on the other.8 It may be surprising to think about the following human features from such an evolutionary perspective, but social phenomenon such as morality, reputation, gossip and even, it has been speculated, language may all have evolved in order to allow for better in-group cooperation and to reduce unsocial and defective behaviour.


So what does all this amount to? The Social Brain Hypothesis is an attempt to bring together insights from evolutionary biology, anthropology, comparative anatomy and neuroscience to explain why it is we spend so much of our time engaging in social relationships. Its primary claim is that our ancestors evolved larger brain to allow for greater computational power, which in turn allows us to live in complex social groups.
So, the next time you are messaging on Facebook, gossiping with you friends or flirting with your crush, you can stop for a moment and thank the complexities of your evolved, social brain.

1. Dunbar RIM. 2006. Taking social intelligence seriously. In: Peel RA, Zeki M, eds. Genetic and environmental influences on human ability pp. 47-50. London: Galton Institute.

2. Lavenex et al. 2000. Sex Differences, but No Seasonal Variations in the Hippocampus of FoodCaching Squirrels: A Stereological Study. The Journal of Comparative Neurology 425:152–166

3. Dunbar RIM. 1992. Neocortex size as a constraint on group size in primates. Journal of Human Evolution. 22 (6): 469–493.

4. Dunbar RIM. 1993. Coevolution of neocortical size, group size and language in humans. Behavioral and brain sciences 16 (04), 681-694

5. Dunbar RIM. 2010. The social role of touch in humans and primates: behavioural function and neurobiological mechanisms. Neuroscience & Biobehavioral Reviews 34 (2), 260-268

6. Dunbar RIM. 2012 The social brain meets neuroimaging. Trends in Cognitive Science. 16(2):101-2.

7. Shultz et al. 2004. A community-level evaluation of the impact of prey behavioural and ecological characteristics on predator diet composition. Proc R Soc Lond 271B:725-732.

8. Trivers R. 2013 Deceit and Self-Deception. Penguin Books


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