As humans, we aren't born with formidable armaments or defenses, nor are we the strongest, fastest, or biggest species, yet despite this we are amazingly successful. For a long time it was thought that this success was because our enlarged brains allows each of us to be smarter than our competitors: better at abstract thinking, better with tools and better at adapting our behavior to those of our prey and predators. But are these really the most significant skills our brains provide us with?
Another possibility is that we are successful because we can form long-lasting relationships with many others in diverse and flexible ways, and that this, combined with our native intelligence, explains why homo sapiens came to dominate the planet. In every way from teaching our young to the industrial division of labour we are a massively co-operative species that relies on larger and more diverse networks of relationships than any other species.
In 1992 British anthropologist Robin Dunbar published an article showing that, in primates, the ratio of the size of the neo-cortex to that of the rest of the brain consistently increases with increasing social group size. For example, the Tamarin monkey has a brain size ratio of about 2.3 and an average social group of size of about 5 members. On the other hand, a Macaque monkey has a brain size ratio of around 3.8 but a very large average group size of about 40 members. From this work Dunbar put forward what is now known as the “social brain hypothesis.” The relative size of the neo-cortex rose as social groups became larger in order to maintain the complex set of relationships necessary for stable co-existence. Most famously, Dunbar suggested that given the human brain ratio we have an expected social group size of around 150 people, about the size of what Dunbar called a “clan.”
Now, in the journal Proceedings of the Royal Society B, Dunbar and his colleagues have shown that the size of each individual’s social network is linearly related to the neural volume in a frontal region of each individual’s brain, the orbital prefrontal cortex. This provides strong support for Dunbar’s original conjecture at the individual level for what was previously proposed based on species-level data: Our brains are not as large as they are in order to provide each of us with the raw computational power to think our way out of a sticky situation, instead our brain size helps each of us to deal with the large and complex network of relationships we rely on to thrive.
What the authors have also been able to show is that it's more than just the availability of raw neural material in the right area of the brain that is needed. Within the same study they gave each subject a psychological test of their social skills and what they found was that even if a subject had a larger orbital prefrontal cortex this didn’t necessarily correlate with larger social networks, it also needed the subjects to have developed certain psychological skills, particularly an ability to understand another person’s state of mind. This cognitive skill is called a “theory of mind,” and it is the ability to recognize that others have their own mental states, such as factual knowledge, emotions or beliefs, and that these can be different from our own. It turns out that humans have the most highly developed “theory of mind” amongst the primates. Michael Tomasello and his colleagues have previously shown that while chimpanzees, one of our closest genetic relatives, posses a “theory of mind” it is not nearly as powerful, and this lack of social cognitive ability, combined with Dunbar’s work, suggests that it is this lack that results in their smaller social groups.