In the 1990s the British evolutionary anthropologist Robin Dunbar championed an idea known as the Social Brain Hypothesis. He found that mammals who lived in the largest social groups often had the largest neocortex to brain ratio. Since the neocortex — composed chiefly of gray matter that forms the outermost “rind” of our cantaloupe-sized stuff of thought — is associated with sensory perception and abstract reasoning, Dunbar hypothesized that the demands of group living resulted in a selection pressure that promoted the expansion of neocortical growth.
In 2009 I co-authored a study in the Journal of Human Evolution with colleagues Evan MacLean, Nancy Barrickman, and Christine Wall of Duke University that found no relationship between relative brain size and group size in lemurs (a clade of strepsirrhine primates that last shared a common ancestor with the haplorhine monkeys and apes about 75 million years ago). However, where it comes to these more recently evolved haplorhines, the data is remarkably consistent with Dunbar’s interpretation (see Figure 1 below).
Primates, and humans in particular, are such good social cooperators because we can empathize with others and coordinate our activities to build consensus. It is what also makes us so remarkably deceitful, allowing us to manipulate other members of our group by intentionally making them think we will behave one way when our actual plans are quite different. A successful primate is therefore one who can keep track of these subtle details in behavior and anticipate their potential outcome.
But therein lies a chicken-and-egg problem. How do we know whether it’s the social networks that have promoted an increase in neocortical growth or whether that same expansion of gray matter simply allowed these social networks to expand? A new study published in the November 4th edition of Science addressed this question by housing monkeys in different sized groups to find out if their neocortical gray matter increased as the number of individuals grew.