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Social Networks Matter: Friends Increase the Size of Your Brain

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Let's face it, as a species we're obsessed with ourselves. The vast majority of us spend our days at work or school where a considerable amount of time is taken up not discussing the important issues of the day, but rather the juicy details of one another's personal lives. Then we go home only to sign on to social network services like Facebook, Twitter, or Google Plus and continue where we left off. In this respect we're fairly typical primates. Most of our simian relatives, particularly our great ape cousins the chimpanzees and bonobos, like nothing better than keeping a watchful eye on what other members of their troop are up to. But our species has taken this preoccupation one step further.

Human beings are the most social of the primates and have the largest group sizes of any species in our order. For about 90% of our existence we lived in hunter-gatherer societies with populations that likely clustered around 150-200 individuals. By way of comparison, baboons come in a distant second with an average of about 50 group members. Now, thanks to modern industrial agriculture, our species has pushed that range well into the millions, a development that has resulted in considerable stress on our slightly above average primate brains. Of course, all organisms need to successfully predict and navigate their environments in order to relay their genes on to the next generation. It's just that this becomes increasingly complicated when there are many individuals all interacting in the same environment simultaneously. Merely keeping track of these relationships requires a considerable amount of time and energy, not to mention brain power.

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).

As average group size increases in monkeys and apes, so does neocortex ratio.

Figure 1. As average group size increases in monkeys and apes, so does neocortex ratio.
Reproduced from Dunbar and Shultz (2007).


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. A team of neuroscientists led by Jérôme Sallet and Matthew Rushworth of the University of Oxford in England randomly assigned 34 rhesus macaques to separate social groups ranging in size from 1 to 7. The researchers conducted magnetic resonance imaging (MRI) scans on 23 of the monkey's brain structures both before they were placed into their various groups and again after more than a year had passed.

Their analysis revealed a clear, linear relationship between the size of a monkey's social network and an increase of neocortical gray matter in regions involved with social cognition (such as the mid-superior temporal sulcus, rostral prefrontal cortex as well as the frontal and temporal cortex). Previous research has shown that these regions are important for a variety of social behaviors, such as interpreting facial expressions or physical gestures, "theory of mind," and predicting the behavior of other group members. Overall the monkeys demonstrated an expansion of gray matter ranging from 3-8% (depending on the brain region) for each additional member of their social network. In other words, monkeys that lived in the most socially complex group had an average increase of 20% more neocortical growth than monkeys housed individually.

Gray matter increased with social network size

Figure 2. Gray matter increased with social network size; P < 0.005.

Reproduced from Sallet et al. (2011).


In order to make sure that the increased brain growth corresponded with more successful social behaviors, the research team also tested whether there was a correlation between gray matter volume and a monkey's rank within their group (as in many other primates, rank in rhesus macaques is a strong predictor of reproductive success). Once again the researchers found a linear relationship, at a ratio of 3-to-1, between a monkey's dominance behavior and the growth of key regions in their neocortex. This means there was individual (potentially genetic) variation that allowed certain monkeys to experience greater neocortical growth than other group members that were living in an identical environment. This strongly suggests that it is the cognitive demands of a larger social network that has resulted in the growth of brain regions beneficial to social behavior in primates.

"Social network size, therefore, contributes to changes both in brain structure and function," said Sallet. "Individual variation in brain anatomy should have implications for an individual's success within the social group." Crucially, these individual differences remained consistent for more than four months. Certain individuals happened to be better suited for dealing with the demands of larger social groups, but they had to first live in that environment before their natural abilities could emerge.

This raises a provocative question. Individual variation is the raw material on which natural selection operates. But in a rapidly changing environment -- like in many human societies ever since the invention of agriculture 10,000 years ago -- there will be many new adaptive opportunities that may never have existed throughout most of human evolution. Consider those individuals who have made successful careers (and had large families) through their skill as novelists, DJs, or computer programmers. Certain aspects of their skill sets would certainly have been based in our long history of hominin evolution, but other parts may have had little or no adaptive value at any other time than the present.

It is this capacity that was the focus of a study published last month in Proceedings of the Royal Society that investigated the biological variability in another form of social behavior: online social networking. In a collaboration between neuroscientists and anthropologists led by Ryota Kanai and Geraint Rees from the Institute of Cognitive Neuroscience at University College London, the researchers investigated social media users, specifically Facebook, for the same kinds of biological variation that distinguished certain social monkeys over others.

"These services allow individuals to articulate and make visible their friendship networks," explained Kanai, "and it is apparent that there is considerable variability in the size of such networks."

By comparing the differences between individuals and the size of their online network of friends, real-world friends, as well as the size of neocortical brain regions involved in social behavior, the researchers were able to identify a strong correlation between the volume of three neocortical regions and the number of that individual's Facebook friends. Crucially, these brain regions (the right superior temporal sulcus, left middle temporal gyrus, and entorhinal cortex, areas previously implicated in social perception and associative memory) had no relationship to the real-world social networks of these individuals. There was only one area, the amygdala, that showed a correlation between gray matter density and both forms of social networking. The other brain regions seemed to be, quite literally, wired for the web.

However, unlike the study with monkey social networks, there was no way to determine whether it was the number of an individual's Facebook friends that had pushed this neocortical growth or if it was actually the other way around. But given the similarities in function, it is certainly a tempting conclusion to reach. Could it be that online technology has allowed some individuals to express (and expand) a form of social behavior that emerged for other adaptive reasons but which has been underutilized until now?

Given the regular jeremiads from self-appointed cultural guardians over what they see as the danger of our increasing reliance on online networks at the expense of real-world ones, the possibility that we may actually be enhancing untapped potential is a refreshing idea. At the same time, however, it's probably a good idea to wait until we know for sure before sharing the news with any other primates. The last thing I need is a slew of hairy faces crowding my wall. I have enough trouble keeping track of my online network of friends as it is.

Originally posted at The Primate Diaries hosted by Scientific American.