Tuesday, February 15, 2011

The Social Brain: Bigger is Better?

Humans are an incredibly social species.  How did we become that way?  According to the "social brain hypothesis," specific areas within the brain were selectively enlarged during evolution to support enhanced social functioning.

Humans are a social species
The precise underlying changes in the brain are not known, but many scientists are focusing on one particular area: the amygdala.  With its extensive connectivity throughout the brain and well-known involvement in a variety of social behaviors, including fear, anger, and certain types of learning and memory, the amygdala is an ideal candidate for study.  According to the social brain hypothesis, we would predict that a larger amygdala means increased social capacity.

Now a report in the latest issue of Nature Neuroscience provides new support for this hypothesis.  Using MRI brain imaging and a metric called the Social Network Index, Kevin Bickart and colleagues report that the volume of the amygdala is strongly correlated with both the size and complexity of an individual's social network.  

The size of the amygdala is greater in people
with larger and more complex social networks
(image from scienceblogs.com/neurotopia)
This work builds upon previous studies in non-human primates showing that the more social a group is, the larger the amygdala.  Here the results are extended to humans, and the focus is on the significant variation that occurs between individuals within a group.  The anatomical differences the authors observed were very specific - no correlation was seen with the size of the hippocampus or other brain regions.  Rather, the amygdala seems to have a uniquely key role in the social brain.
When interpreting such results, it's important to remember that correlation does not equal causation.  The social brain hypothesis would suggest that increased amygdala size facilitates greater social functioning.  However, it's possible these are co-occurring but unrelated phenomena.  Alternatively, causation could work in the opposite direction: maybe being more social increases the size of the amygdala, much like a muscle grows with increased use.

How do we define a social network?
Another consideration is how we quantify a social network.  The method used here is the Social Network Index (SNI), in which the primary metric is number of "regular contacts"-  defined as people spoken to (by phone or in person) at least once every 2 weeks.  However, in the age of Facebook and virtual communication, the definition of a social network is rapidly changing.  I personally can think of several close friends who I haven't spoken to in weeks, but we have ongoing conversations by Facebook or text messages.  The SNI, which was created in 1997 for a study of resistance to the common cold, seems overly simplistic and out of date in its definition of "regular contact."  As a result the size of certain individuals' social networks may by underestimated, which could skew the overall findings. (to see the actual test, click here

In the brain, is bigger really better?
Those issues aside, the most significant caveat in the current study is this: we don't actually know if bigger means better.  Increased size of a brain area does not necessarily mean greater functional capacity.  In some cases, dysfunction results when the brain is unable to effectively downsize.  Autism is one typical example.  And in fact, another study previously showed that the amygdala is larger in young children with autism.  So in contrast to what Bickart and colleagues observed, a bigger amygdala in certain scenarios might actually be detrimental to one's social abilities.

Despite these caveats, the present results are intriguing and provide further support for the social brain hypothesis and the idea that the amygdala plays a central role in social functioning.  The study's authors are quick to point out that their results don't really provide answers but rather should be viewed as preliminary findings that "provok[e] an interesting question."  The real answers await future studies that will continue to elucidate the complex entangled ways in which our brains and social capacities evolved. 

Original article:  Bickart KC, Wright CI, Dautoff RJ, Dickerson BC, Barrett LF (2011). Amygdala volume and social network size in humans. Nature Neuroscience 14(2): 163-164.


  1. "Bigger brain" necessarily means more synapses and/or cells?

  2. can be either, or both. unfortunately it's impossible to distinguish without doing something much more invasive (i.e. autopsy)

  3. Mens brains are a bit larger than womens yet it doesnt make much of a difference when you take into account our body size differences. I am studying the glia that seems to fascinate me.