Jan 3, 2011

Amygdala Volume and Social Network Size in Humans

The Social Network, the recent film that tells the story of the founders of the Facebook social networking page reminds us of the impact of online connections and the complexity of interpersonal relationships in today's world. For many, the number of friends in Facebook represents the ability to communicate effectively with many people, a measure of success in the world today. With this in mind, it is interesting to ask whether the brain responds to indicators of socialization as it does other kinds of measures, such as motor development in the case of musicians. The amygdala, a small almond-shaped structure in the brain, is involved in interpersonal functions such as memory and facial recognition. Could the size of the amygdala predict the ability to form social ties in the form of the number of Facebook friends and the effectiveness of online relationships as suggested by the article in NatureNeuroscience last month?

The article by Kevin Bickart –a fellow doctorate student at Boston University-and a team of researchers from Massachusetts General Hospital and Northeastern University in Boston, show that in humans, the size of the amygdala is correlated with the size and complexity of social networks (notonly Facebook, but social networks in general). To establish the Social Network Index, the researchers asked 58 volunteers to list the number of close contacts they maintain (size) and the number of groups their contacts belong to (complexity index). Using non-invasive techniques of magnetic resonance imaging (MRI), the team examined the volume of the amygdala and of other brain regions and found a strong correlation between the size of the first with measures of socialization.

In the world today close contacts are a difficult concept to define (in fact the article does not). Some people are limited to a small group of friends and relatives, while rock stars have thousands of contacts on their lists. However, the average Facebook user has 130 friends, which is comparable to the number of friends people keep in close contact with, even before the Facebook era; and it is the number predicted by some sociology studies. This means that online social tools have not increased the number of relationships a person can have, but rather, the physical distance between the user and his friends. Before, all the friends were in a small geographical area, but now can reside anywhere in the world.

However, if the size of the contact list has not changed much with Facebook, the complexity of the networks probably has. The number of groups the contacts belong to, can be equal to the number of contacts themselves. If we consider that our ability to make friends was limited to our daily activities, then the number of groups was restricted to school, work, family and a hobby or two. On the other hand, through Facebook, people can be in active contact with others in any context, as physical restraints are not a problem. Now, how does this make the network more or, conversely, less complex? According to the article criteria, it would make it more complex because the number of groups would be larger. However in that case, most contacts would not be related and would not interact with one another, which would render the user's relationships with each person much simpler, something quite contrary to what happens in small towns, for example.

The answer to the level of complexity of the interactions is unpredictable, but this study could help answer these questions. The article places the results in an evolutionary context in which the pressure of living in social groups naturally selected for larger and more complex brains, an idea called "Social Brain Hypothesis." In particular, comparative studies with non-human primates indicate that the amygdala and its connections with other brain regions have evolved to maintain more complex social lifestyles. Moreover in the study by Bickard, the amygdala shows non-evolutionary differences that respond to the complexity of social networks among people from the same time period. If this is true, then the amygdala would reflect the pressure of complex social relationships, due to the demands of a globalized world, and the analysis of its size would show a dichotomy between the generations of Internet users, or even Facebook users. It would also be possible to determine whether the type of relationships through social networking sites are, in fact, a driving force; or conversely, an artificial selection tool for interpersonal relationships more numerous but simpler.

However, the results of the paper should be considered carefully because the correlations do not say anything about the dynamics of interaction: a larger amygdala leads to larger and more complex social networks?; or, Having more friends increases the size of the amygdala?; or, does the found association is simple coincidence?. In addition, other individual characteristics of social interactions, such as memory, empathy and trust can independently influence the size of the amygdala. Although the interpretation and discussion of results in the article is rather short (except for supplemental material), the study clearly shows the relationship between the size of the amygdala and the social networks and opens the doors a variety of investigations, from the effects of social networking sites in healthy individuals to the implication in diseases affecting the ability to interact socially such as autism, depression and social phobias.


  1. This is a very nice summary and set of questions for future research. I suspect that what might be driving the link between amygdala size and social network size is the increased demands imposed by forming and maintaining larger, more complex social networks. A larger amygdala might better equip certain individuals to process more social signals from more people in more social contexts in order to coordinate effective interpersonal behavior. Research in healthy humans has not answered these questions yet, but the literature on amygdala size in autism might be a good place to look for an understanding of amygdala size and social dysfunction. I am currently conducting a number of follow up studies to understand whether it is the amygdala in isolation or a network of strongly connected regions that further explains the link reported here.

    As for online social networks, I suspect that a measure that estimates the demands of managing online relationships would be important in studying links between online social networks and amygdala size for future research - another interesting and open question. Another important measure might be the number of interactions with friends online, or number of high-contact friends, or different subnetworks of friends (as mentioned here by Veronica), or number of interconnections between one's friends.

    Other ideas here?

  2. Thank you Kevin for your input. I was recently reading about William's Syndrome, besides a number of symptoms, children with this disease are extremely friendly and do not distinguish between friends and strangers. The amygdala of these individuals also shows wide variation as is the case in autistic kids. This is even more evidence that the amygdala is at the center of the social being!