Do you ever look at people on Facebook and wonder why they have so many—or so few—friends? It turns out that the number of friends you list online may be linked to your brain structure. New research in Proc B has found that the amount of grey matter in specific areas of our brain is correlated to the number of friends we have on Facebook, even though there's no correlation with real-world social network size.

Researchers already knew that the size of our real-life social network is related to amygdala size, presumably because the volume of this area of the brain limits the amount of social information we can process; those with large amygdalae tend to have larger groups of friends and acquaintances than those with smaller amygdalae. Since Facebook and other social networks have become so popular, researchers have begun to wonder whether this relationship holds true for online networks as well, or whether other regions of the brain are more closely correlated to the number of friends we have online.

A group of British undergraduates served as as the subjects for this experiment, since college students are particularly active in online social networks. These 125 students were given MRI scans, and the results of these scans were compared to the number of friends the subjects listed on their Facebook page. Grey matter densities in three areas of the brain—the left middle temporal gyrus, the right posterior superior temporal sulcus, and the right entorhinal cortex—were positively correlated to the size of the participants’ online social network. Amygdala volume was similarly correlated to the number of Facebook friends.

These areas of the brain are functionally tied to social cognition. The right posterior superior temporal sulcus, for example, helps us anticipate and understand others' intentions. The right entorhinal cortex is associated with the formation of memories between pairs of things, such as names and faces.

To replicate the findings of previous work, the researchers also determined the real life social network size of the subjects by asking a set of questions. These questions asked how many friends' numbers were in their cell phone, how many people they think would do them a favor, and how many people were at their last birthday party. This confirmed that in this sample, as in previous research, amygdala size was related to real life social network size. No other focal regions of the brain predicted the number of friends the participants had in real life.

So, if the regions of the brain associated with online social network size are implicated in social cognition, why don’t they also predict the size of real life social networks? There isn’t a great answer to this question yet. The researchers behind the study believe that different aspects of social cognition may mediate online relationships and real life relationships. For instance, social networks are often larger than networks in real life, so associative memory may play a larger role online.

There are still a lot of unknowns here; for instance, we don’t yet know if these relationships hold true for other age groups. We also don’t have any idea how much of this association is due to nature and how much to nurture. In any case, it’s a good start to understanding what cognitive skills are important in online relationships compared to real relationships, and how our brains may dictate the sizes of these networks.

Proceedings of the Royal Society B, 2011. DOI: 10.1098/rspb.2011.1959 (About DOIs).