How many friends do you have? A rough answer can be predicted by the size of a small, almond-shaped brain structure that is present in a wide range of vertebrates, scientists report today in Nature Neuroscience. The researchers studied the amygdala, which is involved in inter-personal functions such as interpreting emotional facial expressions, reacting to visual threats and trusting strangers. Inter-species comparisons in non-human primates have previously shown that amygdala volume is associated with troop size, suggesting that the brain region supports skills necessary for a complex social life1. On the basis of these past findings, psychologist Lisa Feldman Barrett of Northeastern University in Boston, Massachusetts, wondered whether a larger amygdala size allows some humans to build a richer social world.
Barrett's team measured the amygdala volume in 58 healthy adults using brain images gathered during magnetic resonance imaging sessions. To construct social networks, the researchers asked the volunteers how many people they kept in regular contact with, and how many groups those individuals belonged to. They found that participants who had bigger and more complex social networks had larger amygdala volumes. This effect did not depend on the age of the volunteers or their own perceived social support or life satisfaction, suggesting that happiness is not the underlying causal factor that links the size of this brain structure in an individual to their number of friends2.