Social neuroscience is a rapidly growing discipline that examines the relationship between the brain and social behavior. The “social brain hypothesis” posits that, over evolutionary time, living in large, social groups favored the physical growth of brain regions important for social behavior. In non-human primates, some evidence indicates that the size of the amygdala is related to social behavior. Little is known, however, about this relationship in humans. A provocative new study finds that the volume of a key component of the social brain, the amygdala, is directly related to the size and complexity of social networks in adult humans.