The social brain hypothesis has suggested that social network size (and structure) is constrained by a combination of cognitive processes and the time required to service social relationships. We test this hypothesis in humans using a unique 18-month mobile phone dataset by examining changes in the structure of social networks across a major change in subjects' social and geographical circumstances.  (...) This provides the first direct evidence that social networks are constrained by a combination of cognitive constraints and the time individuals have available for social interaction, confirming one of the key assumptions of the social brain hypothesis.

The persistence of social signatures in human communication

J. Saramaki, E. A. Leicht, E. Lopez, S. G. B. Roberts, F. Reed-Tsochas, R. I. M. Dunbar

http://arxiv.org/abs/1204.5602