  The social brain hypothesis predicts that humans have an average of about 150
relationships at any given time. Within this 150, there are layers of friends
of an ego, where the number of friends in a layer increases as the emotional
closeness decreases. Here we analyse a mobile phone dataset, firstly, to
ascertain whether layers of friends can be identified based on call frequency.
We then apply different clustering algorithms to break the call frequency of
egos into clusters and compare the number of alters in each cluster with the
layer size predicted by the social brain hypothesis. In this dataset we find
strong evidence for the existence of a layered structure. The clustering yields
results that match well with previous studies for the innermost and outermost
layers, but for layers in between we observe large variability.
