Like many topics of psychological research, the explanation for
friendship is at once intuitive and difficult to address empirically.
These difficulties worsen when one seeks, as we do, to go beyond
"obvious" explanations ("humans are social creatures") to ask deeper
questions, such as "What is the evolved function of human friendship?"
In recent years, however, a new window into human behavior has opened
as a growing fraction of people’s social activity has moved online,
leaving a wealth of digital traces behind.  One example is a feature
of the MySpace social network that allows millions of users to rank
their "Top Friends."  In this study, we collected over 10 million
people’s friendship decisions from MySpace to test predictions made by
hypotheses about human friendship.  We found particular support for
the alliance hypothesis, which holds that human friendship is caused
by cognitive systems that function to create alliances for potential
disputes.  Because an ally’s support can be undermined by a stronger
outside relationship, the alliance model predicts that people will
prefer partners who rank them above other friends.  Consistent with
the alliance model, we found that an individual’s choice of best
friend in MySpace is strongly predicted by how partners rank that
individual.