  Theoretical models suggest that social networks influence the evolution of
cooperation, but to date there have been few experimental studies.
Observational data suggest that a wide variety of behaviors may spread in human
social networks, but subjects in such studies can choose to befriend people
with similar behaviors, posing difficulty for causal inference. Here, we
exploit a seminal set of laboratory experiments that originally showed that
voluntary costly punishment can help sustain cooperation. In these experiments,
subjects were randomly assigned to a sequence of different groups in order to
play a series of single-shot public goods games with strangers; this feature
allowed us to draw networks of interactions to explore how cooperative and
uncooperative behavior spreads from person to person to person. We show that,
in both an ordinary public goods game and in a public goods game with
punishment, focal individuals are influenced by fellow group members'
contribution behavior in future interactions with other individuals who were
not a party to the initial interaction. Furthermore, this influence persists
for multiple periods and spreads up to three degrees of separation (from person
to person to person to person). The results suggest that each additional
contribution a subject makes to the public good in the first period is tripled
over the course of the experiment by other subjects who are directly or
indirectly influenced to contribute more as a consequence. These are the first
results to show experimentally that cooperative behavior cascades in human
social networks.
