  Cooperation among unrelated individuals is frequently observed in social
groups when their members combine efforts and resources to obtain a shared
benefit that is unachievable by an individual alone. However, understanding why
cooperation arises despite the natural tendency of individuals towards selfish
behavior is still an open problem and represents one of the most fascinating
challenges in evolutionary dynamics. Recently, the structural characterization
of the networks in which social interactions take place has shed some light on
the mechanisms by which cooperative behavior emerges and eventually overcomes
the natural temptation to defect. In particular, it has been found that the
heterogeneity in the number of social ties and the presence of tightly knit
communities lead to a significant increase in cooperation as compared with the
unstructured and homogeneous connection patterns considered in classical
evolutionary dynamics. Here, we investigate the role of social-ties dynamics
for the emergence of cooperation in a family of social dilemmas. Social
interactions are in fact intrinsically dynamic, fluctuating, and intermittent
over time, and they can be represented by time-varying networks. By considering
two experimental data sets of human interactions with detailed time
information, we show that the temporal dynamics of social ties has a dramatic
impact on the evolution of cooperation: the dynamics of pairwise interactions
favors selfish behavior.
