The coevolution of loyalty and cooperation

Humans are inclined to engage in long-lasting relationships whose stability does not only rely on cooperation, but often also on loyalty - our tendency to keep interacting with the same partners even when better alternatives exist. Yet, what is the evolutionary mechanism behind such irrational behavior? Furthermore, under which conditions are individuals tempted to abandon their loyalty, and how does this affect the overall level of cooperation? Here, we study a model in which individuals interact along the edges of a dynamical graph, being able to adjust both their behavior and their social ties. Their willingness to sever interactions is determined by an individual characteristic and subject to evolution. We show that defectors ultimately loose any commitment to their social contacts, a result of their inability to establish any social tie under mutual agreement. Ironically, defectors' constant search for new partners to exploit leads to heterogeneous networks in which cooperation survives more easily. Cooperators, on the other hand, develop much more stable and long-term relationships. Their loyalty to their partners only decreases when the competition with defectors becomes fierce. These results indicate how our innate commitment to partners is related to mutual agreement among cooperators and how this commitment is evolutionary disadvantageous in times of conflict, both from an individual and a group perspective.