Repeated interactions provide a prominent but paradoxical hypothesis for human cooperation in one-shot interactions. Intergroup competitions provide a different hypothesis that is intuitively appealing but heterodox. We show that neither mechanism reliably supports the evolution of cooperation...

An apology is a strong and cheap device to restore social or economic relationships that have been disturbed. In a laboratory experiment we find that harmdoers use apologies in particular if they fear punishment and when their intentions cannot be easily inferred. After offenses with ambiguous...