The emergence of commitments and cooperation

Introduction Over the last few years, several mechanisms have been pointed out to promote the emergence and maintenance of cooperation. From group and kin relations, memory and reputation based reciprocity mechanisms, to social diversity and context based reactions, grounded or not on incipient levels of cognition, there has been a large improvement on our capacity to understand the roots of animal and human cooperation (Axelrod and Hamilton, 1981; Axelrod, 1984; Hofbauer and Sigmund, 1998; Nowak, 2006b; Santos et al., 2008; Sigmund, 2010; Han et al., 2011a,b). They are certainly hugely important, but are they sufficient? Or, as many have suggested (Boehm, 1999; Hirshleiffer, 1999; Humphrey, 1999), might there be other routes to social behavior that have been neglected? Certainly there are. Commitment, which amounts to expressing an intention rather than having it recognized, may stand as another route to cooperation, even in its simplest form, as we purport to show here.