Can kinship ever inhibit the evolution of cooperation?

A 2-person game-theoretic model of the evolution of cooperation is presented. The model compares the fate of cooperation when pairs are formed by randomly selecting individuals and when groups are composed of close kin. Results indicate that some cost/benefit scenarios, kinship can inhibit, rather than facilitate, the evolution of cooperation. Although kinship increases between-group variance (and thus facilitates cooperation), it also causes cooperators to continue to cooperate when they wouldn't otherwise, therby providing a large payoff to ''free-riding'' cheaters. The model presented here, in conjunction with other recent theoretical developments, should serve as a caveat that although kinship often facilitates sociality, under some reasonable conditions it may, in fact, do just the opposite. Rather than simply assume that relatedness per se selects for sociality, we need to step back and consider the population structure, as well as the rules that are available to animals, when examining the evolution of cooperation, and more generally, social behavior.