P (see "Prisoner's Dilemma", above) it cooperates; if S or T it defects. In a 2006 paper Nowak listed five mechanisms by which natural selection can lead to cooperation. In addition to kin selection and direct reciprocity, he shows that: Indirect reciprocity is based on knowing the other player's reputation, which is the player's history with other players. Cooperation depends on a reliable history being projected from past partners to future partners. Network reciprocity relies on geographical or social factors to increase the interactions with nearer neighbors; it is essentially a virtual group. Group selection assumes that groups with cooperators (even altruists) will be more successful as a whole, and this will tend to benefit all members. The payoffs in the Prisoner's Dilemma game are fixed, but in real life defectors are often punished by cooperators. Where punishment is costly there is a second-order dilemma amongst cooperators between those who pay the cost of enforcement and those who do not. Other work has shown that while individuals given a choice between joining a group that punishes free-riders and one that does not initially prefer the sanction-free group, yet after several rounds they will join the sanctioning group, seeing that sanctions secure a better payoff. In small populations or groups there is the possibility that indirect reciprocity (reputation) can interact with direct reciprocity (e.g. tit for tat) with neither strategy dominating the other. The interactions between these strategies can give rise to dynamic social networks which exhibit some of the properties observed in empirical networks If network structure and choices in the Prisoner's dilemma co-evolve, then cooperation can survive. In the resulting networks cooperators will be more centrally located than defectors who will tend to be in the periphery of the network. In "The Coevolution of Parochial Altruism and War" by Jung-Kyoo Choi and Samuel Bowles. From their summary: Altruism—benefiting fellow group members at a cost to oneself —and parochialism—hostility towards individuals not of one's own ethnic, racial, or other group—are common human behaviors. The intersection of the two—which we term "parochial altruism"—is puzzling from an evolutionary perspective because altruistic or parochial behavior reduces one's payoffs by comparison to what one would gain from eschewing these behaviors. But parochial altruism could have evolved if parochialism promoted intergroup hostilities and the combination of altruism and parochialism contributed to success in these conflicts.... [Neither] would have been viable singly, but by promoting group