[The emergence of indirect reciprocity: evolutionary foundation of altruistic behavior based on "strict...

Although there have been a number of studies that theoretically and empirically examined altruism based on direct reciprocity, few have been conducted on how altruism based on indirect reciprocity emerges. Recent advances in biological research, however, have suggested possible answers to the question. For instance, Nowak and Sigmund (1998a, b) proposed that what they called image scoring strategy made indirect reciprocity possible. After critically examining their work, Leimar and Hammerstein (2001) pointed out several limitations to the theory, and instead proposed standing strategy as an explanation. Although careful attempts to replicate the findings by them and Panchanathan and Boyd (2003) supported the arguments against image scoring, we reveal that standing strategy was not a satisfactory answer either. Based on a series of evolutionary simulations, we propose a new strategy, which we call strict discriminator, as an alternative. Strict discriminators are discriminating altruists, similar to the altruists with image scoring or standing strategy, but they are different in that its criterion for discrimination is stricter: unconditional altruists are excluded from their reciprocity.