Learning to cooperate via indirect reciprocity

Cooperating in the Prisoner's Dilemma is irrational and some supporting mechanism is needed to stabilize cooperation. Indirect reciprocity based on reputation is one such mechanism. Assessing an individual's reputation requires first-order information, i.e. knowledge about its previous behavior, as it is utilized under image scoring. But there seems to be an agreement that in order to successfully stabilize cooperation, higher-order information is necessary, i.e. knowledge of others' previous reputations. We show here that such a conclusion might have been premature. Tolerant scoring, a first-order assessment rule with built-in tolerance against single defections, can lead a society to stable cooperation.