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Evolution of indirect reciprocity by image scoring


Darwinian evolution has to provide an explanation for cooperative behaviour. Theories of cooperation are based on kin selection (dependent on genetic relatedness)1,2, group selection3,4,5 and reciprocal altruism6,7,8. The idea of reciprocal altruism usually involves direct reciprocity: repeated encounters between the same individuals allow for the return of an altruistic act by the recipient10,11,12,13,14,15,16. Here we present a new theoretical framework, which is based on indirect reciprocity17 and does not require the same two individuals ever to meet again. Individual selection can nevertheless favour cooperative strategies directed towards recipients that have helped others in the past. Cooperation pays because it confers the image of a valuable community member to the cooperating individual. We present computer simulations and analytic models that specify the conditions required for evolutionary stability18 of indirect reciprocity. We show that the probability of knowing the ‘image’ of the recipient must exceed the cost-to-benefit ratio of the altruistic act. We propose that the emergence of indirect reciprocity was a decisive step for the evolution of human societies.

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Figure 1: Cooperation wins in a computer simulation of indirect reciprocity.
Figure 2: Long-term evolution of indirect reciprocity under mutation and selection.
Figure 3: Indirect reciprocity with incomplete information about the image score of other players.
Figure 4: A further dimension is added to the game if donors base their decision to cooperate not only on the image score of the recipient but also on their own score.


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We thank M. Dawkins, A. Kacelnik, J. Krebs and R. May for discussion. Support from the Wellcome Trust is gratefully acknowledged. Part of this work was done at IIASA (Laxenburg).

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Correspondence to Martin A. Nowak.

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Nowak, M., Sigmund, K. Evolution of indirect reciprocity by image scoring. Nature 393, 573–577 (1998).

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