Letters to Nature

Nature 393, 573-577 (11 June 1998) | doi:10.1038/31225; Received 11 November 1997; Accepted 31 March 1998

Evolution of indirect reciprocity by image scoring

Martin A. Nowak1,2 & Karl Sigmund3

  1. Department of Zoology, University of Oxford, Oxford OX1 3PS, UK
  2. Institut für Mathematik, Universität Wien, Strudlhofgasse 4, A-1090 Wien, Austria
  3. Present address: Institute for Advanced Study, Olden Lane, Princeton, New Jersey 08540, USA

Correspondence to: Martin A. Nowak1,2 Correspondence and requests for materials should be addressed to M.A.N. (e-mail: Email: martin.nowak@zoo.ox.ac.uk).

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.