Summary: vol. 175, no. 2 the american naturalist february 2010
The Evolution of Reciprocity: Social Types or Social Incentives?
Ecologie et Evolution, Unite´ Mixte de Recherche 7625, Centre National de la Recherche Scientifique, Universite´ Pierre et Marie Curie,
Ba^timent A, 7e`me e´tage, Case 237, 7 Quai Saint Bernard, F-75252 Paris Cedex 05, France
Submitted May 26, 2009; Accepted October 14, 2009; Electronically published December 16, 2009
Online enhancement: appendix.
abstract: The vast majority of human beings regularly engage in
reciprocal cooperation with nonrelated conspecifics, and yet the cur-
rent evolutionary understanding of these behaviors is insufficient.
Intuitively, reciprocity should evolve if past behavior conveys infor-
mation about future behavior. But it is not straightforward to un-
derstand why this should be an outcome of evolution. Most evo-
lutionary models assume that individuals' past behavior informs
others about their stable social type (defector, cooperator, recipro-
cator, etc.), which makes it sensible to reciprocate. In this article,
after describing the central source of difficulty in the evolutionary
understanding of reciprocity, I put forward an alternative explanation
based on a work by O. Leimar. It consists of taking into account the
fact that the payoffs to individuals in social interactions can change