Human cooperation is an evolutionary puzzle. Unlike other creatures, people frequently cooperate with genetically unrelated strangers, often in large groups, with people they will never meet again, and when reputation gains are small or absent. These patterns of cooperation cannot be explained by the nepotistic motives associated with the evolutionary theory of kin selection and the selfish motives associated with signalling theory or the theory of reciprocal altruism. Here we show experimentally that the altruistic punishment of defectors is a key motive for the explanation of cooperation. Altruistic punishment means that individuals punish, although the punishment is costly for them and yields no material gain. We show that cooperation flourishes if altruistic punishment is possible, and breaks down if it is ruled out. The evidence indicates that negative emotions towards defectors are the proximate mechanism behind altruistic punishment. These results suggest that future study of the evolution of human cooperation should include a strong focus on explaining altruistic punishment.

Ernst 'ehr and Simon 01chter 3niversity of 7urich 9nstitute for Empirical Research in Economics =l>mlisalpstrasse 10 CB-D00E 7urich e-mail: efehrGieHIuniJhIchK LaechterGieHIuniJhIch http:MMHHHIuniJhIchMieHMLrpMfehrMindeNIhtml OorPinL Qaper RoI 10 June 1TTT U Vhis paper is part of the E3-VWR Research RetHorP ERXEYR ('WR[-CVTD-02]D)I Support from the SHiss Rational Science 'oundation under pro_ect number 1214-0b1000IT7 and from the WacYrthur 'oundation RetHorP on Economic Environments and the Evolution of 9ndividual Qreferences and Social Rorms is Lratefully acPnoHledLedI Oe received valuable comments by seminar participants at the WacYrthur-'oundation WeetinL in Stanford, the OorPshop in ENperimental Economics in =erlin, the YSSY-WeetinLs in ReH erleans and ReH forP, the 9YREQ conference in

. 35. A sea-level rise Dsl pushes the entire oceanic lithosphere down; thus, the resulting sea-level rise Dsl is less than the changing mean ocean basement depth Dh, which is computed without this effect. 36. P. L. Heller, C. L. Angevine, Earth Planet. Sci. Lett. 75, 417 (1985). 37. G. C. Bond, Tectonophysics 61, 285 (1979 We document the widespread existence of antisocial punishment, that is, the sanctioning of people who behave prosocially. Our evidence comes from public goods experiments that we conducted in 16 comparable participant pools around the world. However, there is a huge cross-societal variation. Some participant pools punished the high contributors as much as they punished the low contributors, whereas in others people only punished low contributors. In some participant pools, antisocial punishment was strong enough to remove the cooperation-enhancing effect of punishment. We also show that weak norms of civic cooperation and the weakness of the rule of law in a country are significant predictors of antisocial punishment. Our results show that punishment opportunities are socially beneficial only if complemented by strong social norms of cooperation.

We study the importance of conditional cooperation in a one-shot public goods game by using a variant of the strategy-method. We find that a third of the subjects can be classified as free riders, whereas 50% are conditional cooperators.