Re typified by high levels of reciprocity (12?5), implying that mutual acceptance of new links is the social norm. Our study builds upon this work in three ways. First, our design is fully endogenous, allowing NS-018 price individuals to decide with whom they will make and break ties. As we explain below, the resulting effect sizes are much larger than in previous studies of dynamic networks (8, 9), reaching close to 100 cooperation in some cases. Second, we consider an extremely wide range of update rates, affording us a much clearer understanding of the importance of varying rates. We find no evidence of the hypothesized threshold effect (9, 10), instead finding significant and positive increases in cooperation at rates well below those previously reported. Finally, and in contrast to both previous studies that considered only one set of payoffs, we manipulate the payoff structure itself, effectively varying the attractiveness of the “outside option” (16), meaning roughly the payoff associated with choosing not to interact with a potential partner. We find that only in the presence of an attractive outside option do conditional cooperators punish defectors (by proactively deleting ties with them). By contrast, when the outside option is less attractive, we find that cooperators tolerate defecting partners, eventually leading them to defect themselves. Our work is also related more generally to a number of recent experiments that have investigated various aspects of the relationship between cooperation and partner selection, such as unilateral vs. bilateral choice (17, 18), the effect of introducing an outside option of varying attractiveness (16), and the attributes of the individuals (age, sex, race, etc.) as predictors of selection and cooperation (19, 20). Although our treatment of the outside option is consistent with previous work (16), it is distinct in that it extends it to the case of a dynamic network. Finally,Author contributions: J.W., S.S., and D.J.W. designed research; J.W. and S.S. performed research; J.W., S.S., and D.J.W. analyzed data; and S.S. and D.J.W. wrote the paper. The authors declare no conflict of interest.Freely available online through the PNAS open access option.To whom correspondence may be addressed. E-mail: [email protected], suri@ microsoft.com, or [email protected] article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. 1073/pnas.1120867109/-/DCSupplemental.www.pnas.org/cgi/doi/10.1073/pnas.PNAS | September 4, 2012 | vol. 109 | no. 36 | 14363?SOCIAL SCIENCESThis article is a PNAS Direct Submission. M.O.J. is a guest editor invited by the Editorial Board.other related work (21, 22) has examined how individuals select groups or are excluded by them. Although at a high level these papers clearly resemble both the partner selection literature and dynamic updating studies such as ours, they differ substantially from both literatures in that the object of selection (21) or the actor (22) is the group, not the individual. Experimental Setup We conducted a series of online human subjects experiments in which groups of 24 participants played an iterated prisoner’s dilemma (PD) game, where in addition to choosing their action each round–cooperate or GSK2256098 web defect–they also were given the opportunity to update their interaction partners at some specified rate, which was varied across experimental conditions. (See SI Appendix, Figs. S1 and S2 for details of the experimental platform and recr.Re typified by high levels of reciprocity (12?5), implying that mutual acceptance of new links is the social norm. Our study builds upon this work in three ways. First, our design is fully endogenous, allowing individuals to decide with whom they will make and break ties. As we explain below, the resulting effect sizes are much larger than in previous studies of dynamic networks (8, 9), reaching close to 100 cooperation in some cases. Second, we consider an extremely wide range of update rates, affording us a much clearer understanding of the importance of varying rates. We find no evidence of the hypothesized threshold effect (9, 10), instead finding significant and positive increases in cooperation at rates well below those previously reported. Finally, and in contrast to both previous studies that considered only one set of payoffs, we manipulate the payoff structure itself, effectively varying the attractiveness of the “outside option” (16), meaning roughly the payoff associated with choosing not to interact with a potential partner. We find that only in the presence of an attractive outside option do conditional cooperators punish defectors (by proactively deleting ties with them). By contrast, when the outside option is less attractive, we find that cooperators tolerate defecting partners, eventually leading them to defect themselves. Our work is also related more generally to a number of recent experiments that have investigated various aspects of the relationship between cooperation and partner selection, such as unilateral vs. bilateral choice (17, 18), the effect of introducing an outside option of varying attractiveness (16), and the attributes of the individuals (age, sex, race, etc.) as predictors of selection and cooperation (19, 20). Although our treatment of the outside option is consistent with previous work (16), it is distinct in that it extends it to the case of a dynamic network. Finally,Author contributions: J.W., S.S., and D.J.W. designed research; J.W. and S.S. performed research; J.W., S.S., and D.J.W. analyzed data; and S.S. and D.J.W. wrote the paper. The authors declare no conflict of interest.Freely available online through the PNAS open access option.To whom correspondence may be addressed. E-mail: [email protected], suri@ microsoft.com, or [email protected] article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. 1073/pnas.1120867109/-/DCSupplemental.www.pnas.org/cgi/doi/10.1073/pnas.PNAS | September 4, 2012 | vol. 109 | no. 36 | 14363?SOCIAL SCIENCESThis article is a PNAS Direct Submission. M.O.J. is a guest editor invited by the Editorial Board.other related work (21, 22) has examined how individuals select groups or are excluded by them. Although at a high level these papers clearly resemble both the partner selection literature and dynamic updating studies such as ours, they differ substantially from both literatures in that the object of selection (21) or the actor (22) is the group, not the individual. Experimental Setup We conducted a series of online human subjects experiments in which groups of 24 participants played an iterated prisoner’s dilemma (PD) game, where in addition to choosing their action each round–cooperate or defect–they also were given the opportunity to update their interaction partners at some specified rate, which was varied across experimental conditions. (See SI Appendix, Figs. S1 and S2 for details of the experimental platform and recr.
