Reputation drives cooperative behaviour and network formation in human groups

SUBJECT AREAS: Reputation drives cooperative behaviour and network formation in human groups SOCIAL EVOLUTION Jose A. Cuesta1,2, Carlos Gracia-Lázaro1, Alfredo Ferrer1, Yamir Moreno1,3,4 & Angel Sánchez1,2 OPEN COMPLEX NETWORKS 1 Received 2 October 2014 Accepted 15 December 2014 Published 19 January 2015 Correspondence and requests for materials should be addressed to J.A.C. (cuesta@math. Instituto de Biocomputación y Fı́sica de Sistemas Complejos (BIFI), Universidad de Zaragoza, 50018 Zaragoza, Spain, 2Grupo Interdisciplinar de Sistemas Complejos (GISC), Departamento de Matemáticas, Universidad Carlos III de Madrid, Avenida de la Universidad 30, 28911 Leganés, Madrid, Spain, 3Departamento de Fı́sica Teórica, Facultad de Ciencias, Universidad de Zaragoza, 50009 Zaragoza, Spain, 4Complex Networks and Systems Lagrange Lab, Institute for Scientific Interchange, Turin, Italy. Cooperativeness is a defining feature of human nature. Theoreticians have suggested several mechanisms to explain this ubiquitous phenomenon, including reciprocity, reputation, and punishment, but the problem is still unsolved. Here we show, through experiments conducted with groups of people playing an iterated Prisoner’s Dilemma on a dynamic network, that it is reputation what really fosters cooperation. While this mechanism has already been observed in unstructured populations, we find that it acts equally when interactions are given by a network that players can reconfigure dynamically. Furthermore, our observations reveal that memory also drives the network formation process, and cooperators assort more, with longer link lifetimes, the longer the past actions record. Our analysis demonstrates, for the first time, that reputation can be very well quantified as a weighted mean of the fractions of past cooperative acts and the last action performed. This finding has potential applications in collaborative systems and e-commerce. uc3m.es) H uman life would be inconceivable without the high levels of cooperation that our species is capable of1–3. Our complex society strongly relies on this defining and intrinsic feature of our nature. The study of human cooperation is often framed in the stylized, strict form of a Prisoner’s Dilemma, a situation which is vulnerable to exploitation by free riders. It is well established that, in repeated prisoner’s dilemma games, the global cooperation level rapidly decays down to values around 10%4,5. In order to explain how this effect is circumvented in social interactions, several mechanisms have been proposed that allow cooperators to assort and avoid free riders. Reciprocity6, reputation7, or punishment8,9 are prototypical. Somewhat less obvious is network reciprocity10, a mechanism that has attracted a lot of interest motivated by the increasing importance of networks in social research. This mechanism enhances the assortment of cooperators by limiting interactions to one’s social neighbours (see Ref. 11 for a review). However, recent experiments with human subjects revealed that simply structuring a population is not enough to foster and sustain cooperation, no matter the underlying social network12–16. The reason is that individuals do not base their decision on payoffs17, but behave with their neighbours just as they do in public good games on groups—cooperating conditionally on the number of cooperative acts they receive18 as well as on their own previous action15–17,19. This inevitably leads to the decay of cooperation irrespective of the underlying network structure20. To be more specific, the networks used in the above experiments are static, predefined structures on which individuals are constrained to interact. Similar experiments have been carried out with dynamic networks, where individuals are allowed to change their social ties as they play. The latter mechanism is able to induce and sustain high levels of cooperation21–24 (note that in Ref. 21 players choose independent actions for every partner, opposite to all other experiments). Dynamic rewiring introduces a mechanism for punishment, namely to break links as a response to defective partners, and indeed, it has been found that the promotion of cooperation depends on the rate at which connections can be modified22,23. However, an important point which seems to have been overlooked is that in all cases participants received, to some extent, information on their opponents’ past actions, and therefore, the possibility that it is this information that is driving the rise of cooperation can not be excluded. Experiments. To address this issue, we conducted 24 experiments involving 243 participants distributed in groups of between 17 and 25 individuals. Initially, participants in any of the experiments were randomly assigned to the nodes of a ring with links to nearest and next-nearest neighbours. Sub-sequently, they were allowed to change their ties along the experiment. The game consisted of 25 rounds (although this number was unknown to the participants). Each round had two phases. In phase one, each participant played a multi-player Prisoner’s Dilemma with all participants she was linked to (neighbours) by choosing one action: cooperate (C) or SCIENTIFIC REPORTS | 5 : 7843 | DOI: 10.1038/srep07843 1 www.nature.com/scientificreports defect (D). The payoff they obtained was proportional to the number nC of neighbours who chose C: if they cooperated they received 7nC points, whereas if they defected they received 10nC points. These payoffs correspond to the same payoff matrix used in previous experiments with fixed networks15,16, and were so chosen for the sake of comparison. In phase two, they were allowed to break any of their current ties and to propose up to five new ones with participants they were not yet linked to (this is the protocol used in Ref. 23). To make their decisions they were informed of a certain number (m 5 0, 1, 3, 5) of past actions of every participant in the experiment (henceforth referred to as ‘‘memory’’). In the case of memory m 5 0, nothing identified the participants, not even those they were connected with (i.e., they knew nC but not who among their neighbours cooperated or defected). At the end of phase two, all participants decided whether to accept or reject each received proposal for a new link. Every participant ran two (and only two) consecutive sessions with different values of m. (For further details see Supplementary Information). Our results establish clearly that reputation governs the way individuals change their ties and, as a consequence, cooperation can be sustained, but only when information on others’ past actions is available. We have also found that people assign reputation to partners based mainly on their last action and their average cooperation. Thus, the results of this study suggest that, in e-commerce and related contexts, adding information on the last action performed to the usual practice of providing the average reliabilit (...truncated)