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. 2010 Mar 23;107(12):5334-8.
doi: 10.1073/pnas.0913149107. Epub 2010 Mar 8.

Cooperative behavior cascades in human social networks

James H Fowler  1 , Nicholas A Christakis
Affiliations

Cooperative behavior cascades in human social networks

James H Fowler et al. Proc Natl Acad Sci U S A. .

Abstract

Theoretical models suggest that social networks influence the evolution of cooperation, but to date there have been few experimental studies. Observational data suggest that a wide variety of behaviors may spread in human social networks, but subjects in such studies can choose to befriend people with similar behaviors, posing difficulty for causal inference. Here, we exploit a seminal set of laboratory experiments that originally showed that voluntary costly punishment can help sustain cooperation. In these experiments, subjects were randomly assigned to a sequence of different groups to play a series of single-shot public goods games with strangers; this feature allowed us to draw networks of interactions to explore how cooperative and uncooperative behaviors spread from person to person to person. We show that, in both an ordinary public goods game and in a public goods game with punishment, focal individuals are influenced by fellow group members' contribution behavior in future interactions with other individuals who were not a party to the initial interaction. Furthermore, this influence persists for multiple periods and spreads up to three degrees of separation (from person to person to person to person). The results suggest that each additional contribution a subject makes to the public good in the first period is tripled over the course of the experiment by other subjects who are directly or indirectly influenced to contribute more as a consequence. These results show experimentally that cooperative behavior cascades in human social networks.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Example of a network drawn from the Fehr–Gaechter public goods game experiments (36). Here we abstract from the numerous interactions that take place between individuals in these experiments to focus on a specific set of pathways from alters’ alters to alters to egos. An “ego” is the focal subject (in this example we focus on subject A in period 3); “alters” are the subjects in the ego's group in the previous period (E, I, and M in period 2). The ego has a direct network connection to alters because s/he sees each of their contributions to the public good before proceeding to the next period. “Alter's alters” are the individuals in the alters’ groups in the period before the previous period (F, G, H, J, K, L, N, O, and P in period 1). Note that the ego has no direct network connection to any of the alters’ alters and has not seen any of their contributions. However, the ego is indirectly connected to the alters’ alters by two degrees of separation via the alters (E, I, and M in period 2). The requirement that no two subjects be placed in the same group twice guarantees that we can draw a network like this for all 24 subjects in period 3.
Fig. 2.
Fig. 2.
The raw data from the Fehr–Gaechter public goods game experiments (both the simple version and the version with punishment) show a relationship between alter giving in period t (x-axis) and ego giving in period t+1 (y-axis). Individuals who gave the maximum or minimum are removed from the data to avoid floor and ceiling effects. Vertical bars show 95% confidence intervals based on SEM.
Fig. 3.
Fig. 3.
The total effect of alter's contribution to the public good on ego's contribution is significant and extends up to three degrees of separation. For each 1 MU contributed by alter, ego contributes an additional 0.19 MUs (0.18 MUs in the version with punishment) in the next period. For each 1 MU contributed by alter's alter (a contribution ego did not observe), ego contributes an additional 0.07 MUs (0.05 MUs in the version with punishment) two periods later. For each 1 MU contributed by alter's alter's alter (three degrees of separation), ego contributes an additional 0.06 MUs in the public good game with punishment three periods later. Mediation analyses show that indirect total effects are mediated by the direct effect of alter on ego (SI Appendix). Alters are randomly assigned to egos, and they are assessed only at the minimum degree of separation at each point in time. Estimates are from interval regressions, controlling for multiple observations of the same ego, multiple observations of the same alter, the ego's initial contribution in the period in which alter's contribution was observed, and period fixed effects. Vertical bars show 95% confidence intervals.
Fig. 4.
Fig. 4.
The total effect of alter's giving on ego's giving persists beyond the initial period. (Upper) Alter significantly influences ego's behavior up to four periods later in the public goods game (Left) and up to five periods later in the public goods game with punishment (Right). (Lower) Alter's alter (two degrees of separation) significantly influences ego's behavior up to four periods later in the public goods game (Left) and up to three periods later in the public goods game with punishment (Right). Estimates are from interval regressions, controlling for multiple observations of the same ego, multiple observations of the same alter, the ego's initial contribution in the period in which alter's (or alter's alter's) contribution was made, and period fixed effects. Vertical bars show 95% confidence intervals.
Fig. 5.
Fig. 5.
A hypothetical cascade. This diagram illustrates the difference between the spread of the interpersonal effects across individuals and the persistence of effects across time. We abstract from the numerous interactions that take place between individuals in these experiments to focus on a specific, illustrative set of pathways. Cooperative behavior spreads three degrees of separation: if Eleni increases her contribution to the public good, it benefits Lucas (one degree), who gives more when paired with Erika (two degrees) in period 2, who gives more when paired with Jay (three degrees) in period 3, who gives more when paired with Brecken in period 4. The effects also persist over time, so that Lucas gives more when paired with Erika (period 2) and also when paired with Lysander (period 3), Bemy (period 4), Sebastian (period 5), and Nicholas (period 6). There is also persistence at two degrees of separation, because Erika gives more not only when paired with Jay (period 3) but also when paired with Harla (period 4) and James (period 5). All the paths in this illustrative cascade are supported by significant results in the experiments, and it is important to note that if Eleni decreases her initial contribution, her uncooperative behavior can spread and persist as well.
See this image and copyright information in PMC

References

    1. Nowak MA. Five rules for the evolution of cooperation. Science. 2006;314:1560–1563. - PMC - PubMed
    1. Ohtsuki H, Hauert C, Lieberman E, Nowak MA. A simple rule for the evolution of cooperation on graphs and social networks. Nature. 2006;441:502–505. - PMC - PubMed
    1. Lieberman E, Hauert C, Nowak MA. Evolutionary dynamics on graphs. Nature. 2005;433:312–316. - PubMed
    1. Santos FC, Santos MD, Pacheco JM. Social diversity promotes the emergence of cooperation in public goods games. Nature. 2008;454:213–216. - PubMed
    1. Skyrms B, Pemantle R. A dynamic model of social network formation. Proc Natl Acad Sci USA. 2000;97:9340–9346. - PMC - PubMed

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