Contagious Cooperation

Interconnected Trees

Humans, and other animals, tend to cooperate with each other in a variety of social situations. Without working together toward a common goal, species as a whole would suffer and evolution may have been short-lived. In general, humans tend to be most cooperative and apt to help others when the costs are small to the donor, but the benefits are great to the recipient. Accordingly, this leads to a disconnect between what is best for the group and what is best for the individual. A survival-of-the-fittest mentality would assume that individual needs should prevail, since only the strong survive. However, new research asserts that human cooperation is contagious, with a likely evolutionary basis, and a pay-it-forward mentality spreads through social connections like a virus.

A diverse set of phenomena have been shown to spread from person-to-person, much like contagions: obesity, happiness, ideas and beliefs. Emotional states, as well as yawning and smiling, are deeply rooted in the neurophysiological processes, but the spread of other behaviors, including altruism, arise from a combination of social norms and other psychosocial processes, such as innate mimicry. Based on observations of public goods games, researchers at Harvard University and the University of California, San Diego, report that witnessing other people, even strangers, performing cooperative or uncooperative behavior influenced future altruistic and cooperative behavior.

In the public goods game experiments, individuals were given 20 money units (MUs). Each participant could decide how many to keep (0 to 20) or contribute (0 to 20) to a group project. Each 1 MU donated yielded 0.4 MU to each of the four group members in the experiment. Therefore, donating was costly to the individual, but beneficial to the group. If each member kept all of his MUs, he would still have a total of 20 MUs at the end of the experiment; if each member donated all of his MUs to the group, he would have a total of 32 MUs at the end. The contributions of each individual were revealed at the end of the game. It was repeated six times.

In an alternate game, once the contributions of each individual were revealed at the end of each round, each participant had the opportunity to punish the other members who contributed less. Each participant was allowed to spend up to 10 MUs; each MU reduced the target’s total by 3 MUs. Overall, in both game scenarios, individuals were influenced by the other participants’ behaviors. Contributions tended to decline over the course of the six rounds in the first game, while contributions increased in the game with punishment. However, conflicting research has reported that in similar experiments, reward tends to increase contributions and total earnings, while punishment has no effect on contribution and leads to decreased total earnings.

The authors of the current research, which is published in the Proceedings of the National Academy of Sciences, additionally report that the behavior influences not only the individual involved in the decision, but up to three degrees of separation from him. People mimic the behaviors of those around them, and this type of mimicry can cause cooperative behaviors to spread to dozens, or even hundreds, of people in a social network.

Such contagious behavior has been studied previously in situations like workplace charity donations or organ donation decisions. However, it has not been extensively studied in real-world situations. It is not known why people engage in cooperative or altruistic behavior. Are they only cooperative when a punishment is at stake? Altruistic behavior does influence the brain’s reward center, so humans may have developed and adapted to engage in this particular behavior. But, natural selection would seem to favor non-cooperators — those who get the most benefit for the least amount of work, or those who seek individual strength over group needs. Some research does suggest that “winners” of the public good games — individuals with the highest total payoff — tend not to use costly punishment. Such punishment is reportedly maladaptive.

Cooperation is often at odds with self-interest. Such behavior is driven by the amount of reward, and the immediacy of the reward, as well as future reputation. Humans, and many animal species, are understandably more apt to help their family members, or help others because their family members will receive benefits. They are less likely to help when cooperation is examined across diverse social or demographic lines.

Has doing a good deed evolved as a survival mechanism? The answer is unclear. But, what does seem to be clear is that behavior — cooperative or uncooperative — is contagious. See a good deed, pay it forward, and watch the altruism spread.


Dreber A, Rand DG, Fudenberg D, & Nowak MA (2008). Winners don’t punish. Nature, 452 (7185), 348-51 PMID: 18354481

Fowler, J. (2005). Altruistic punishment and the origin of cooperation Proceedings of the National Academy of Sciences, 102 (19), 7047-7049 DOI: 10.1073/pnas.0500938102

Fowler, J., & Christakis, N. (2010). Cooperative behavior cascades in human social networks Proceedings of the National Academy of Sciences, 107 (12), 5334-5338 DOI: 10.1073/pnas.0913149107

Helbing, D., & Johansson, A. (2010). Cooperation, Norms, and Revolutions: A Unified Game-Theoretical Approach PLoS ONE, 5 (10) DOI: 10.1371/journal.pone.0012530

Santos FC, Santos MD, & Pacheco JM (2008). Social diversity promotes the emergence of cooperation in public goods games. Nature, 454 (7201), 213-6 PMID: 18615084:

Stevens JR, & Hauser MD (2004). Why be nice? Psychological constraints on the evolution of cooperation. Trends in cognitive sciences, 8 (2), 60-5 PMID: 15588809

Rand DG, Dreber A, Ellingsen T, Fudenberg D, & Nowak MA (2009). Positive interactions promote public cooperation. Science (New York, N.Y.), 325 (5945), 1272-5 PMID: 19729661

Jennifer Gibson, PharmD

Jennifer Gibson, PharmD, is a practicing clinical pharmacist and medical writer/editor with experience in researching and preparing scientific publications, developing public relations materials, creating educational resources and presentations, and editing technical manuscripts. She is the owner of Excalibur Scientific, LLC.
See All Posts By The Author

Do not miss out ever again. Subscribe and get latest Brain Blogger articles straight to your inbox.