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A joint study from the National Institute for Mathematical and Biological Synthesis (NIMBioS) at UT Knoxville and the University of Oxford sheds new light on the evolutionary roots of group cooperation. Researchers say that leaders in group-living species may bully their own to get what they want, but they also bully outsiders for the overall betterment of their own group.

“As far as within-group interactions are concerned,” said Sergey Gavrilets, associate director for scientific activities for NIMBioS and the study’s lead author, “the alpha males and females are ‘bad guys’ taking various resources from their group-mates. However, in between-group conflicts they become ‘good guys’ and their presence and effort benefit everybody else.”

The researchers used a series of mathematical models to uncover a mechanism for explaining how conflict between groups influences cooperation within groups and how genes for this behavior might be maintained in the population by natural selection.

Humans are unique in their innate ability and willingness to cooperate within groups ranging in size from small families to nations of millions of individuals. Yet, according to the study, cooperation has its downsides as it can lead to what scientists call “the collective action problem,” which says that if individual effort is costly and a group member can benefit from the action of others, then there is an incentive to “free-ride,” whereby effort is reduced or withdrawn completely. If a number of individuals follow this logic, all group members suffer. The collective action problem also occurs in conflicts between groups: everyone benefits from the group’s success, but achieving success requires costly contributions by members of the group.

The study, issued today in the journal Nature Communications, shows that the collective action problem can be overcome in groups that have a hierarchical structure and high inequality. When hierarchy and inequality are well-established within a group, higher-ranking individuals effectively spend their effort on competition with their peers in outside groups. This competition then results in a seemingly altruistic behavior of the higher-ranking individuals as they make stronger effort, pay higher costs, and get smaller net benefit than their lower-ranking group mates who free-ride and contribute nothing. The study also found that the total effort that a group directs toward conflict with other groups typically increases with the degree of hierarchy and inequality within the group.

The results are consistent with observations in nature across a range of species. The study cites chimpanzees, for example, whose high-ranking males travel further into the periphery of the group during border patrols, and ring-tail lemurs and blue monkeys whose high-ranking females participate more in the defense of communal feeding territories.

The open-access study may be read online.

NIMBioS brings together researchers from around the world to collaborate across disciplinary boundaries to investigate solutions to basic and applied problems in the life sciences. It is sponsored by the National Science Foundation, the US Department of Homeland Security, and the US Department of Agriculture, with additional support from UT. For more information, visit the website.

C O N T A C T :

Sergey Gavrilets, NIMBioS (865-974-8136,

Catherine Crawley, MINBioS (865-974-9350,

Rebekah Winkler UT Media Relations (865-974-8304,