"If the help and support of a community significantly affects the well-being of its members, then the threat of withdrawing that support can keep people in line and maintain social order," said Karthik Panchanathan, a UCLA graduate student whose study appears in Nature. "Our study offers an explanation of why people tend to contribute to the public good, like keeping the streets clean. Those who play by the rules and contribute to the public good will be included and outcompete freeloaders."
This finding -- at least in part -- may help explain the evolutionary roots of altruism and human anger in the face of uncooperative behavior, both of which have long puzzled economists and evolutionary biologists, he said.
"If you put two dogs together, and one dog does something inappropriate, the other dog doesn't care, so long as it doesn't get hurt," Panchanathan said. "It certainly wouldn't react with moralistic outrage. Likewise, it wouldn't experience elation if it saw one dog help out another dog. But humans are very different; we're the only animals that display these traits."
The study, which uses evolutionary game theory to model human behavior in small social groups, is the first to show that cooperation in the context of the public good can be sustained when freeloaders are punished through social exclusion, said co-author Robert Boyd, a UCLA professor of anthropology and fellow associate in UCLA's Center for Behavior, Evolution and Culture.
"Up to this point, social scientists interested in the evolutionary roots of cooperative behavior have been hard-pressed to explain why any single individual would stick his neck out to punish those who fail to pull their weight in society," Boyd said. "But without individuals willing to mete out punishment, we have a hard time explaining how societies develop and sustain cooperative behavior. Our model shows that as long as it is socially permissible, withholding help from a deadbeat actually proves to be in an individual's self-interest."
With funding from the National Science Foundation, Panchanathan set out to recreate mathematically a small community in which people participate in a public good, such as an annual clearing of a mosquito-infested swamp, which takes time from their day but which saves the entire community time down the line because the work prevents them from getting sick. He assumed that individuals in the close-knit community frequently swap favors, like helping neighbors repair their homes after a storm. He also assumed that no single individual or agency was being paid to keep individuals in line. Community members had to do it themselves, much as our evolutionary ancestors would have done.
In his mathematical model, Panchanathan pitted three types of society members:
- "Cooperators," or people who always contribute to the public good and who always assist individual community members in the group with the favors that are asked of them.
- "Defectors," who never contribute to the public good nor assist other community members who ask for help.
- "Shunners," or hard-nosed types who contribute to the public good, but only lend aid to those individuals with a reputation for contributing to the public good and helping other good community members who ask for help. For members in bad standing, shunners withhold individual assistance.
During the course of the game, both cooperators and shunners helped to clear the swamp. The benefits from the mosquito-free swamp, however, flowed to the whole community, including defectors. When the researcher took only this behavior into account, the defectors come out on top because they enjoyed the same benefits the other types, but they paid no costs for the benefits.
But when it came to getting help in home repair, the defectors didn't always do so well. The cooperators helped anyone who asked, but the shunners were selective; they only help those with a reputation for clearing the swamp and helping good community members in home repair. By not helping defectors when they ask for help, shunners were able to save time and resources, thus improving their score. If the loss that defectors experienced from not being helped by shunners was greater than the cost they would have paid to clear the swamp, then defectors lost out.
After these social interactions went on for a period of time that might approximate a generation, individuals were allowed to reproduce based on accumulated scores, so that those with more "fitness points" had more children. Those children were assumed to have adopted their parents' strategy.
Eventually, Panchanathan found that communities end up with either all defectors or all shunners.
"Both of those end points represent 'evolutionarily stable equilibriums'; no matter how much time passes, the make-up of the population does not change," Panchanathan said.
In a community with just cooperators and defectors, defectors -- not surprisingly -- always won. Also when shunners were matched against cooperators, shunners won.
"The cooperators were too nice; they died out," Panchanathan said. "In order to survive, they had to be discriminate about the help they gave."
But when shunners were matched against defectors, the outcome was either shunners or defectors. The outcome depended on the initial frequency of shunners. If enough shunners were present at the beginning of the exercise, then shunners prevailed. Otherwise, defectors prevailed, potentially pointing to the precarious nature of cooperative society.
"We know that people pay their taxes and engage in all kinds of other cooperative behaviors in modern society because they're afraid they'll get punished," Panchanathan said. "The problem for the social scientist becomes how did the propensity to punish get started? Why do I get angry if someone doesn't contribute? Isn't it just better to say, 'It's their business,' and let everybody else in the group get angry? After all, punishing someone else will take time and energy away from activities that are more directly important to me and I may get hurt."
"By withdrawing my support from a freeloader, I benefit because every time I do something nice for someone, it costs me something," Panchanathan said. "By withdrawing that support, I'm spared the energy, time or whatever costs are entailed. I retain my contribution, but the deadbeat is punished."
In practice, however, cooperative societies hold defectors in line through a series of measures, Panchanathan said. "The first level is disapproval: you say, 'That wasn't cool' or you give a funny look," he said. "Then you withdraw social support. Finally, you lower the boom and either physically hurt the defector or run him out of town."
Ultimately, he admits, this model is "a very simple and crude approximation" of the real world. "For example, in my model, only defectors or shunners can persist. They cannot coexist," he said. "But we know that some people are generally cooperative, playing by society's rules, while others are not. This type of modeling doesn't explain everything. Instead, it boils down a complex social world and tries to understand one small piece. In this case, we found that cooperation can persist if people need to maintain a good reputation in their community."