Explaining cooperation is one of the greatest problems for evolutionary biology. Cooperation often involves shared resources, and can therefore be tainted by conflicts of interest. Why should an individual carry out a costly cooperative behavior that benefits other individuals? Cooperation is a particularly notable puzzle in species such as humans, where there is frequent cooperation between non-relatives.

People behave less cooperatively when they think they are in direct "local" competition with each other, and more cooperatively under circumstances of "global"-scale competition. The findings hold implications for our understanding of the evolution of social animals, as well as our understanding of factors that influence the cooperative (and cheating) behavior of individuals and groups in business, government, and academia.

Numerous possible solutions have been suggested for the problem of cooperation between non-relatives, including punishment, policing, and various forms of reciprocity. The solution most commonly suggested is that individuals will interact repeatedly. This allows reciprocal cooperation to evolve: individuals cooperate because it will lead to others cooperating with them in the future. Another way of looking at reciprocal cooperation is that participation avoids any punishment that would be handed out to non-collaborators.

The impact of repeated interactions has been examined in the past by observing humans asked to play the Prisoner's Dilemma game. The dilemma in this game is that not cooperating (that is, cheating) is the best short-term option, but cooperation by both players gives greater rewards than cheating by both players. It is well known that cooperation can be favored in this game if players have repeated interactions - such a scenario sets up the possibility of reciprocal cooperation.

In the new study, conducted by Stuart A. West from the School of Biological Sciences at University of Edinburgh and colleagues from Queen's University, students played the Prisoner's Dilemma game in small groups, with cash rewards manipulated to make competition either "local" or "global." In local competition, the top player in each group of three won money; this was found to lead to selfish conflict within groups. In global competition, the top scorers in the room won money; this was found to lead to reciprocal cooperation within groups.

The findings show that the spatial scale of competition influences cooperative behavior, and may drive evolutionary changes that affect the social behavior of a given species. "We found that individuals were less likely to cooperate when competition was relatively local," authors write. "Cooperation between humans will therefore be most likely when repeated interactions take place on a local scale between small numbers of people, and competition for resources takes place on a more global scale among large numbers of people."

The authors point out that the responses observed in their study could be exploited by engineering the structure of human groups to favor cooperation and the benefits of competition in diverse realms of human activity.