The same tension runs through climate action and social movements: the shared payoff arrives only when enough people pitch in. In a new study published in the Proceedings of the National Academy of Sciences, two Dartmouth researchers find that rewarding the people who consistently cooperate does more to sustain that cooperation than punishing the ones who don’t. “Our contribution is that we’ve looked at how incentive structures affect cooperation when there are multiple rounds. In other words, what makes people more willing to consistently cooperate over and over again?” “We found that when players who have consistently cooperated are rewarded, people branch into two distinct groups: ‘volunteers’ who consistently cooperate, and ‘free-riders,’ who do not,” Fu says.