Prisoner’s Dilemma Experiment
In a classic PD, players choose independently whether to contribute toward a mutually desirable outcome. Cooperation comes at a cost, so that (for example) a dollar’s contribution yields a return of more than a dollar to the group but less than a dollar to the individual contributor. Thus contributing is optimal for the group and suboptimal for the individual. Not contributing is a dominant strategy, which is to say, standard game theory predicts defection. If players prefer higher to lower monetary playoffs, and if other things are equal so far as a player is concerned, a player will not contribute.
And yet, among the more robust findings to come out of experimental economics is that human players do not behave as game theory predicts. In laboratory PDs, individual contributions toward the group good are can be substantial.