One landmark application of evolutionary game theory is the study of social dilemmas. This literature explores why people cooperate even when there are strong incentives to defect. Much of this literature, however, assumes that interactions are symmetric. Individuals are assumed to have the same strategic options and the same potential payoffs. Yet many interesting questions arise once individuals are allowed to differ. Here, we study asymmetry in simple coordination games. In our setup, human participants need to decide how much of their endowment to contribute to a public good. If a group’s collective contributions reach a pre-defined threshold, all group members receive a reward. To account for possible asymmetries, individuals either differ in their endowments or their productivities. According to our theoretical equilibrium analysis, such games tend to have many possible solutions. In equilibrium, group members may contribute the same amount, different amounts, or nothing at all. According to the behavioral experiment, however, humans favor the equilibrium in which everyone contributes the same proportion of their endowment. We use these experimental results to highlight the nontrivial effects of inequality on cooperation, and we discuss to which extent models of evolutionary game theory can account for these effects.

Experimental setting: 

There are five treatments: full equality, moderate endowment inequality, strong endowment inequality, moderate productivity inequality, and strong productivity inequality. Each participant is assigned to one treatment only. Each treatment includes two games: participants play Game 1 first and then Game 2. At the beginning of Game 1, players are randomly matched in pairs, where in each pair one player is assigned to Role 1 and the other is Role 2. They then play 20 rounds of the asymmetric threshold public goods game (TPGG). Throughout a game, the pair of players and their roles do not change. The players receive their fixed endowments and independently decide an integer amount out of their endowment to contribute to a public good. A player's effective contribution is their contribution multiplied by their individual productivity factor. If the sum of the players' effective contributions exceeds a threshold, then both of them receive a reward. The payoff in each round  is the sum of the remaining amount of the initial endowment and the reward if the threshold is reached. At the end of each round, players receive information about the contributions and payoffs of themselves and their partners, as well as the total effective contribution (that is, whether they have reached the  threshold). After Game 1, players engage in Game 2. Again, they play 20 rounds of the asymmetric TPGG but now with a different partner and in the opposite role. Finally, players are asked to finish a questionnaire on what they consider to be the fair contribution pattern.

The experiment was conducted in the computer labs of Beijing Normal University. All the 558 subjects were undergraduates and graduates recruited from Beijing Normal University. The interactions were anonymous, and took place via computers.The experimental platform software was built using o-Tree (version 5.3).

Please refer to the ReadMe file.