Isolating and identifying motivations: A voluntary contribution mechanism experiment with interior Nash equilibria

What motivates subjects in their decision making is a lingering issue in public goods experiments. Using a nonlinear payoff function and a two-subject model, we create a one-toone correspondence between contributions and motivations, enabling us to isolate and identify the following three possible motivations: Nash, cooperative, and altruistic motivations. The experimental results show that Nash- motivated behavior accounts for more than 70% of all decisions. Some subjects reveal a cooperative motivation when they know the other subject's payoff information. Altruistic motivation is found to be rare throughout the experiment.