Even 'cooperative' game theory does not model cooperation, but treats it as a black box: the values of coalitions in a cooperative game are taken as given, and it is not explained how coalitions produce these values. In Roemer and Silvestre (1993), the authors proved the existence, for quite general economic environments, of an allocation they called the proportional solution (PS): an allocation of goods and labor which is Pareto efficient, and in which each receives goods whose value (at supporting efficiency prices) is proportional to the value of their expended labor. In particular, if such an allocation could be realized, it would rectify the inefficiencies exhibited in the Nash equilibrium known as the tragedy of the commons.