Public Goods Game

The public goods game is an experimental game in which players decide how much of a private endowment to contribute to a shared pool that benefits all group members. The pool is typically multiplied by the experimenter and divided equally, regardless of individual contribution. The Nash equilibrium prediction is zero contribution: each player's contribution costs them personally but benefits everyone equally, so the individually rational strategy is to free-ride on others' contributions while contributing nothing oneself.

In practice, players contribute substantial amounts in early rounds — typically 40-60% of their endowment — and contributions decline only gradually over time. The pattern is consistent across cultures and experimental designs: initial optimism, conditional cooperation, and decay as defectors exploit cooperators. The decline is not evidence that players are learning to be selfish; it is evidence that they are learning whom they can trust. When high-contribution players can identify and exclude low-contribution players, cooperation stabilizes at high levels.

The public goods game is structurally identical to the free rider problem and the tragedy of the commons. It reveals that cooperation fails not because people are selfish but because the institutional structure does not protect cooperators from exploitation. The solution is not to appeal to morality but to redesign the game — through mechanism design, reputation systems, or peer punishment — so that contributing becomes the individually rational strategy.