Reward structures in public good experiments

We contrast results from a laboratory experiment in voluntary contributions for a public good using two reward structures with interior equilibria. The first induces a set of Nash equilibria, all sharing the same total donations. The second induces a unique dominant-strategy equilibrium. Apparently, donor confusion over the Nash concept and coordination problems explain at most a small portion of the 'excessive' giving observed in those previous experiments that use an interior Nash design. Thus, we strengthen the case for interpreting experimental results as consistent with altruistic preferences that are not easily overcome by experimental induction.