Manipulation through Bribes

We consider allocation rules that choose both an outcome and transfers, based on the agents' reported valuations of the outcomes. Under a given allocation rule, a bribing situation exists when agent j could pay agent i to misreport his valuations, resulting in a net gain to both agents. A rule is bribe-proof if such opportunities never arise. The central result is that when a bribe-proof rule is used, the resulting payoff to any one agent is a continuous function of any other agent's reported valuations. We then show that on connected domains of valuation functions, if either the set of outcomes is finite or each agent's set of admissible valuations is smoothly connected, then an agent's payoff is a constant function of other agents' reported valuations. Finally, under the additional assumption of a standard domain-richness condition, we show that a bribe-proof rule must be a constant function. The results apply to a very broad class of economies.