Coalition-proof full efficient implementation

The Vickrey-Clarke-Groves and d'Aspremont-Gerard-Varet mechanisms implement efficient social choice by compensating each agent for the externalities that his report imposes on all other agents. Instead of aggregate compensations, which may lead to profitable coalitional deviations, this paper provides an alternative mechanism, in which each pair of agents directly compensate each other for the pairwise externalities they impose. Under the assumption of independent private values, any agent is guaranteed to receive his ex ante efficient payoff by reporting truthfully, regardless of others' strategies. This absence of ex ante externalities makes the mechanism coalition-proof, and makes all equilibria efficient.