Multiplicity and Uniqueness of Stationary Expected Payoffs in Coalitional Bargaining

Abstract

I study uniqueness of stationary expected payoffs in random-proposer coalitional bargaining with transferable utility. Proposer independence of coalition surplus does not guarantee uniqueness, even when the surplus is strictly monotone and every equilibrium features immediate agreement. For $M^\natural$-concave characteristic functions, including non-monotone ones, I show that stationary expected payoffs are unique and continuous in the primitives, provided that every equilibrium features immediate agreement. To prove uniqueness, I identify an equitable grouping property of maximizing coalitions and show that $M^\natural$-concavity implies it at every no-delay candidate payoff vector. Using equitable grouping, I show that every no-delay equilibrium payoff vector is isolated and has positive local index. For parameter changes that preserve immediate agreement in every equilibrium, a uniform increase in a player's contribution to every coalition containing her strictly increases her expected payoff, while an increase in her discount factor weakly increases it. I apply these results to joint ventures with coordination costs and government formation with ministerial expertise.