Information Aggregation and Optimal Market Size

This paper studies a rational expectations model of trading where strategic traders face information asymmetries and endowment shocks. We show that negative partici- pation externalities arise due to an endogenous interaction between information aggre-gation and multiple trading motives. Moreover, the negative externalities are strong enough to make optimal market size ?nite. In a decentralized process of market for- mation, multiple markets can survive due to the negative externalities among traders. The model also predicts: (i) that only in a su¢ ciently large market the equilibrium multiplicity due to self-ful?lling trading motives can arise, (ii) that a high correlation in endowment shocks can make markets extremely illiquid.