The aim of this paper is to investigate how the capacity of an economic system to absorb shocks depends on the specific pattern of interconnections established among financial firms. The key trade-off at work is between the risk-sharing gains enjoyed by firms when they become more interconnected and the large-scale costs resulting from an increased risk exposure. We focus on two dimensions of the network structure: the size of the (disjoint) components into which the network is divided, and the "relative density" of connections within each component. We find that when the distribution of the shocks displays "fat" tails extreme segmentation is optimal, while minimal segmentation and high density are optimal when the distribution exhibits "thin" tails. For other, less regular distributions intermediate degrees of segmentation and sparser connections are also optimal. We also find that there is typically a conflict between efficiency and pairwise stability, due to a "size externality" that is not internalized by firms who belong to components that have reached an individually optimal size. Finally, optimality requires perfect assortativity for firms in a component.JEL Classification: D85, C72, G21.

Do Walrasian markets function orderly in the presence of adverse selection? In particular, is their outcome efficient? This paper addresses these questions in the context of a Rothschild and Stiglitz insurance economy. We identify an externality associated with the presence of adverse selection as a special form of consumption externality. Consequently, we show that while competitive equilibria always exist, they are not typically incentive efficient. However, as markets for pollution rights can internalize environmental externalities, markets for consumption rights can be designed so as to internalize the consumption externality due to adverse selection. With such markets competitive equilibria exist and are always incentive efficient. Moreover, any incentive efficient allocation can be decentralized as a competitive equilibrium.

For a general class of pure exchange OLG economies under uncertainty, we provide a complete characterization of the efficiency properties of competitive equilibria when markets are only sequentially complete and the criterion of efficiency is conditional Pareto optimality. We also consider a particular case in which markets fail to be even sequentially complete and provide a characterization when the criterion of efficiency is weakened to ex post Pareto optimality. Journal of Economic Literature Classification Numbers: D52, D61.

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In this article we examine the competitive equilibria of a dynamic stochastic economy with complete markets and collateral constraints. We show that, provided the sets of asset pay-offs and of collateral levels are sufficiently rich, the equilibrium allocations with sequential trades and collateral constraints are equivalent to those obtained in Arrow-Debreu markets subject to a series of limited pledgeability constraints. We provide both necessary and sufficient conditions for equilibria to be Pareto efficient and show that when collateral is scarce equilibria are not only Pareto inefficient but also often constrained inefficient, in the sense that imposing tighter borrowing restrictions can make everybody in the economy better off. We derive sufficient conditions for the existence of Markov equilibria and, for the case of two agents, for the existence of equilibria that have finite support. These equilibria can be computed with arbitrary accuracy and the model is very tractable. Abstract In this paper we examine the competitive equilibria of a dynamic stochastic economy with complete markets and collateral constraints. We show that, provided the sets of asset payoffs and of collateral levels are sufficiently rich, the equilibrium allocations with sequential trades and collateral constraints are equivalent to those obtained in Arrow-Debreu markets subject to a series of limited pledgeability constraints.We provide both necessary and sufficient conditions for equilibria to be Pareto efficient and show that when collateral is scarce equilibria are not only Pareto inefficient but also often constrained inefficient, in the sense that imposing tighter borrowing restrictions can make everybody in the economy better off.We derive sufficient conditions for the existence of Markov equilibria and, for the case of two agents, for the existence of equilibria that have finite support. These equilibria can be computed with arbitrary accuracy and the model is very tractable. * We thank seminar participants at various universities and conferences and in particular Arpad Abraham,