Herakles Polemarchakis

Idiosyncratic risk and financial policy

In economies subject to uninsurable idiosyncratic risks, competitive equilibrium allocations are constrained inefficient: reallocations of assets support Pareto superior allocations. This is the case even if the asset market for the allocation of aggregate risks is complete.

Equilibrium and arbitrage in incomplete asset markets with fixed prices

At arbitrary prices of commodities and assets, fix-price equilibria exist under weak assumptions: endowments need not satisfy an interiority condition, utility functions need only satisfy very weak monotonicity requirement, and the asset return matrix allows for redundant assets. Prices of assets may permit arbitrage. At equilibrium, though restricted through endogenously determined trading constraints, arbitrage possibilities may persist; in an example, an individual holds an arbitrage portfolio.

Sovereign debt and incentives to default with uninsurable risks

We show that sovereign debt is unsustainable if debt contracts are not supported by direct sanctions and default carries only a ban from ever borrowing in financial markets even in the presence of uninsurable risks and time-varying interest rate. This extension of Bulow and Rogoff (1989) requires that the present value of the endowment be finite under the most optimistic valuation. We provide examples where this condition fails and sovereign debt is sustained by the threat of loss of insurance opportunities upon default, despite the fact that the most pessimistic valuation of the endowment, the natural debt limit, is finite.

The identification of attitudes towards ambiguity and risk from asset demand

Individuals behave differently when they know the objective probability of events and when they do not. The smooth ambiguity model accommodates both ambiguity (uncertainty) and risk. For an incomplete, competitive asset market, we develop a revealed preference test for asset demand to be consistent with the maximization of smooth ambiguity preferences ; and we show that ambiguity preferences constructed from finite observations converge to underlying ambiguity preferences as observations become dense. Subsequently, we give sufficient conditions for the asset demand generated by smooth ambiguity preferences to identify the ambiguity and risk indices as well as the ambiguity probability measure. We do not require ambiguity beliefs to be observable : in a generalized specification, they may not even be defined. An ambiguity free asset plays an important role for identification.

An argument for positive nominal interest

In a dynamic economy, money provides liquidity as a medium of exchange. A central bank that sets the nominal rate of interest and distributes its profit to shareholders as dividends is traded in the asset market. A nominal rates of interest that tend to zero, but do not vanish, eliminate equilibrium allocations that do not converge to a Pareto optimal allocation.

Markets and efficiency

Intervention improves on competitive outcomes.

Convergence to agreement and the role of public information

When states of the world are normally distributed, the sequential exchange and revision of beliefs converges to agreement in finitely many rounds of communication. Public information may reduce the information shared by individuals after the revision of beliefs.