We consider a production economy with a finite number of heterogeneous, infinitely lived consumers. We show that, if the economy is smooth enough, equilibria are locally unique for almost all endowments. We do so by converting the infinite dimensional fixed point problem stated in terms of prices and commodities into a finite dimensional Negishi problem involving individual weights in a social value function. By adding a set of artificial fixed factors to utility and production functions, we can write the equilibrium conditions equating spending and income for each consumer entirely in terms of time zero factor endowments and derivatives of the social value function.

Previous authors have argued that the optimal monetary policy is contractionary. If buyers value consumption substantially more than sellers, there is some randomness and informational constraints make asset trading useful, we show that there is an incentive compatible expansionary policy that dominates all incentive compatible contractionary policies.

In a monetary model, it is shown that if there is a unique Pareto inefficient barter equilibrium, then a monetary equilibrium exists when traders are sufficiently patient.

We develop a theory of general equilibrium with endogenous debt limits in the form of individual rationality constraints similar to those in the dynamic consistency literature. If an agent defaults on a contract, he can be excluded from future contingent claims markets trading and can have his assets seized. He cannot be excluded from spot markets trading, however, and he has some private endowments that cannot be seized. All information is publicly held and common knowledge, and there is a complete set of contingent claims markets. Since there is complete information, an agent cannot enter into a contract in which he would have an incentive to default in some state. In general there is only partial insurance: variations in consumption may be imperfectly correlated across agents; interest rates may be lower than they would be without constraints; and equilibria may be Pareto ranked.

We characterize equilibria of general equilibrium models with externalities and taxes as solutions to optimization problems. This characterization is similar to Negishi’s characterization of equilibria of economies without externalities or taxes as solutions to social planning problems. It is often useful for computing equilibria or deriving their properties. Frequently, however, finding the optimization problem that a particular equilibrium solves is difficult. This is especially true in economies with multiple equilibria. In a dynamic economy with externalities or taxes there may be a robust continuum of equilibria even if there is a representative consumer. This indeterminacy of equilibria is closely related to that in overlapping generations economies.

This paper uses a simple general equilibrium model in which agents use money holdings to self insure to address the classic question: What is the optimal rate of change of the money supply? The standard answer to this question, provided by Friedman, Bewley, Townsend, and others, is that this rate is negative. Because any revenues from seigniorage in our model are redistributed in lump-sum form to agents and this redistribution improves insurance possibilities, we find that the optimal rate is sometimes positive. We also discuss the measurement of welfare gains or losses from inflation and their quantitative significance.

Arrow (1962) argued that since a monopoly restricts output relative to a competitive industry, it would be less willing to pay a fixed cost to adopt a new technology. Arrow’s idea has been challenged and critiques have shown that under different assumptions, increases in competition lead to less innovation. We develop a new theory of why a monopolistic industry innovates less than a competitive industry. The key is that firms often face major problems in integrating new technologies. In some cases, upon adoption of technology, firms must temporarily reduce output. We call such problems switchover disruptions. If firms face switchover disruptions, then a cost of adoption is the forgone rents on the sales of lost or delayed production, and these opportunity costs are larger the higher the price on those lost units. In particular, with greater monopoly power, the greater the forgone rents. This idea has significant consequences since if we add switchover disruptions to standard models, then the critiques of Arrow lose their force: competition again leads to greater adoption. In addition, we show that our model helps explain the accumulating evidence that competition leads to greater adoption (whereas the standard models cannot).

Typical models of bankruptcy and collateral rely on incomplete asset markets. In fact, bankruptcy and collateral add contingencies to asset markets. In some models, these contingencies can be used by consumers to achieve the same equilibrium allocations as in models with complete markets. In particular, the equilibrium allocation in the debt constrained model of Kehoe and Levine (2001) can be implemented in a model with bankruptcy and collateral. The equilibrium allocation is constrained efficient. Bankruptcy occurs when consumers receive low income shocks. The implementation of the debt constrained allocation in a model with bankruptcy and collateral is fragile in the sense of Leijonhufvud’s “corridor of stability,” however: If the environment changes, the equilibrium allocation is no longer constrained efficient.

Intellectual property protection involves a trade-off between the undesirability of monopoly and the desirable encouragement of creation and innovation. As the scale of the market increases, due either to economic and population growth or to the expansion of trade through treaties such as the World Trade Organization, this trade-off changes. We show that, generally speaking, the socially optimal amount of protection decreases as the scale of the market increases. We also provide simple empirical estimates of how much it should decrease.