Discrete time dynamics in a random matching monetary model

Summary. Under take-it-or-leave-it offers, dynamic equilibria in the discrete time random matching model of money are a “translation” of dynamic equilibria in the standard overlapping generations model. This formalizes earlier conjectures about the equivalence of dynamic behavior in the two models and implies the indeterminacy of dynamic equilibria in the random matching model. As in the overlapping generations model, the indeterminacy disappears if an arbitrarily small utility to holding money is introduced. We introduce a different pricing mechanism, one that puts into sharp focus that agents are forward-looking when they interact. Received: January 18, 2001; revised version: May 25, 2001     

On characterizing equilibria of economies with externalities and taxes as solutions to optimization problems

Summary 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.An earlier version of this paper, entitled Externalities and Taxes in General Equilibrium, was presented at the North American meetings of the Econometric Society, June 1988, at the University of Minnesota. We are grateful to David Backus, Kenneth Judd, Patrick Kehoe, and Rodolfo Manuelli for helpful conversations. National Science Foundation grants SES 86-18325 and SES 87-08616 provided financial support.The views expressed herein are those of the authors and not necessarily those of the Federal Reserve Bank of Minneapolis or the Federal Reserve System.     

Renewable Resource Use and Nonseparable Amenity Benefits

We incorporate amenity benefits into an overlapping generations model with a renewable resource as a factor of production, source of amenity benefits and store of value. Unlike the conventional renewable resource problems studied under the assumption of additive consumption and amenity benefits, we let amenity benefits affect the utility of consumers in a nonseparable fashion. We examine the role that weights given to consumption and amenities have for harvesting and the resource stock. We characterize dynamics and stability of steady state equilibria with a logistic resource growth function. We demonstrate in parametric and numerical models that the weights given to consumption and amenities in the utility function matter substantially for the steady state equilibrium stock and its stability and dynamics. Both conventional saddle point equilibria and indeterminacy with infinite number of equilibria and saddle-node bifurcation is possible depending on the weights given to consumption and amenities. In addition, we show that for each inefficient equilibrium stock, there is a unique subsidy rate that can move the economy from an inefficient equilibrium to an efficient one. The presence of indeterminacy provides a challenge to resource policies, because the system becomes unpredictable. Therefore, expectations and market psychology may play an important role in resource utilization and provision of amenities.     

Indeterminacy of intertemporal equilibria under imperfect competition

Summary. It is known that overlapping generations models with imperfectly competitive firms may exhibit a continuum of stationary equilibria. The reason of this indeterminacy is that different price expectation functions of consumers lead to different objective demand functions against which firms maximize. All these expectation functions fulfill perfect foresight in the equilibrium, but they can be arbitrary off the equilibrium. In this paper it is shown that it is not this arbitrariness which is responsible for the indeterminacy, but that the continuum of stationary equilibria emerges even if expectation functions are rational. Received: March 25, 1999; revised version: February 16, 2000     

Endogenous public policy and multiple equilibria

One of the well known roles of public policy in models with indeterminacy is to reduce the set of equilibria. However, agents' expectations regarding future policy may be self fulfilling when public policy is endogenized. We show that a simple overlapping generations economy with public education may yield multiple equilibria. Under a laissez-faire system, our model has a unique equilibrium. Thus, the presence of public policy may generate, rather than eliminate, multiple equilibria.     

Indeterminacy in dynamic models: When Diamond meets Ramsey

We consider an aggregate two-periods overlapping generations model with endogenous labor, consumption in both periods of life, homothetic preferences and productive external effects coming from the average capital and labor. We show that under realistic calibrations of the parameters, in particular a large enough share of first period consumption over the wage income, local indeterminacy of equilibria cannot occur with capital externalities alone. It can nevertheless occur when there are only, even very small, vanishing labor externalities provided that the elasticity of capital-labor substitution and the wage elasticity of the labor supply are large enough. We also show that if labor externalities are slightly stronger, but still small enough to be plausible, and the elasticity of labor supply is larger, local indeterminacy occurs in a Cobb-Douglas economy. Finally, we show that a locally indeterminate steady state is generically characterized by an under-accumulation of capital. It follows therefore that while agents live over a finite number of periods, the conditions for the existence of locally indeterminate equilibria are very similar to those obtained within infinite horizon models and that from this point of view, Diamond meets Ramsey.     

Education and growth with endogenous debt constraints

When future human capital cannot be alienated, households are allowed to borrow up to the point where it is in their own interest not to default. In such a framework, endogenous borrowing limits arise as the outcome of individual rationality constraint. In a model where education is the engine of growth, we show that endogenous borrowing constraints imply global indeterminacy. Comparing outcomes across the various equilibria we show that the relation between growth and yields is hump-shaped. Maximum growth can arise in an equilibrium with binding borrowing constraints, specially if the elasticity of human capital to education spending is large. Deepening financial markets promotes long-run growth in the case of a poverty trap, but not necessarily otherwise. On the methodological side, our approach stresses the importance of studying borrowing limits in general equilibrium, not only in small open economies. Philippe Michel passed away on July 22, 2004. His death is a great loss for his friends and for the overlapping generations and optimal control community.     

Endogenous Debt Constraints in Lifecycle Economies

We characterize competitive equilibria with perfect foresight in a deterministic, three-period pure-exchange overlapping generations economy with perfect information and no commitment to loan contracts. Commitment is replaced by an enforcement mechanism that excludes defaulters from asset markets for one period. For hump-shaped endowment profiles, young individuals face endogenous debt constraints that ration current consumption. Changes in current and future yields affect these constraints, inducing an additional income effect on rationed household demand that makes current and future consumption complements. This mechanism can lead to multiple steady states, persistent indeterminacy and regime switching. We show that sensitivity to shocks and complex dynamic behaviour are consistent with endogenous debt limits but not with exogenous liquidity constraints.     

Indeterminacy Arising in Multi-sector Economies

We characterize a large class of constant-returns-to-scale economies with standard Cobb–Douglas production technologies, which, when perturbed to incorporate external effects, exhibit indeterminacy or multiple equilibria. The perturbations are constrained to maintain overall constant returns to scale. We characterize the magnitude of the external effects that yield multiple equilibria in terms of the parameters of the unperturbed economy. We show that it is very easy to construct large and plausible classes of economies that exhibit indeterminacy with constant returns to scale, and with external effects that are arbitrarily small.
JEL Classification Numbers: E00, E3, O40.     

Trade using assets divisible at a cost

The consequences of costly divisibility of assets are studied using a model with the following features. The demand for assets is generated from an overlapping generations model with a continuum of agents in each generation and with intrageneration trade (intermediation) ruled out. There is a once-for-all supply of a stock of nonnegative-dividend assets in a large size, and there is a costly technology for dividing them into smaller sizes. Stationary equilibria are shown to exist. In contrast with similar models with costless divisibility of assets, competitive equilibria are not necessarily desirable; there can be Pareto-ordered equilibria.     

Equilibrium and optimality: Some imprints of David Gale

The paper is a review of some of the themes to which David Gale made lasting contributions. It touches upon a number of the fundamental issues in the Walrasian equilibrium theory (existence, uniqueness and stability), the overlapping generations model (non-optimality and indeterminacy of competitive equilibria), the von Neumann equilibrium (as a turnpike), and in the theory of decentralized intertemporal allocation through competitive prices (efficiency and golden rules, duality and existence of optimal programs).     

Time and Uncertainty in Overlapping Generations Economies

This paper shows the general reversibility of every perfect foresight equilibrium of an overlapping generations economy. It then shows and characterizes the existence of reversible sunspot equilibria in these economies as well, which seems to be at odds with our intuition about the irreversibility of a tree of events. Although the paper establishes also that such reversible stochastic equilibria constitute a negligible subset of all the equilibria of their class, their mere existence may be considered somewhat puzzling for this intuition. Journal of Economic Literature Classification Numbers: D50, D80, D90.     

The determinacy of equilibrium in economies of overlapping generations

We study the problem of uniqueness of equilibrium paths in the overlapping generations model. We show that, despite local calculation based on counting equations and unknowns, the equilibrium path may be unique. We do this by constucting an example of an economy of overlapping generations with just one equilibrium up to time shift, beside the steady states. Time is either discrete or continuous; in either case, it extend into the infinite future and, possibly, the infinite past. There is one, non-storable commodity at each date. The economy is stationary; intertemporal preferences are logarithmic; the endowments and discount factors of individuals need not depend continuously on time. With continuous time, equilibrium paths of prices are smooth; this, even for endowments and discount factors of individuals that do not depend continuously on time. With discrete time, as the number of periods in the life-span of individuals increases, equilibrium paths converge to the continuous time solutions. If time extends infinitely into the infinite past as well as into the infinite future, in continuous time, all non-stationary equilibrium paths of prices are time-shifts of a single path; in addition, there are two stationary solutions; in discrete time, there is a one dimensional family of non-stationary solutions, up to time-shift; however, the indeterminacy vanishes as the number of periods in the life span of individuals tends to infinity. If, alternatively, time has a finite starting point, in discrete time the degree of indeterminacy increases with the life-span of individuals, and, in continuous time, it is infinite; however, these are families of exponentially decreasing oscillations which, although they may exhibit pseudo-chaotic behaviour for a while, as time tends to infinity they all get damped, and asymptotic behaviour is that of the economy that originates in the infinite past. Stefano Lovo made interesting comments.     

Rational expectations in the overlapping generations model

This paper reports results on the character of the rational expectations equilibria of a stochastic overlapping generations model with heterogenous markets. The model considered is a stationary overlapping generations model in which the endowments of young agents are subject to i.i.d. random shocks. The main result shown is that if there are l > 1 commodities traded in every period, then for most preferences, the rational expectations equilibrium stochastic process of prices and allocations necessarily exhibits serial correlation. This is in marked contrast to the one commodity model in which there always exists an equilibrium which is measure isomorphic to the endowment process.     

Indeterminacy of equilibrium in stochastic OLG models

Summary. This paper studies the equilibria of a stochastic OLG exchange economies consisting of identical agents living for two periods, and having the opportunity to trade a single infinitely-lived asset in constant supply. The agents have uncertain endowments and the stochastic process determining the endowments is Markovian. For such economies, the literature has focused on studying strongly stationary equilibria in which quantities and prices are functions of the exogenous states of nature which describe the uncertainty: such equilibria are generalizations of deterministic steady states, and this paper investigates if they have the same special status as asymptotic limits of other equilibrium paths. The difficulty in extending the analysis of equilibria beyond the class of strongly stationary equilibria comes from the presence of indeterminacy: we propose a procedure for overcoming this difficulty which can be decomposed into two steps. First backward induction arguments are used to restrict the domain of possible prices; then if some indeterminacy is left, expectation functions are introduced to make the forward equilibrium equations determinate. The properties of the resulting trajectories, in particular their asymptotic properties, can then be studied. For the class of models that we study this procedure provides a justification for focusing on strongly stationary equilibria. For the model with positive dividends (equity or land) the justification is complete, since we show that the strongly stationary equilibrium is the unique equilibrium. For the model with zero dividends (money) there is a continuum of self-fulfilling expectation functions resulting in a continuum of equilibrium paths starting from any admissible initial condition: under conditions given in the paper, these equilibrium paths converge almost surely to one of the strongly stationary equilibria-either autarchy or the stochastic analogue of the Golden Rule. Received: November 19, 2001; revised version: March 22, 2002 RID="*" ID="*" We are grateful for the stimulating environment and research support provided by the Cowles Foundation at Yale University during the Fall 2000 when this paper was first conceived. We are also grateful to the participants of the SITE Workshop at Stanford University and the Incomplete Markets Workshop at SUNY Stony Brook during the summer 2001 for helpful discussions. Correspondence to: M. Magill     

On the nonexistence of recursive equilibrium in stochastic OLG economies

We study a class of stochastic Overlapping generations (OLG) economies that have one-memory equilibria, that is equilibria that are determined by the current and past realizations of the states of uncertainty. This class is negligible, but important. In particular, it contains all the known examples of nonexistence of recursive equilibrium. We show that, within this restricted domain, the existence of recursive equilibrium is actually typical, and such examples are therefore nonrobust. We thank Felix Kubler, Herakles Polemarchakis and Steve Spear for many useful discussions. This paper was started while the first author was visiting Columbia Business School.     

Existence of equilibria in the overlapping generations model: the nontransitive case

Summary This paper investigates the existence of competitive equilibria in dynamic exchange models with countably many periods and countably many agents. At each period the commodity space can be finite or infinite dimensional. The preferences of agents are not assumed to be transitive or complete. A first equilibrium existence theorem is established under the classical assumption that there exists a finite set of non-negligible agents. In the particular case of an overlapping generations model, a second existence theorem allows simultaneously for finite-lived assets and infinite-lived assets and limits the previous assumption to infinite-lived assets. This theorem covers obviously the standard case of an overlapping generations model where the agents have no endowment outside their lifetime.     

Indeterminacy in incomplete market economies

Summary It is shown that in a two-period economy with a continuum of states and real assets, the following holds: (1) if the asset structure is complete, then generically the number of equilibria is finite; (2) if there are a finite number of real assets (this can approximate completeness arbitrarily close) then, for a nonempty open set of economies, there are a continuum of distinct equilibria. Asymptotic versions (on the number of states and on the number of assets) of the result are also given. It is argued, therefore, that incompleteness, by itself, may be a leading source of indeterminacy.I want to thank R. A. Dara, J. Geanakoplos and the audience of a larger number of presentations (at Brown, Stanford, Columbia, Harvard, Paris,...) for useful comments.     

Price dynamics in overlapping generations environments

Summary This work examines the existence, uniqueness and computation of competitive equilibria in a class of overlapping generations environments. This set of environments represents a broad generalization of the overlapping generations model considered by Aliprantis and Plott [1]. Two types of results are presented in this paper. First, some general characteristics of perfect foresight competitive equilibrium price paths are developed for economies with finite or countably infinite time horizons and agents with finite lifetimes. The results establish the conditions leading to locally monotonic and locally stable equilibrium prices given arbitrarily many exogenous parameter shifts. Second, these results are strengthened when consideration is focused on a single parametric shift in a finite economy. Existence of a unique equilibrium price path is established. A simple set of rules are given to facilitate computation of this price path for any given shift.The authors wish to express their thanks to Donald Brown, Diego Moreno, Charles Plott, Vernon Smith, and Mark Walker for their comments and suggestions regarding this research.