John K.-H. Quah

COMPARATIVE STATICS, INFORMATIVENESS, AND THE INTERVAL DOMINANCE ORDER

We identify a new way to order functions, called the interval dominance order, that generalizes both the single crossing property and a standard condition used in statistical decision theory. This allows us to provide a unified treatment of the major theorems on monotone comparative statics with and without uncertainty, the comparison of signal informativeness, and a non-Bayesian theorem on the completeness of increasing decision rules. We illustrate the concept and results with various applications, including an application to optimal stopping time problems where the single crossing property is typically violated. Copyright 2009 The Econometric Society.

Aggregating the Single Crossing Property

The single crossing property plays a crucial role in economic theory, yet there are important instances where the property cannot be directly assumed or easily derived. Difficulties often arise because the property cannot be aggregated: the sum or convex combination of two functions with the single crossing property need not have that property. We introduce a new condition characterizing when the single crossing property is stable under aggregation, and also identify sufficient conditions for the preservation of the single crossing property under multidimensional aggregation. We use our results to establish properties of objective functions (convexity, logsupermodularity), the monotonicity of optimal decisions under uncertainty, and the existence of monotone equilibria in Bayesian games.

The Monotonicity of Individual and Market Demand

This paper studies the interplay between two types of conditions guaranteeing the monotonicity of market demand : conditions on individual preferences and conditiosn on the distribution of income.

The law of demand when income is price dependent

This paper establishes a set of conditions for the uniqueness and stability of the equilibrium price in exchange and production economies. Building on the earlier work of J. M. Grandmont and W. Hildenbrand, it shows that increasing heterogeneity in preferences (in some well-defined sense) causes aggregate Engel curves to become increasingly linear. So sufficient dispersion, together with the assumption that preferences and endowments are independently distributed, leads to the aggregate excess demand function satisfying the law of demand. Uniqueness and stability of the equilibrium price follows.

The Law of Demand and Risk Aversion

This note proposes a necessary and sufficient condition on a preference to guarantee that the demand function it generates satisfies the law of demand. It shows that the law of demand may be succinctly characterized by differences in an agents level of risk aversion when she is confronted with different lotteries composed of commodity bundles.

Discounting, Values, and Decisions

How do discount rates affect agents’ decisions and valuations? This paper provides a general method to analyze this question, allowing stochastic and managed cash flows, stochastic discount rates, and time inconsistency and including arbitrary learning and payoff or utility processes. We show that some of these features can lead to counterintuitive answers (e.g., “a more patient agent stops earlier”), but we also establish, under simple conditions, theorems yielding robust predictions concerning the impact of discount rates on control and stopping decisions and on valuations. We apply our theory to models of search, experimentation, bankruptcy, optimal growth, investment, and social learning.

Homothetic or Cobb-Douglas Behavior Through Aggregation

A common theme in the theory of demand aggregation is that market demand can acquire properties which are not always individually present among the agents who make up that market, a phenomenon we call heteroiosis in this paper. This paper focusses on the well known result that with a suitable distribution of demand behavior (arising perhaps from the underlying distribution of preferences), market demand can become an approximately linear function of income or even take on approximately Cobb-Douglas properties. We highlight the mathematical arguments underpinning these models and show that in the right context, it is possible to carry the arguments further and achieve exact, rather than just approximate, results: exact Cobb-Douglas market demand or exact linearity of market demand with respect to income.

Market demand and comparative statics when goods are normal

Abstract This paper examines the impact of the normality assumption on the structure of market demand and on general equilibrium comparative statics. We define a new notion of comparative statics which is fundamentally related to normality and examine its incidence in exchange, production and financial economies.