Market clearing, utility functions, and securities prices

Summary. We prove the existence of equilibrium in a continuous-time finance model; our results include the case of dynamically incomplete markets as well as dynamically complete markets. In addition, we derive explicitly the stochastic process describing securities prices. The price process depends on the risk-aversion characteristics of the utility function, as well as on the presence of additional sources of wealth (including endowments and other securities). With a single stock, zero endowment in the terminal period, and Constant Relative Risk Aversion (CRRA) utility, the price process is geometric Brownian motion; in essentially any other situation, the price process is not a geometric Brownian motion.JEL Classification Numbers: D52.This paper is part of my Dissertation (UC Berkeley). I am very grateful to my advisor Professor Robert M. Anderson. I also would like to thank Steve Evans, Roger Purves, Jacob Sagi, Chris Shannon and the participants of the 2002 NBER General Equilibrium Conference at the University of Minnesota (Minneapolis) for very helpful discussions and comments. This work was supported by Grant SES-9710424 from the National Science Foundation.     

Optimal monetary policy and economic growth

A question at the center of many analyses of optimal monetary policy is, why do central banks never implement the Friedman rule? To the list of answers to this question, we add neoclassical production (specifically, the Tobin effect) as one possible explanation. To that end, we study an overlapping generations economy with capital where limited communication and stochastic relocation create an endogenous transactions role for fiat money. We assume a production function with a knowledge externality (Romer style) that nests economies with endogenous growth (AK form) and those with no long-run growth (the Diamond model). The Tobin effect is shown to be always operative. Under CRRA preferences, a mild degree of social increasing returns is sufficient (but not necessary) for some positive inflation to dominate zero inflation and for the Friedman rule to be sub-optimal, irrespective of the degree of risk aversion.     

Consumption Externalities,Habit Formation and Equilibrium Efficiency*

We analyze the welfare properties of the competitive equilibrium in a capital accumulation model where individual preferences are subject to both habit formation and consumption spillovers. Using an additive specification for preferences, according to which the argument in the utility function is a linear combination of present and past values of own consumption and consumption spillovers, we analyze the circumstances under which these spillovers are a source of inefficiency. It is shown that consumption externalities have to interact with habits in order to generate an inefficient dynamic equilibrium. Finally, we characterize optimal tax policies aimed at restoring efficient decentralized paths.     

Sustained positive consumption in a model of stochastic growth: The role of risk aversion

In a stochastic economy, long run consumption and output may not be bounded away from zero even when productivity is arbitrarily high near zero and uncertainty is arbitrarily small. In the one-sector stochastic optimal growth model with i.i.d. production shocks, we characterize the nature of preferences that lead to this phenomenon for a stochastic Cobb–Douglas technology. For the general version of the model, we outline sufficient conditions under which the economy expands its capital stock near zero and long run consumption is bounded away from zero with certainty. Our conditions highlight the important role played by risk aversion for small consumption levels.     

Enterprise diversification in US dairy: impact of risk preferences on scale and scope economies

Enterprise diversification has recently become a prominent feature of US dairy farms. Scope economies and risk aversion are two forces that simultaneously determine diversification. We jointly estimate scope economies and determine risk preferences under price uncertainty. We reject risk neutrality in favour of Increasing Absolute Risk Aversion (IARA) and Increasing Relative Risk Aversion (IRRA). Scope economies are significant, but diminish with farm size. Increasing returns to scale exist in the production of multiple enterprises and diminish with size. Large farms operate under decreasing returns to scale. Ignoring risk preferences, a common practice in empirical work, results in an underestimate of the effect of scope economies for large farms.     

Wealth inequality and dynamic stability

We explore the link between wealth inequality and stability in a two-sector neoclassical growth model with heterogeneous agents. We show that when the inverse of absolute risk aversion (or risk tolerance) is a strictly convex function, wealth inequality is a factor that favors instability. In the opposite case, inequality favors stability. Our characterization also shows that whenever absolute risk tolerance is linear, as when preferences exhibit hyperbolic absolute risk aversion (HARA), wealth heterogeneity is neutral.     

Uniqueness of asset prices in an exchange economy with unbounded utility

Summary. This paper studies conditions under which the price of an asset is uniquely determined by its fundamental value – i.e., no bubbles can arise – in Lucas-type asset pricing models with unbounded utility. After discussing Gilles and LeRoy's (1992) example, we construct an example of a two-period, representative agent economy to demonstrate that bubbles can arise in a standard model if utility is unbounded below, in which case the stochastic Euler equation may be violated. In an infinite horizon framework, we show that bubbles cannot arise if the optimal sequence of asset holdings can be lowered uniformly without incurring an infinite utility loss. Using this result, we develop conditions for the nonexistence of bubbles. The conditions depend exclusively on the asymptotic behavior of marginal utility at zero and infinity. They are satisfied by many unbounded utility functions, including the entire CRRA (constant relative risk aversion) class. The Appendix provides a complete market version of our two-period example. Received: January 22, 1996; revised version: February 18, 1997     

Optimal portfolio positioning under ambiguity

This paper analyzes the optimality of financial portfolios when the investor has a utility with ambiguity aversion. It provides a general result about the optimal portfolio profile under ambiguity, in the Anscombe–Aumann framework, using the Maccheroni et al. (2006) approach which includes Gilboa and Schmeidler's (1989) multiple prior preferences and Hansen and Sargent's (2011) multiplier preferences. The paper then details the CRRA case with an ambiguity index based on relative entropy. Such findings have practical applications in structured portfolio management. Indeed, it is important to take account of uncertainty about the true values of financial parameters when determining the best portfolio profile.     

再分配倾向的决定:对既有文献的考察

本文对再分配倾向决定领域的既有文献进行详尽考察,认为从内在动力角度来理解各种决定因素才是根本,归纳提炼了再分配倾向决定的四大动机:经济利益动机、风险规避动机、公平信念动机和声誉理想动机。再分配倾向决定的四大动机通过不尽相同的理论逻辑机制和多种实证影响因素对人们的再分配倾向产生影响,四大动机的框架为实证研究中的因素筛选提供了有力的工具。对于该领域进一步的研究,本文提出应用实验手段的重要性,并应注意宏观变量对再分配倾向的内生性影响。     

Optimal climate and fiscal policy in an OLG economy

This paper develops a climate–economy model to study the joint design of optimal climate and fiscal policies in economies with overlapping generations (OLGs). I demonstrate how capital taxation, if optimal, drives a wedge between the market costs of carbon (the net present value of marginal damages using the market interest rate) and the Pigouvian tax (the net present value of marginal damages using the consumption discount rate of successive OLGs). In contrast to deterministic infinitely lived representative agent models, at the optimum, the capital income tax is positive, the carbon price equals the market costs of carbon but it falls short of the Pigouvian tax when (i) preferences are not separable over consumption and leisure; and (ii) labor income taxes cannot be age-dependent. I also show that restrictions on climate change policy provide a novel rationale for positive capital income taxes.     

Wealth inequality, preference heterogeneity and macroeconomic volatility in two-sector economies

We explore the link between wealth inequality, preference heterogeneity and macroeconomic volatility in a two-sector neoclassical growth model. First we prove that, if agents have homogeneous preferences, when the absolute risk tolerance is a strictly convex (concave) function, sufficiently high (low) levels of wealth inequality may lead to endogenous fluctuations in the neighborhood of the steady state. Second, we consider the effects of preference heterogeneity when agents are homogeneous with respect to their wealth. We show that when the utility function belongs to the HARA class, sufficiently high levels of preference heterogeneity may lead to endogenous fluctuations in the neighborhood of the steady state if the elasticity of intertemporal substitution in consumption is greater than one.     

Dynamic optimization with a nonsmooth, nonconvex technology: the case of a linear objective function

This paper studies a one-sector optimal growth model with linear utility in which the production function is only required to be increasing and upper semicontinuous. The model also allows for a general form of irreversible investment. We show that every optimal capital path is strictly monotone until it reaches a steady state; further, it either converges to zero, or reaches a positive steady state in finite time and possibly jumps among different steady states afterwards. We establish conditions for extinction (convergence to zero), survival (boundedness away from zero), and the existence of a critical capital stock below which extinction is possible and above which survival is ensured. These conditions generalize those known for the case of S-shaped production functions. We also show that as the discount factor approaches one, optimal paths converge to a small neighborhood of the capital stock that maximizes sustainable consumption.This paper is dedicated to Professor Mukul Majumdar on his 60th birthday. His research with various co-authors in the late 70s and the 80s pioneered innovative techniques for the analysis of nonconvex dynamic optimization models – both deterministic and stochastic. Roy considers himself particularly fortunate for having had the opportunity to learn economic theory and mathematical economics from Professor Majumdar. This paper has benefited from helpful comments and suggestions by an anonymous referee. Financial support from the 21st Century COE Program at GSE and RIEB, Kobe University, is gratefully acknowledged.     

DYNAMIC ASSET ALLOCATION AND CONSUMPTION CHOICE IN INCOMPLETE MARKETS*

We study the optimal investment and consumption problem of a CRRA investor when the drift and volatility of the stock are driven by a correlated factor. The myopic and non‐myopic components of the optimal portfolio process are characterised in terms of the market price of traded and non‐traded risk of the minimax martingale measure. We find that the optimal policies depend crucially on the nature of the agent, aggressive versus conservative, and the market incompleteness, improving versus deteriorating investment opportunities. Furthermore, we show that the original problem cannot be decomposed into a pure consumption and a pure terminal wealth problem, unless the market is complete.     

Knightian uncertainty and moral hazard

This paper presents a principal-agent model in which the agent has imprecise beliefs. We model this situation formally by assuming the agent?s preferences are incomplete as in Bewley (1986) [2]. In this setting, incentives must be robust to Knightian uncertainty. We study the implications of robustness for the form of the resulting optimal contracts. We give conditions under which there is a unique optimal contract, and show that it must have a simple flat payment plus bonus structure. That is, output levels are divided into two sets, and the optimal contract pays the same wage for all output levels in each set. We derive this result for the case in which the agent?s utility function is linear and then show it also holds if this utility function has some limited curvature.     

Disutility of pollution and endogenous growth

Endogenous growth is generally built on a positive externality hypothesis which is the opposite of a negative externality caused by pollution. We study a linear technology with simple assumption: an aggregate capital stock which represents a learning by doing effect and a pollution flow proportional to production. In this framework, we analyse the precise effects on growth of the disutility of pollution and its interaction with the utility of consumption in an economy without abatement technology. The decentralized equilibrium always leads to unlimited growth, but optimal growth is often limited (the negative effect of pollution dominating the positive effect of learning by doing). In this case, the optimal policy which leads the decentralized economy to follow the optimal growth path is to tax capital; in contrast with the optimal subsidy policy in an economy without pollution. When an abatement technology is introduced, the optimal solution can lead the economy to unlimited growth, whatever the form of the utility function.     

The undesirability of randomized income taxation under decreasing risk aversion

For the standard specification of the utilitarian optimal income tax problem with hidden characteristics, the paper shows that randomized tax schemes are undesirable if preferences exhibit a property of weakly decreasing risk aversion according to the multidimensional risk aversion concept of Hellwig [Hellwig, M.F., 2004, Risk Aversion in the Small and the Large with Multidimensional Outcomes. Max Planck Institute for Research on Collective Goods, Bonn. Preprint 2004/6, http://www.mpp-rdg.mpg.de/pdf_dat/2005_23online.pdf]. This property of decreasing risk aversion also implies that the optimal income tax schedule is unique and depends continuously on the data of the problem.     

A Case Against Zero Capital-Income Taxation

This paper develops a dynamic general equilibrium model to investigate the optimal level of capital income taxation in light of stochastic endogenous economic growth. Although endogenous human capital is incorporated into our model, we restrict our investigation to the issue of optimal physical capital income tax; and the labor supply is also endogenously determined. This paper proves that the optimal capital income tax should be zero provided exogenous government expenditure on production; however, capital income should be taxed if we consider endogenous government consumption.     

The creative class, its preferences, and unbalanced growth in an urban economy

We theoretically analyze unbalanced growth in an urban economy arising from the preferences of the creative class concerning the relative desirability of agricultural, manufacturing, and service goods. We study two cases. In the first case, the production functions for the three categories of goods are identical. Our analysis leads to four results. First, we compute the equilibrium physical to creative capital ratios and the relationships between the neutral productivity shifters and the output prices. Second, we show that agricultural and services consumption are a constant multiple of manufacturing consumption. Third, we note that under certain conditions, an equilibrium in which all sectors of our urban economy grow at a constant rate does not exist. Fourth, we show that a constant growth path (CGP) equilibrium exists in which, across the three sectors, the pattern of consumption changes and there is a reallocation of creative capital. In the second case, the production functions for the three categories of goods are dissimilar. In this more realistic setting, we study generalizations of the previously described four results.     

Preservation and exogenous uncertain future preferences

Summary. We extend the Beltratti, Chichilnisky and Heal's (1993) and (1998) continuous-time stochastic dynamic framework to analyze the optimal depletion of an asset whose consumption is irreversible, in the face of uncertainty about future preferences. Their model is rather general and so the results are general qualitative theorems. We show that in some interesting cases it is possible to solve their model analytically. The cases involve constant elasticity utility functions and the assumption of a Poisson process for the evolution of preferences. Received: September 13, 1999; revised version: November 23, 1999     

Stochastic technology shocks in an extended Uzawa–Lucas model: closed-form solution and long-run dynamics

We add stochastic technological progress, modelled as a geometric Brownian motion with drift, to an augmented Uzawa–Lucas growth model. Under a particular combination of parameters we derive a closed form solution to the model and analytical expressions which show that uncertainty reduces the optimal levels of consumption and increases the proportion of human capital devoted to producing new human capital.