Intertemporal substitution, risk aversion and ambiguity aversion

Summary. This paper axiomatizes a form of recursive utility on consumption processes that permits a role for ambiguity as well as risk. The model has two prominent special cases: (i) the recursive model of risk preference due to Kreps and Porteus [18]; and (ii) an intertemporal version of multiple-priors utility due to Epstein and Schneider [8]. The generalization presented here permits a three-way separation of intertemporal substitution, risk aversion and ambiguity aversion.Received: 5 August 2003, Revised: 12 March 2004, JEL Classification Numbers: D80, D81, D90.I am grateful to Larry Epstein for his guidance and invaluable advice, and to a referee for helpful comments and suggestions.     

Subjective random discounting and intertemporal choice

This paper provides an axiomatic foundation for a particular type of preference shock model called the random discounting representation where a decision maker believes that her discount factors change randomly over time. For this purpose, we formulate an infinite horizon extension of [E. Dekel, B. Lipman, A. Rustichini, Representing preferences with a unique subjective state space, Econometrica 69 (2001) 891-934], and identify the behavior that reduces all subjective uncertainties to those about future discount factors. We also show uniqueness of subjective belief about discount factors. Moreover, a behavioral comparison about preference for flexibility characterizes the condition that one's subjective belief second-order stochastically dominates the other. Finally, the resulting model is applied to a consumption-savings problem.     

Information,endogenous uncertainty and risk aversion

The importance of incomplete information and risk aversion for the allocation of economic resources is shown to depend critically on whether uncertainty is exogenously imposed or endogenously related to the ability of the price system to aggregate and disseminate the information possessed by the agents in a decentralized market economy. The specific example analysed in this paper is a two-period exchange model with competitive markets and a homogeneous product.Comments and suggestions by anonymous referees, Jacques Drèze, Jan Rose Sørensen, and financial support from the Danish Social Science Research Council are gratefully acknowledged.     

Past experience of uncertainty affects risk aversion

In an experiment with more than 500 participants we study how past experience of uncertainty (imperfect knowledge of the state space) affects risk preferences. Participants in our experiment choose between a sure outcome and a lottery in 32 periods. All treatments are exactly identical in periods 17–32 but differ in periods 1–16. In the early periods of the risk treatment there is perfect information about the lottery; in the ambiguity Treatment participants perfectly know the outcome space but not the associated probabilities; in the unawareness treatment participants have imperfect knowledge about both outcomes and probabilities. We observe strong treatment effects on behavior in periods 17–32. In particular, participants who have been exposed to an environment with very imperfect knowledge of the state space subsequently choose lotteries with high (low) variance less (more) often compared to other participants. Estimating individual risk attitudes from choices in periods 17–32 we find that the distribution of risk attitude parameters across our treatments can be ranked in terms of first order stochastic dominance. Our results show how exposure to environments with different degrees of uncertainty can affect individuals’ subsequent risk-taking behavior.     

Endogenous factor market distortion, risk aversion, and international trade under input uncertainty

In the context of non-diversifiable and sector-specific risks in labour markets, we show that the resulting factor market distortion – attributable to an endogenous intersectoral wage differential – can provide a possible rationale that explains why larger wage dispersion prevails in developing nations. We also demonstrate how endogenous wage distortions spill over to capital markets, with capital-poor economies offering lower rates of returns. In addition, we show that inequality in the distribution of wealth further deviates factor allocation away from first-best and impairs intersectoral mobility of the poor.
Ce mémoire montre que la distorsion dans le marché des facteurs qui résulte de risques non diversifiables et spécifiques à certains secteurs (et qui se traduit par un différentiel de salaire endogène entre secteurs) peut expliquer pourquoi on observe une dispersion plus grande des salaires dans les pays en voie de développement.On montre aussi comment des distorsions endogènes de salaires débordent vers les marchés de capitaux,ce qui fait que les pays pauvres en capital ont des rendements plus faibles.De plus, on montre que l'inégalité dans la distribution de la richesse contribue à faire dévier l'allocation des ressources de son optimum de premier ordre et nuit à la mobilité inter-sectorielle des pauvres.     

Consequences of irreversibilities on optimal intertemporal CO2 emission policies under uncertainty

This paper investigates how irreversibilities affect the optimal intertemporal accumulation of greenhouse gases in the atmosphere under uncertainty. More precisely, the evolution of the future temperature is assumed to follow an Itô-process with the drift provided by greenhouse gas emissions. This paper considers two different kinds of irreversibilities: of emissions (i.e., CO₂ once dissolved into the air cannot be collected later) and of stopping. These issues are investigated first (in the tradition of the real option literature) as pure stopping problems and then allowing for a continuous choice of emissions. Implications for global warming are: an irreversible stopping of greenhouse gas emissions is never optimal in a continuous framework and yields in the real option framework a less conservationist stopping rule in which uncertainty increases the stopping threshold (i.e. works against conservation).     

Expropriation risk and discounting

This paper focuses on time preference and individual measures of expropriation risk. We use survey data on Italian households and on the 50+ population in eleven European countries. The surveys provide measures of discount rate and proxies for expropriation due to criminal offenses. We show that impatience increases with expropriation risk.     

Mean-dispersion preferences and constant absolute uncertainty aversion

We axiomatize, in an Anscombe–Aumann framework, the class of preferences that admit a representation of the form V(f)=μ−ρ(d)$V(f)=\mu -\rho (d)$, where μ is the mean utility of the act f with respect to a given probability, d   is the vector of state-by-state utility deviations from the mean, and ρ(d)$\rho (d)$ is a measure of (aversion to) dispersion that corresponds to an uncertainty premium. The key feature of these mean-dispersion   preferences is that they exhibit constant absolute uncertainty aversion. This class includes many well-known models of preferences from the literature on ambiguity. We show what properties of the dispersion function ρ(⋅)$\rho (\cdot )$ correspond to known models, to probabilistic sophistication, and to some new notions of uncertainty aversion.     

Ambiguity,uncertainty aversion and equilibrium welfare

Summary. In order to analyse the effect of ambiguity and uncertainty aversion on equilibrium welfare, a two period, pure exchange one good economy is considered. Agents are Choquet-expected-utility maximizers with same convex capacity and strictly concave utility index. It is proven that equilibrium is indeterminate whenever several probabilities in the core of the capacity minimize the expected value of aggregate endowment and not all agents have same expected endowments under those probabilities. It is further shown that small changes in aggregate endowment may have drastic welfare implications. A more general model is considered in the case of no aggregate uncertainty: agents have a set of priors and are uncertainty averse as modelled by Gilboa-Schmeidler [1989]. In the case of complete markets, it is shown that assets have a spread of equilibrium prices similar to the spread of no-arbitrage prices compatible with absence of arbitrage in markets with imperfections.Received: 2 June 2000, Revised: 27 March 2003, JEL Classification Numbers: D46, D59,D60, G12.I have benefited from conversations with L. Epstein, F. Magnien and J. M. Tallon.     

On the correspondence between multivariate risk aversion and risk aversion with state-dependent preferences

This paper establishes the correspondence between multivariate risk aversion and risk aversion with state-dependent preferences. It shows that the prerequisite for comparability of risk aversion in the multivariate case, namely, identical ordinal preferences on the commodity space, corresponds to identical, properly defined, reference sets in the case of state-dependent preferences. For comparable decision makers the condition that the utility function of one is a concave transformation of that of the other on the commodity space corresponds to the condition that the expected utility of one is a concave transformation of that of the other on the reference set.     

First-order risk aversion and non-differentiability

Summary First-order risk aversion happens when the risk premium a decision maker is willing to pay to avoid the lottery , is proportional, for smallt, tot. Equivalently, 0$$ " align="middle" border="0"> . We show that first-order risk aversion is equivalent to a certain non-differentiability of some of the local utility functions (Machina [7]).We are grateful to the Social Sciences and Humanities Research Council of Canada for financial support and to Kim Border, Larry Epstein, Mark Machina and Joe Ostroy for helpful discussions and suggestions.     

Housing and relative risk aversion

This paper derives closed-form and numerical solutions for relative risk aversion in a standard consumption-based model enriched with housing. The presence of housing enables the household to hedge against unexpected shocks and may decrease relative risk aversion. In addition, housing may generate state-dependent, time-varying risk aversion.     

Core and equilibria under ambiguity

This paper introduces new core and Walrasian equilibrium notions for an asymmetric information economy with non-expected utility preferences. We prove existence and incentive compatibility results for the notions we introduce. We also discuss a framework for ex ante, interim and ex post preferences.     

Saving and retirement behavior under quasi-hyperbolic discounting

This paper investigates saving and retirement behavior using a quasi-hyperbolic discounting model with endogenous labor supply. The behavior of quasi-hyperbolic-discounting consumers is compared with optimal behavior, which is obtained under exponential discounting. The quasi-hyperbolic discounters, whether naïve or sophisticated, under-save and retire early compared with an exponential discounter, if and only if the present-biased marginal utility of future consumption decreases with stronger present bias. Logarithmic utility functions and constant-absolute-risk-aversion utility functions can both exhibit this property. In other words, quasi-hyperbolic discounting explains why, consistent with previous empirical studies, under-savers might also be early retirers. Under logarithmic utility, a wage tax and an interest subsidy can counteract the under-saving and early retirement and improve consumer welfare.     

Relative risk aversion, relative prudence and comparative statics under uncertainty: The case of (μ, σ)‐preferences

From the expected‐utility approach, relative risk aversion being smaller than one and relative prudence being smaller than two emerge as preference restrictions that fully determine the optimal responses of decisions under uncertainty to certain shifts in probability distributions. We characterize the magnitudes of relative risk aversion and relative prudence in terms of the two‐parameter, mean‐standard deviation approach. We demonstrate that this characterization is instrumental in obtaining comparative static results in the two‐parameter setting. We further relate our findings to the results in the expected‐utility framework.     

State-price densities under heterogeneous beliefs, the smile effect, and implied risk aversion

It has been widely noted in the empirical literature that state-price densities implicit in financial asset prices are not log-normal. This paper shows that this phenomenon can be caused by heterogeneity in investors’ beliefs. It derives the state-price density under heterogeneous beliefs in closed form and demonstrates that heterogeneous beliefs can give rise to multimodal state-price densities. Consequences for the “smile effect” in implied option volatility and for measures of risk aversion inferred from empirical state-price densities are discussed.