Rohit Rahi

Informed Trading, Investment, and Welfare*

This paper studies the welfare economics of informed trading in a stock market. We provide a model in which all agents are rational and trade either to exploit information or to hedge risk. We analyze the effect of more informative prices on investment, given that this dependence will itself be reflected in equilibrium prices. Agents understand that asset prices may affect corporate investment decisions, and condition their trades on prices. We present both a general framework, and a parametric version that allows a closed-form solution. We show that in rational expectations equilibrium with price-taking competitive behaviour, and in the presence of risk-neutral uninformed agents, uninformed traders cannot lose money on average to informed traders. However, some agents with superior information may be willing to lose money on average, in order to improve their hedging possibilities. While a higher incidence of informed speculation always increases firm value through a more informative trading process, the effect on agents welfare depends on how revelation of information that agents wish to insure against reduces their hedging opportunities (the Hirshleifer effect). On the other hand, early revelation of information that is uncorrelated with hedging needs allows agents to construct better hedges.

Information revelation and market incompleteness

This paper introduces a theory of market incompleteness based on the information transmission role of prices and its adverse impact on the provision of insurance in financial markets. We analyse a simple security design model in which the number and payoff of securities are endogenous. Agents have rational expectations and differ in information, endowments, and attitudes toward risk. When markets are incomplete, equilibrium prices are typically partially revealing, while full relevation is attained with complete markets. The optimality of complete or incomplete markets depends on whether the adverse selection effect (the unwillingness of agents to trade risks when they are informationally disadvantaged) is stronger or weaker than the Hirshleifer effect (the impossibility of trading risks that have already been resolved), as new securities are issued and prices reveal more information. When the Hirshleifer effect dominates, an incomplete set of securities is preferred by all agents, and generates a higher volume of trade.

Adverse Selection and Security Design

This paper studies the problem of optimal security design by a privately informed entrepreneur. In the context of a simple parametric model, it is shown that the entrepreneur does not find it profitable to float an asset that affords her an informational advantage. The reason is that, with rational, uninformed outside investors, the entrepreneur faces adverse selection in the security market, which prevents her from exploiting her position as an insider. This is true whether or not she has market power in trading the asset.

Partially revealing rational expectations equilibria with nominal assets

This paper provides an existence theorem for rational expectations equilibria that aggregate information imperfectly. It studies a general equilibrium model of a static exchange economy with incomplete asset markets and finitely revealing equilibria exist, provided assets have nominal payoffs (in terms of units of account). In fact, any structure of information revealed by prices, that is consistent with the absence of arbitrage, can be embedded in a rational expectations equilibrium. This is in sharp contrast to the case of real assets, in which prices are generically fully revealing.

Strategic Financial Innovation in Segmented Markets

We study a model with restricted investor participation in which strategic arbitrageurs reap profits by exploiting mispricings across different market segments. We endogenize the asset structure as the outcome of a security design game played by the arbitrageurs. The equilibrium asset structure depends realistically upon considerations such as depth and gains from trade. It is neither complete nor socially optimal in general; the degree of inefficiency depends upon the heterogeneity of investors.

Value of Information in Competitive Economies with Incomplete Markets

A substantial literature addresses the negative eect on welfare of the release of information in a competitive market economy. We show that the value of information in this setting is typically positive if asset markets are suciently incomplete. More specically, for any competitive equilibrium of a generic economy, we can nd a ner information structure such that an allocation that is resource feasible and measurable with respect to this information ex- post Pareto dominates the given equilibrium allocation.

Efficiency properties of rational expectations equilibria with asymmetric information

In this paper we provide a characterization of the welfare properties of rational expectations equilibria of economies in which, prior to trading, agents have some information over the realization of uncertainty. We study a model with asymmetrically informed agents, treating symmetric information as a limiting case. Trade takes place in asset markets that may or may not be complete. We show that equilibria are characterized by two forms of inefficiency, price inefficiency and spanning inefficiency, and that generically both of them are present. Price inefficiency arises whenever equilibrium prices reveal some information. It formalizes and generalizes the so-called Hirshleifer effect, by showing that generically an interim Pareto improvement is possible even conditional on the information that is available to agents in equilibrium; the primary source of the inefficiency is a pecuniary externality. Spanning inefficiency, on the other hand, arises if prices are not fully revealing and markets are incomplete relative to the uncertainty faced by agents in equilibrium. In this case, an ex-post improvement can generically be implemented by providing agents with more information, thus expanding their risk-sharing opportunities and reducing informational asymmetries, even though this additional information restricts the set of allocations that are incentive compatible and individually rational.

Information Acquisition, Price Informativeness, and Welfare

We consider the market for a risky asset with heterogeneous valuations. Private information that agents have about their own valuation is reflected in the equilibrium price. We study the learning externalities that arise in this setting, and in particular their implications for price informativeness and welfare. When private signals are noisy, so that agents rely more on the information conveyed by prices, discouraging information gathering may be Pareto improving. Complementarities in information acquisition can lead to multiple equilibria.

Walrasian Foundations for Equilibria in Segmented Markets

We study a CAPM economy with segmented financial markets and competitive arbitrageurs who link these markets. We show that the equilibrium of the arbitraged economy is Walrasian in the sense that it coincides with the equilibrium of an appropriately defined competitive economy with no arbitrageurs. This characterization serves to clarify the role that arbitrageurs play in integrating markets.

Walrasian foundations for equilibria in segmented markets

We study a CAPM economy with segmented financial markets and competitive arbitrageurs who link these markets. We show that the equilibrium of the arbitraged economy is Walrasian in the sense that it coincides with the equilibrium of an appropriately defined competitive economy with no arbitrageurs. This characterization serves to clarify the role that arbitrageurs play in integrating markets.