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This paper studies the welfare economics of informed trading in a stock market. We model the effect of more informative prices on investment, given that this dependence will itself be reflected in equilibrium prices. 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. We provide a parametric model that allows a closed-form solution and a complete welfare analysis. 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 changes risk-sharing opportunities in the market.

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.

A number of economists have supported the taxation of speculation in financial markets. We examine the welfare economics of such a tax in a model of trading in a financial market where some agents have superior information. We show that in some cases a tax on speculators may actually increase speculative profits. This occurs if the speculators* benefit from less informative prices offsets the costs of the tax. The effect on the welfare of other agents depends on how revelation of information changes risk-sharing opportunities in the market. It is possible for the introduction of a tax to cause a Pareto improvement .