[It is generally agreed that speculators can make profits from insider trading or from the release of false information. Both forms of stock-price manipulation have now been made illegal. In this article, we ask whether it is possible to make profits from a different kind of manipulation, in which an uninformed speculator simply buys and sells shares. We show that in a rational expectations framework, where all agents maximize expected utility, it is possible for an uninformed manipulator to make a profit, provided investors attach a positive probability to the manipulator being an informed trader.]

The paper describes a Walrasian theory of markets with adverse selection and shows how refinements of equilibrium can be used to characterize uniquely the equilibrium outcome. Equilibrium exists under standard conditions. It is shown that, under certain conditions, a stable set exists and is contained in a connected set of equilibria. For generic models there exists a stable outcome, that is, all the equilibria in the stable set have the same outcome. These ideas are applied to markets with one-sided and two-sided uncertainty. Under standard monotonicity conditions, it is shown that the stable outcome is separating and implies a particular pattern of matches of buyers and sellers.

The cost of gathering information about unfamiliar securities may lead to gains from standardization: firms issue a particular security because it is used by other firms. To support standardization as an equilibrium phenomenon, information must be non-transferable (otherwise it might be revealed by prices or the observation of other agents' decisions) and it must be generic (useful in evaluating a number of securities). A competitive equilibrium in which standard contracts are used may be subject to coordination failure: while there always exists a constrained efficient equilibrium, there may also exist Pareto-ranked equilibria.

It is generally agreed that speculators can make profits from insider trading or from the release of false information. Both forms of stock-price manipulation have now been made illegal. In this article, we ask whether it is possible to make profits from a different kind of manipulation, in which an uninformed speculator simply buys and sells shares. We show that in a rational expectations framework, where all agents maximize expected utility, it is possible for an uninformed manipulator to make a profit, provided investors attach a positive probability to the manipulator being an informed trader.