This paper seeks to cast some light on the importance of adverse selection in competitive markets by examining the market for the sale of slaves in pre-Civil War New Orleans. Estimates of the degree of adverse selection in the New Orleans market are obtained by examining the relative prices of slaves from different regions of origin. These estimates indicate that slaves brought to market may on average have been of 20 percent to 40 percent lower quality than the slave population in general, and that good slaves were perhaps three times less likely to be sold than low quality ones.

This paper presents a simple, general framework for analyzing externalities in economies with incomplete markets and imperfect information. By identifying the pecuniary effects of these externalities that net out, the paper simplifies the problem of determining when tax interventions are Pareto improving. The approach indicates that such tax interventions almost always exist and that equilibria in situations of imperfect information are rarely constrained Pareto optima. It can also lead to simple tests, based on readily observable indicators of the efficacy of particular tax policies in situations involving adverse selection, signaling, moral hazard, incomplete contingent claims markets, and queue rationing equilibria.