This paper argues that adverse selection in the labour market, when viewed as part of a three-way interaction among workers, their current employers and a universe of alternative employers, may seriously impair a worker's freedom to change jobs. When current employers are better informed about the abilities of their workers than potential alternative employers, they will presumably concentrate their efforts to prevent turnover on their better workers. If these efforts lead to fewer quits among better workers, the stream of job changers should be composed disproportionately of less able ones. This will inhibit turnover in two ways. First, firms should be unwilling to hire from the job-changing pool except at low wages. Second, workers who change jobs are marked by being part of an inferior group, which lowers their future bargaining power and wages. Models of these phenomena can be made to account for many aspects of observed labour market behaviour.

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.