This paper examines the optimal tax design problem in the presence of wage uncertainty. The wage has a continuous distribution, individuals are ex ante identical, preferences are separable in labor supply and goods, public policy aims at providing the population with social insurance, and the only restriction on the tax instruments is that emanating from lack of public observability of realized wages and labor supplies. We show that optimal tax structures depend crucially on whether it is labor supply or goods that consumers have to commit to before the resolution of uncertainty. Specifically, we prove that, in the absence of commitment, the social insurance problem collapses to the traditional optimal tax problem. Second, if labor supply is precommitted, it would be possible to effect a first-best outcome. Third, commitment to goods would make indirect taxation a useful instrument of tax policy even in the presence of a general income tax; it requires differential tax treatment of committed and noncommitted goods. Finally, if preferences are separable between the two types of goods, precommitted goods must be taxed at a uniform rate lower than that on the noncommitted goods. Copyright 1999 by Blackwell Publishing Inc.
