The optimal design of a tax system is a topic that has long fascinated economic theorists
and flummoxed economic policymakers. This paper explores the interplay between tax theory
and tax policy. It identifies key lessons policymakers might take from the academic literature on
how taxes ought to be designed, and it discusses the extent to which these lessons are reflected in actual tax policy
We begin with a brief overview of how economists think about optimal tax policy, based
largely on the foundational work of Ramsey (1927) and Mirrlees (1971). We then put forward
eight general lessons suggested by optimal tax theory as it has developed in recent decades: 1)
Optimal marginal tax rate schedules depend on the distribution of ability; 2) The optimal
marginal tax schedule could decline at high incomes; 3) A flat tax, with a universal lump-sum
transfer, could be close to optimal; 4) The optimal extent of redistribution rises with wage
inequality; 5) Taxes should depend on personal characteristics as well as income; 6) Only final
goods ought to be taxed, and typically they ought to be taxed uniformly; 7) Capital income ought
to be untaxed, at least in expectation; and 8) In stochastic, dynamic economies, optimal tax
policy requires increased sophistication. For each lesson, we discuss its theoretical underpinnings
and the extent to which it is consistent with actual tax policy.