The marginal cost of public funds is one at the optimal tax system

This paper develops a Mirrlees framework with skill and preference heterogeneity to analyze optimal linear and nonlinear redistributive taxes, optimal provision of public goods, and the marginal cost of public funds (MCF). It is shown that the MCF equals one at the optimal tax system, for both lump-sum and distortionary taxes, for linear and nonlinear taxes, and for both income and consumption taxes. By allowing for redistributional concerns, the marginal excess burden of distortionary taxes is shown to be equal to the marginal distributional gain at the optimal tax system. Consequently, the modified Samuelson rule should not be corrected for the marginal cost of public funds. Outside the optimum, the marginal cost of public funds for distortionary taxes can be either smaller or larger than one. The findings of this paper have potentially important implications for applied tax policy and social cost-benefit analysis.