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2012～2016年度科学研究費補助金[基盤研究(S)]「長期デフレの解明」(研究代表者 東京大学経済学研究科・渡辺努, 課題番号：24223003)

We consider an economy where individuals face uninsurable risks to their human capital accumulation and analyze the optimal level of linear taxes on capital and labor income together with the optimal path of government debt. We show that in the presence of such risks, it is beneficial to tax both labor and capital and to issue public debt. We also assess the quantitative importance of these findings, and show that the benefits of government debt and capital taxes both increase with the magnitude of idiosyncratic risks and the degree of relative risk aversion. (JEL D52, H21, H24, H25, H63, J24) Human capital is an important component of wealth both at the individual and aggregate level, and its role has been investigated in various fields in economics. In public finance, Jones, Manuelli, and Rossi (1997) show that the zero-capital-tax result of Chamley (1986) and Judd (1985) 1 can be strengthened if human capital accumulation is explicitly taken into account. Specifically, they demonstrate that, in a deterministic economy with human capital accumulation, in the long run not only capital but also labor income taxes should be zero, hence the government must accumulate wealth—that is, public debt be negative—to finance its expenditure.

We study the dynamic Ramsey problem of finding optimal public debt and linear taxes on capital and labor income within a tractable infinite horizon model with incomplete markets. With zero public expenditure and debt, it is optimal to tax the risky labor income and subsidize capital, while a positive amount of public debt is welfare improving. A steady state optimality condition is derived which implies that the tax on capital is positive, when savings are sufficiently inelastic to returns. A calibration of our model to the US economy indicates positive optimal taxes and a small but positive optimal debt level.

How should capital and labor be taxed when individuals' labor income is subject to unin- surable idiosyncratic risks? To address this question, we develop a tractable infinite horizon model with incomplete markets and consider a dynamic optimal taxation problem with linear taxes on the wage and interest income. We derive two general principles for public policy in such an environment: (i) providing an insurance for the idiosyncratic income risks; and (ii) allocating tax burdens efficiently over time. The first principle calls for taxing the labor income. The second principle clarifies when accumulating government debt is welfare improving, and also when the tax rate on physical capital needs to be strictly positive in the long run. We also calibrate our model to the U.S. economy and find that the presence of idiosyncratic income risks significantly affects the optimal tax rates and the optimal amount of the government debt.