On the Sign of the Optimum Marginal Income Tax
研究在Mirrlees最优所得税模型中，仅基于凹效用主义时最优边际所得税是否非负，发现原结论需效用可加分离，并在更弱条件下证明边际税严格为正。
This paper studies a central aspect of optimal income taxation as modelled in Mirrlees' original paper on the topic (identical leisure/consumption preferences, qualitatively homogeneous "skills" in production), namely, whether given concave utilitarianism alone the optimal marginal income tax will be non-negative. A well-known positive answer to this question (in weak inequality form) was given in the said paper which, however, we show requires additive separability of individual utility (in the ordinal and cardinal senses). Our main result here is to derive the required (strict) positivity of the marginal tax under weak conditions, slightly wider than noninferiority of consumption and leisure, with preferences otherwise arbitrary.