Mirrlees meets Diamond-Mirrlees: simplifying nonlinear income taxation

We show that the Diamond and Mirrlees (1971) linear tax model contains the Mirrlees (1971) nonlinear tax model as a special case. In this sense, the Mirrlees model is an application of Diamond-Mirrlees. We also provide a simple derivation of the Mirrleesian optimal income tax formula from the Diamond-Mirrlees commodity tax formula. In the Mirrlees model, the relevant compensated cross-price elasticities are zero, providing a situation where an inverse elasticity rule holds. We provide four extensions that illustrate the power and ease of our approach, based on Diamond-Mirrlees, to study nonlinear taxation. First, we consider annual taxation in a lifecycle context. Second, we include human capital investments. Third, we incorporate more general forms of heterogeneity into the basic Mirrlees model. Fourth, we consider an extensive margin labor force participation decision, alongside the intensive margin choice. In all these cases, the relevant optimality condition is easily obtained as a direct application of the general Diamond-Mirrlees linear tax formula.