Balancing Equity and Efficiency in Mirrlees Optimal Tax Rule

Designing an optimal income tax system involves balancing a fundamental trade-off: redistributing resources to boost equity while minimizing distortions to labor supply and productivity. This challenge is particularly crucial under asymmetric information, where policymakers cannot directly observe individuals innate abilities, creating incentives for high-skilled workers to underreport their productivity to avoid higher taxes. The Mirrlees model revolutionized this field by formalizing how non-linear tax schedules, where marginal tax rates vary with income, can reconcile these goals through incentive compatibility constraints. These constraints ensure that individuals voluntarily reveal their true skill levels, preventing welfare-reducing misreporting. However, using this theory into practical policy means dealing with complex trade-offs, especially when abilities are discrete rather than continuously distributed. In this analysis, we first reconstruct Mirrlees theoretical framework, emphasizing the mathematical derivation of the “no distortion at the top” result, where the highest earners face a zero marginal tax rate, and its implications for social welfare maximization. We then extend this logic to a simplified economy with two distinct skill groups, demonstrating how optimal marginal tax rates for high-ad low- ability workers emerge from balancing equity concerns against labor supply elasticities.