Do progressive marginal income tax rates discourage self-employment? We assume risk neutrality to construct an implicit surtax on stochastic income relative to steady income, arising from a convex tax schedule. It is computed as part of a structural probit model with earnings equations and a tax simulator. The tax convexity variable and the net-of-tax income difference between self- and paid employment have the predicted signs and high levels of statistical significance for the probability of self-employment. A simulated flat tax reform suggests the tax effects are small.