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In this paper, using the inversion of the optimal income tax problem approach, we recover the government beliefs about individual preferences starting from the observed distribution of gross and disposable income within a population and from the observed marginal tax rates as computed in standard microsimulation models. A detailed application is given in the case of France and Spain. The results show that, in the two countries analyzed, the government beliefs about the value of elasticity all along the income range are not uniform. Poor and rich people react less than the middle class. This result highlights the limit of using estimations of average labor supply elasticity in the analysis of fiscal reforms. We show also that the heterogeneity of the treatment of household size, implicit in the tax systems analyzed, influences dramatically the elasticity calculated. This highlights the relevance of defining a theoretical framework allowing for the explicit treatment of multidimensional optimal redistribution. The approach implemented in this paper provides an alternative way of reading marginal tax rates calculations routinely provided by tax-benefit models. In that framework, the issue of the optimality of an existing tax-benefit system may be analyzed by considering whether the individual utility function associated with that system satisfies elementary properties