Abstract : We investigate whether the optimal income tax model of Mirrlees (1971) can reproduce the empirically observed increasing average tax rates. We give a necessary condition and exhibit two examples where the optimal average tax rates are increasing. We wish to thank Mathias Hungerbühler and Laurence Jacquet for stimulating discussions. Citation: HINDRIKS, Jean, Etienne LEHMANN, and Alexis PARMENTIER, (2006) "Optimal income taxation and the shape of average tax rates.