The articles by Allègre et al. (2021) and André & Sireyjol (2021) document in detail, using microsimulations, the redistributive impacts of the familialization of the income tax, and thus contribute to the important debate on this specific system of household income taxation in France. To discuss their results, we first propose to review the history of this specificity, which refers to the question of contributory capacity and its origin in the 1789 Déclaration des Droits de l’Homme et du Citoyen. We question its interpretation through the concepts of decreasing marginal utility and equivalence scale and its scope of application, the income tax or the whole system of taxes and transfers. Finally, we question the unit of evaluation : the ind...