Income tax in the real world is very different from the income tax in theoretical studies where tax liabilities only depend on income. The fact that there are tax groups which are treated differently, in function of non- income attributes, implies a restatement of the conventional results on progressivity and redistribution of income tax and its assessment in terms of social welfare. Our aim in this paper is to study the different treatment between three tax classes in Spanish personal income tax (IRPF) from its introduction in 1979 to the present day, namely: single persons, married couples with one income earner, and married couples with two income earners. For any year in this period, there is no sequential generalised Lorenz dominance. Thus, adopting a social welfare specification à la Atkinson & Bourguignon, and assuming that for the social decision-maker the only needs relevant factors in the IRPF code are marital status and the number of income earners, we find that the Spanish IRPF is at no time welfare superior to an equal-yield proportional tax applied to all income units regardless of marital status.

Keywords: personal income fax, marital status, social welfare, inequality.