In 1969, Shoup postulated that the presence of interrelated taxes in a tax system would reinforce the system of tax penalty ("self-reinforcing penalty system of taxes"). In this paper, we have tried to formally develop this idea. We find that in order that tax re-enforcement holds, it is necessary that the interrelated taxes are administered by a single tax administration, or in the case that they are administered by different tax administrations, the level of collaboration between them has to be high enough. If that is the case, tax evasion in interrelated taxes might be considered as an alternative explanation of the existing gap between the levels of tax evasion that can be guessed in practice and those much lower predicted by the classical theory of tax evasion (Allingham and Sandmo, 1972; Yitzhaki, 1974). Otherwise, the result expected by Shoup might even reverse. Moreover, as long as collaboration is imperfect, the classical results of the comparative statics might change, since in some cases although global tax compliance increases in front of a variation in a tax parameter, it can decrease in a tax.
