What makes personal income taxes progressive? The case of Belgium

In this paper we investigate the progressivity impact of various components of the Belgian personal income tax system, before and after a major reform of this system. The reform reduced the top tax rates, broadened the tax base and increased tax credits. We show that, contrary to the opinion, commonly expressed in public debates, the reform did not reduce aggregate liability progression of the system and that the rate structure is relatively unimportant in explaining progressivity.