Vertical Versus Horizontal Tax Externalities: An Empirical Test

We study taxation externalities in federations of benevolent governments. Where different hierarchical government levels tax the same base, one can observe two types of externalities: a horizontal externality, working among governments of the same level and leading to tax rates that are too low compared to the social optimum; and a vertical externality, working between different levels of government and leading to suboptimally high tax rates. Building on the model of Keen and Kotsogiannis (2002), we derive a discriminating hypothesis to distinguish vertical and horizontal tax externalities based on observable variables. This test is applied to a panel data set on local taxes in a sample of Swiss municipalities that feature direct-democratic fiscal decision making, so as to maximize the correspondence with the "benevolent" governments of the theory. We find that vertical externalities dominate - they are thus an observed empirical phenomenon as well as a notable extension to the theory of tax competition.