The Optimal Design of a Fiscal Union

We study cooperative and non-cooperative fiscal policy in an open economy model where cross-country risk sharing is imperfect and countries face terms of trade externalities. We show that the optimal form of fiscal cooperation, or fiscal union, is defined by one parameter: the Armington elasticity of substitution between goods from different countries. We prove that members of a fiscal union should: (1) harmonize steady state income tax rates when the Armington elasticity is low in order to ameliorate terms of trade externalities; and (2) send fiscal transfers across countries when the Armington elasticity is high in order to improve risk sharing. Our analytical predictions hold both outside of and within currency unions. For standard calibrations, we find that the welfare gain from the optimal fiscal union is as high as 5% of permanent consumption when countries are able to trade safe government bonds, and can approach 20% when countries lose access to international financial markets. We also find that labor mobility significantly improves welfare and alleviates the need for a transfer union entirely.