International migration can significantly increase income per capita in Europe. We estimate that at the given wage and productivity gap between Western and Eastern Europe, migration of 3% of the Eastern population to the West could increase total EU GDP by up to 0.5%. Yet on 1 May 2004, 14 EU countries out of 15 adopted transitional arrangements vis-à-vis the new Member States and national migration restrictions vis-à-vis third country nationals are getting stricter and stricter. In this paper we offer two explanations for this paradox and document their empirical relevance in the case of the EU enlargement. The first explanation is that immigration to rigid labour markets involves a number of negative externalities on the native population. The second explanation is that there are important cross-country spillovers in the effects of migration policies, inducing a race-to-the top in border restrictions with high costs in terms of foregone European output. In light of our results, we discuss, in the final section, the key features of a desirable migration policy to be coordinated at the EU level.