For almost a decade now, there has been a debate among scholars of regional and federal studies about how to explain policy evolution after devolution. Surprisingly, this literature has attached little importance to the policy impact of sub-state nationalism. This article assesses existent institutionalist and societal hypotheses in the case of immigrant integration policy divergence in Belgium after devolution. This empirical test shows that although several of these hypotheses yield valuable insights in explaining integration policy divergence in Belgium, they have difficulties in accounting for a striking feature of this policy divergence, i.e. the different interventionism regarding the cultural dimension of the integration process. This article argues that sub-state nationalism, and in particular the differing degrees of regional government involvement in sub-state nation building, provides explanatory insight into how policy frames diverge.