Regionalization, in the form of a dispersion of political power away from national political centers to regional governments, has been a defining feature of European politics since the 1970s. The article focuses on how institutional regionalization changed citizens' attitudes about the division of competences between the central and regional level. It argues that regional institutions and policies exert a socializing effect on citizens' preference in favor of these institutions through a mechanism of adaptive preferences. First, attitudes are studied across cohorts in a single population to test whether cohorts that came of age in a context of more institutional regionalization are more favorable towards regional decision-making than cohorts that came of age in a centralized state. The analyses indeed show evidence for a socializing effect of institutional regionalization. Second, the article shows how regional elites' discourses may moderate the relationship between institutional regionalization and citizens' attitudes about regionalization. We study Belgium as a crucial case. We use five cross-sectional datasets of the Flemish and Walloon populations during the course of increased regionalization in Belgium (1991-2007).