This research proposal seeks to explore the relationship between national identities and public attitudes toward immigration. According to longstanding frameworks in social psychology, strong attachment to an in-group—such as heightened nationalistic feelings—is often associated with negative attitudes toward out-groups. However, empirical evidence on this relationship remains inconsistent. While some studies find that strong national identities correlate with anti-immigration attitudes, the effect varies depending on how nationalism is measured, across geographical regions (i.e. differences between mostly pro-immigration Scottish, and mostly anti-immigration Flemish nationalism), and over time (i.e. Catalan nationalism being mostly pro-immigration in the last decade, and now going through a hardening turn in latest public opinion data). We refine and advance several pre-registered hypotheses concerning the canonical distinction between civic and cultural conceptions of national identity, bottom-up societal changes vs top-down elite narratives, strategic liberal narratives of minority protection by nationalist movements, historical legacies of trade openness and industrialisation in need of migrant workers, and zero-sum relationships between centre-periphery and immigration conflicts. We will rely on survey vignette experiments embedded in large, well-powered and representative surveys (N = 8,000) in Catalonia fielded by the Centre d'Estudis d’Opinio, comparing the effect of our treatments across Catalan, Spanish, and dual identities. Our study provides a theoretically novel take on national identity which challenges its status as a stable product of long-term socialization, and a rigorous empirical approach based on causal estimates.