Immigration has generated backlash and policy instability across democracies. What determines such attitudes? This project examines whether public opinion on immigration in the United States, Germany, Sweden, and South Korea is primarily shaped by fairness perceptions (rules adherence, humanitarian needs, immigrant contributions), cultural identity threats, or economic competition fears. Using experimental surveys, the study will evaluate these factors systematically across cases. Clarifying the dominant drivers of immigration attitudes can help manage immigration-related conflicts and inform policies that are fair, politically sustainable, and consistent with democratic values.