Why do Muslims born and socialized in Western European democracies radicalize towards Islamism? This is currently one of the most pressing questions plaguing scholars and Western European security agencies alike. Whereas existing studies largely focus on Islamist terrorism, this paper investigates the causes of political Islamism more broadly. As transitions from political to violent Islamism are smooth, I argue that effective prevention efforts should start at the stage of radicalization towards the former. Against this background, the present work asks “what are the causes of political Islamism in Western Europe?” I take up two explanations prominent in the literature on causes of conflict: socio-economic and cultural relative deprivation (RD). In contrast to the bulk of purely theoretical scholarship on the topic, I test both explanations in a structured, focussed comparison by collecting and analysing novel empirical data obtained through interviews and participant observation in five German cities. The article concludes that cultural RD – the perception that one´s primary in-group identity is not recognized as equally worthwhile – is more significant to onset radicalization. In sum, the article seeks to solidify our understanding about the causes of political Islamism and to thus thwart future radicalization in its infancy.