European youth attend classrooms that are religiously diverse, with the importance of religion differing between ethno-religious groups. While religion no longer matters much to many native-origin Christian youth, it is important to many of their immigrant-origin Christian and, especially, Muslim peers. Considering religion as a source of adolescents’ social identity, we examine how religious classroom composition relates to the importance adolescents attach to religion. Optimal Distinctiveness Theory suggests a curvilinear relation, because a group has to be large enough to satisfy the need of belonging but small enough to satisfy the need for differentiation. Using large-scale survey data for 15-year old adolescents from four European countries, we find that this inverted U-shaped relation holds for immigrant-origin Muslim but not for native- and immigrant-origin Christian youth. Instead, for Christian youth religion was more important in classrooms with higher shares of Muslim classmates, thus lending credence to arguments derived from threat theory.
