Does the share of religious ingroup members affect how important religion is to adolescents? Applying Optimal Distinctiveness Theory to four European countries

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.