Social anthropological studies into African loyalty structures have shown that multiple social group membership has a positive effect on intergroup conflict. It was here that social psychologists sought the key to a successful strategy for the reduction, even the elimination, of intergroup discrimination and interethnic conflict. However, as this book shows, this presumed positive effect of multiple group membership is based on limited and even erroneous theoretical assumptions. Empirical studies on the effect of crossed categorization focus mainly on two distinctive patterns of intergroup discrimination. The author presents a theoretical classification that distinguishes 146 models of crossed categorization and does more justice to the reality of the diversity of patterns of intergroup discrimination; in doing so, it demonstrates the limitations of previous research. The results of two crossed categorization studies conducted in the Netherlands are presented in this book. These studies show that crossed categorization is not the key to the reduction, let alone the elimination, of intergroup discrimination it was once thought to be.