Siblings and all the lateral relationships that follow from them are clearly important and their interaction is widely observed, particularly in creative literature. Yet in the social, psychological and political sciences, there is no theoretical paradigm through which we might understand them. In the Western world our thought is completely dominated by a vertical model, by patterns of descent or ascent: mother or father to child, or child to parent. Yet our ideals are ‘liberty, equality and fraternity’ or the ‘sisterhood’ of feminism; our ethnic wars are the violence of ‘fratricide’. When we grow up, siblings feature prominently in sex, violence and the construction of gender differences but they are absent from our theories. This book examines the reasons for this omission and begins the search for a new paradigm based on siblings and lateral relationships. This book will be essential reading for those studying sociology, psychoanalysis and gender studies. It will also appeal to a wide general readership.