Anthropology and sociology have long been fascinated with incest and the origin of the incest taboo. Incest refers to any illicit sexual act, whereas the incest taboo refers to a “thou shalt not” have sexual relations with offspring within the nuclear family, though in many traditional societies the taboo is extended to a broader kinship network. In the nineteenth century, theorizing about incest often began with the idea of a pre-kinship “horde” followed by stage models on the evolution of the family and kinship. Since the late twentieth century, with a resurgence of interest in incest and the taboo, research models have incorporated theoretical ideas not only from anthropology and sociology but also from biology, sociobiology, evolutionary psychology, and primatology in an effort to develop more precise and robust explanations for why incest occurs and when and why incest taboos evolved to regulate sexual activity within kinship systems.