The development of domain-specific knowledge within foundational theories, i.e., intuitive physics, biology, or psychology, has already been examined in cognitive developmental psychology for several years. It is of major importance in this research to find out if children can distinguish between the causal mechanisms appropriate to the different domains (e.g., intentional vs. mechanical causation). The present work focused on the distinction between naive biology and sociology. Distinguishing between these domains children should be aware of the differences between biological relationships (relatives), And purely social relationships (friends). In two studies, 2,5 four-year-old and 25 six-year-old preschoolers worked on little tasks in which they had to distinguish between people in their everyday life (parents, siblings, grandparents, friends, nursery school teachers, doctors) concerning the type of relationship. Do preschoolers know that relatives can only be shared with a defined group of people, but friends can not (study 1)? Do they know that sibling relationships begin and end in a different way compared to friendships (study 2)? The data in these studies reveal a significant developmental progress during the preschool years. While the younger children could not yet distinguish between relatives and friends the older six year olds proved to be sensitive for the special features of family-relationships. The results suggest that only at the end of the preschool years domain-specific biological causality is established in children's thinking.