One of the distinctive features of humans is their unique sociality. Humans live in organized societies that are characterized by a high level of interdependence of group members in various aspects of life, ranging from the economic phenomenon of labour division to providing emotional support to others. Under these circumstances, the capacity to track social connections within and between groups has great adaptive value in managing everyday life. We may understand the importance and adaptive value of tracking the scope of culturally shared knowledge if we consider the importance of cultural norms in guiding behaviour. To become a competent member of their cultural group one must be able to conform to the group's specific behavioural norms and to accumulate culturally shared knowledge. Acquiring this knowledge is essential for successful social interactions. In contrast to current dominant explanatory theories emphasizing that social category formation is simply rooted in humans’ need to belong and affiliate with a group, the aim of this e-book is to provide evidence that, in addition to its affiliative role, children form social categories for epistemic purposes. We show that children use specific cues, like kinship, patterns of resource allocation and consensus to understand group cohesion (Section 1). Once children figured out who is in-group and who is out-group, they show a significant in-group bias in attention, acting and learning (Section 2). Yet, this in-group bias can be attenuated by induced synchronous behavior (Section 3).