Social structures such as families emerge as outcomes of behavioural interactions among individuals, and can evolve over time if families with particular types of social structures tend to leave more individuals in subsequent generations. The social behaviour of interacting individuals is typically analysed as a series of multiple dyadic (pair-wise) interactions, rather than a network of interactions among multiple individuals. However, in species where parents feed dependant young, interactions within families nearly always involve more than two individuals simultaneously. Such social networks of interactions at least partly reflect conflicts of interest over the provision of costly parental investment. Consequently, variation in family network structure reflects variation in how conflicts of interest are resolved among family members. Despite its importance in understanding the evolution of emergent properties of social organization such as family life and cooperation, nothing is currently known about how selection acts on the structure of social networks. Here, we show that the social network structure of broods of begging nestling great tits Parus major predicts fitness in families. Although selection at the level of the individual favours large nestlings, selection at the level of the kin-group primarily favours families that resolve conflicts most effectively.Keywords: social networks; sexual conflict; parent-offspring conflict; begging; social evolution 1. INTRODUCTION Social networks analyses have advanced our understanding of the evolution of animal societies [1,2], cooperation [3][4][5], the transmission of disease [6] and human social [7] and socio-economic [8] systems. Taking a networks approach to the study of social behaviour shifts emphasis away from variation in behaviour among individuals per se to how interactions among individuals shape variation [9]. This more realistically reflects the behaviour of individuals as being both the cause and the effect of their social environment [10,11]. However, all previous research has focused on the importance of social position within a network on the fitness prospects of interacting individuals [12,13] rather than the structure of the network of interactions themselves. As a result, very little is known about the relationship between the structure of social networks and fitness in natural populations, despite the importance of such information in understanding the evolutionary and ecological significance of social networks [9,14].