Why do people have children? Why do they have the number of children they do? These questions are of fundamental importance, but we do not yet have satisfactory answers. I propose to bring an interdisciplinary perspective, involving demography, evolutionary biology, anthropology and psychology, to bear on this topic. The aim is to test the hypothesis that intergenerational influences are important determinants of fertility, from fertility intentions to timing of births to number of children. The theoretical rationale derives from a hypothesis emerging from evolutionary biology that humans are cooperative breeders : mothers need help from others to raise children, because human children are too costly for mothers to raise alone. The support of relatives, particularly intergenerational support, is thus vital to women in determining how many children they have. If so, there is a major gap in our understanding of fertility, since such influences have not been systematically studied. This aim will be achieved using rigorous, empirical methods to analyse data from all world regions, and to use novel methods for collecting new data on fertility. The key to this project is its holistic comparative nature. The ultimate goal will be a novel comparative analysis of data from the full gamut of human societies, from small-scale traditional societies through historical populations to contemporary nations surveyed through large-scale surveys. This will allow us to go beyond simply documenting kin influences, and to understand why particular kin matter under which circumstances. Such a comparative approach has not been used before but is vital if we are to fully understand why fertility varies. This will significantly advance understanding of fertility, and promote interdisciplinary research.