This research program bridges several disciplines in the humanities and (bio)social sciences.
It combines theories from family history and historical kinship studies (family systems, kinship regimes), adaptive ecology and biology (cooperative breeding, dispersal of kin, resource competition), sociology (social learning, socialization, intergenerational transmission), and economics (supply and demand theories of fertility decline) in order to build a novel theoretical framework that better explains spatial and cultural disparities in long-term population change. Furthermore, the program's exploration of the differential role of siblings and of husband-wife relations in influencing fertility patterns is new. If family and kin ties have been taken into account in earlier research on fertility, the focus has been mainly on parent-child relations. Innovative is also the long-term focus including both the first demographic transition (1870-1930) and the second demographic transition (1970-present). The combination of this long-term perspective with a wide European comparative approach makes the research program truly unique. In sum, although often suggested in the literature, theories of family systems have never been empirically tested on their relevance for understanding reproductive change on such a large scale.