The Causal Effect of Sibship Size on Fertility in Adulthood

Sara Cools, BI Norwegian Business School
Rannveig V. Kaldager, Statistics Norway and University of Oslo

Fertility is positively correlated across generations. This correlation could stem from parents’ and children’s shared genetic predispositions and social environment, but it may also reflect a causal effect of sibship size on fertility in adulthood. Using the sex composition of parents’ two first children as an instrumental variable, we estimate the causal effect of sibship size on adult fertility in population data from Norwegian administrative registers. Our study sample is all first- and second-borns to Norwegian couples who had their two oldest children during the 1960s. An additional sibling has a positive effect on male fertility, shifting some men into fatherhood. For women, a negative quantum effect emerges, driven by a preference for two rather than three children among women from three-child families. An additional sibling may cause women to update their beliefs about the disadvantages of having a large family, leading to a preference for smaller families.