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

While fertility is positively correlated across generations, causal drivers – if any – of this relationship are poorly understood. The correlation could stem from the fact that parents and children share genetic predispositions and social environment, but it may also reflect a causal effect of sibship size on fertility in adulthood. Access to resources as well as changes in fertility preferences and beliefs about the consequences of childbearing are all possible mediators of a causal effect. Using the sex composition of the two first-born children as an instrumental variable, we estimate the causal effect of sibship size on adult fertility. Estimations are done on high-quality data from Norwegian administrative registers. Our study sample is all first- or second-borns during the 1960s in Norwegian families with at least two children (approximately 126 000 men and 119 000 women). 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. Having 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.