The causal effect of another sibling on own fertility – an estimation of intergenerational fertility correlations by looking at siblings of twins

Martin Kolk, Stockholm University

In all developed societies family size is correlated across generations. The reasons for these intergenerational correlations are however poorly understood. The current study attempts to differentiate between the causal role of another unexpected child in the parent generation, from the effect of other characteristics that are shared between parent and children, for explaining intergenerational fertility correlations. This is done through an instrumental variable approach, using a twin birth as a source of exogenous variation in family size in the parent generation. Data is drawn from the complete Swedish population using administrative register data on more than 2,000,000 parent-child links. Findings show that little or none of observed fertility correlations can be attributed to the causal affect of growing up with another sibling as such, instead shared characteristics between parents and children such as fertility preferences, ethnicity, religion or socioeconomic background appears to explain observed fertility correlations.