Cohabitation, marriage, and first birth:: The interrelationship of family formation events in Spain

In this paper, we investigate (1) the mutual causal relationship between first union formation and first childbirth, and (2) the existence of constant unmeasured determinants shared by these two events. We argue that these determinants mainly consist of value orientations that are heterogeneous in the population. We apply event-history techniques to retrospective survey data on Spain, allowing for unobserved heterogeneity components which simultaneously affect the two processes. Our findings confirm the existence of a strong selection effect, which influences both union formation and first birth. When controlling for these shared factors, we find that the risk of conception increases immediately at marriage, and it continues to be high during the following four years. Entry into cohabitation, in contrast, produces a much smaller increase in the relative risk of conception. The effect of conception on union formation is particularly strong during pregnancy, but it declines sharply after delivery.