Are people’s decisions about when they have their first child influenced by the people around them? And if so, how strong is this influence? This idea, that individuals’ fertility behavior is not independent of each other, increasingly sparks demographers’ interest. Concerning the first question, pioneering qualitative research from Germany identified the interaction partners parents, siblings, friends, colleagues, and extended kin as being influential through the mechanisms of social learning, social pressure, social support and emotional contagion. The first three chapters of this dissertation statistically test for social interaction effects emanating from parents, siblings, and colleagues using large-scale survey and administrative data. Going beyond mere identification, this cumulative dissertation provides the first estimate for the population-level impact of social interaction effects on fertility. Based on a microsimulation, it converts the three results counterfactually into one measure that researchers, policy-makers, and the wider public may easily understand: birth counts. The main finding is that without the influence of these three interaction partners, around 75,000 fewer first children would have been born in a given year (i.e., 22.5% of all first-borns). This decrease in first children born translates into a decrease of the TFR by 14.6%. Furthermore, ranking the three interaction partners, the population-level impact on first births is strongest for parents’ influence (around 50,000 first children). Colleagues’ influence is less than half of the parents’ impact (around 22,500) and that of siblings is almost negligible at the population level (around 2,500). These results highlight that social interaction affects fertility outcomes very strongly.
