Social science genetics and fertility : Essays on the interplay between genes, social environment and human fertility

To what extent do genes influence the age at which you have your first child and the number of children that you have? Does the social environment influence genetic effects on fertility? Do genes lead to spurious associations between life outcomes such as education and age at first birth? The social sciences have been slow to integrate a genetic approach to the study of fertility choice and behaviour, resulting in theories and findings that are largely socially deterministic. This dissertation investigates genetic and environmental influences on human fertility—as well as their interplay—using both twin data and molecular genetic data of more than 31,000 genotyped individuals from six countries. The central finding from this dissertation is that individual differences in fertility behaviour are associated with genetic differences: this implies ongoing natural selection in humans. However, it is also found that genetic variants associated with fertility can differ across countries and time. Taking the historical and geographic environment into account can increase the explanatory power of genes for fertility up to five times. A twin study shows that the association between education and the age at first birth is largely spurious due to family background effects—not genes. This casts doubt on the idea that the educational expansion in the second half of the 20th century is responsible for the strong fertility postponement all over Europe. Overall, the results demonstrate that an integrative research approach of social sciences and genetics generates important insights into human fertility.