Is Fertility Behavior in Our Genes? Findings from a Danish Twin Study
Hans-Peter Kohler, Joseph L. Rodgers and Kaare Christensen
Abstract: In this paper we investigate the fertility of Danish twins born during the periods 1870--1910 and 1954--64 in order to pursue two central questions for understanding human reproduction: Do genetic dispositions influence fertility and fertility related behavior? Does the relevance of `nature versus nurture' shift over time or with demographic regimes? We find that genetic influences on fertility exist, but that their relative magnitude and pattern is contingent on gender and on the socio-economic environment experienced by cohorts. For females born from 1880--90 and after 1955, about 30--50% of the variance in fertility is due to genetic influences; these influences are substantially smaller for earlier and for interim cohorts. Male fertility is generally subject to smaller genetic and larger shared-environment effects than female fertility. Because genetic effects are most prevalent in situations with consciously controlled fertility and relatively egalitarian opportunities, we propose that the genetic dispositions affect primarily fertility behavior and motivations for having children. Analyses of fertility motivations, measured by age of first attempt to have a child, support this interpretation.