Abstract
Significant shifts in fertility have taken place since the 1950s, alongside great societal changes. Some argue that the changes in fertility are due to weaker norms in society, while others emphasize the transition from the traditional to a gender-equal family. Within the sociogenetic literature, genes are suggested to be of increased importance in periods of weak social norms, or in periods of societal change. This study explores how the genetic influence on fertility behavior changed throughout the Second demographic transition, focusing on Norway which is an interesting case due to its early on progressive welfare system and relatively high fertility rates up until recently. Analyzing cohorts from 1930 to 1977, the study reveals genetic contributions to age at first birth and number of children, with nuanced differences between genders and cohorts. Contrary to existing studies, there is no consistent upward trend in the importance of genes, nor a distinct rise during the shift from traditional to gender-equal family dynamics. The findings highlight the complexity in gene-environment interactions, and the gender differences suggest that differing expectations and opportunities are involved. Thus, I suggest including a broader set of theories of fertility variation and change when theorizing how gene-environment interactions play out across societal changes and across sex.