Unravelling gene-environment interaction and fertility

Jornt Mandemakers, University of Groningen
Felix Tropf, University of Groningen
Nicola Barban, University of Groningen
Melinda Mills, University of Groningen

There has been a massive delay in the age at first birth across Europe since 1970. Explanations for changes in fertility behaviour in demography have almost exclusively relied on socio-environmental factors. The role of genetic endowment and their interaction with the environment (gene-environment interaction (GxE)), has been scarcely investigated. However, an increasing body of literature indicates an interplay between genetics, environment and fertility. Some studies demonstrate the existence of an important genetic component on number of children and age at first child using data from the Danish Twins registers (Kohler et al. 1999; Kohler and Rodgers 2003; Rodgers et al. 2008). Many human traits and behaviours result from both genetic and environmental factors. Genes provide the potential for a trait, but environmental conditions determine whether that potential will be realized. To understand GxE interactions, we must evaluate the estimated heritability of a trait in a particular environment. The extent to which people can modify their fertility behaviour depends on socio-cultural and economic circumstances. To test this hypothesis and to gain further knowledge on what kind of environmental factors are most important for human fertility behaviour we need to undertake heritability analyses of twins in different environments. The aim of this study is extend existing research on this topic by estimating the heritability of fertility behaviour (e.g., age at first birth) in the context of the UK. Specifically, we will compare the heritability in different cohorts, separate in time and space and compare within cohorts of twins born in diverse social backgrounds. The UK is an important case to study since the fertility regime differs from Denmark, it has a less homogeneous and more ethnically diverse population and is a less egalitarian society, likely leading to larger differences in twin environments.