Do Contextual Changes over Cohorts Affect the Relative Importance of Genes in Childbearing Behavior?

Agnes Fauske , University of Oslo
Torkild Hovde Lyngstad , University of Oslo

We investigate how well genetics and environmental factors can explain variation in age at first birth and completed fertility, and how their relative importance change over cohorts. We relate relative explanatory strengths of genetic and environmental factors to changes in Norwegian family policies. In the empirical analysis, we estimate ACE models on rolling sets of cohorts over the period 1967-1977 using data from the Norwegian twin registry linked with Norwegian population register data to construct a sample of 4,009 twin pairs. This way, we use genetically informed data and methods as a measurement device for understanding social change in fertility patterns, and can gauge whether and how family policies moderate the importance of genetic factors for fertility outcomes. We compare the pattern changes to expectations formed from the thesis of a normative change towards individualism, and that this will lead to a greater impact of genetic influences on fertility behavior. Our results show that there is no overall pattern in the changes in genetic importance on age at first birth. When it comes to completed fertility, however, the importance of genes increases over the cohorts, while the importance of shared environment decreases accordingly. These findings are partly consistent with the idea of a larger role for individual preferences in fertility decision-making.

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 Presented in Session 77. Timing of Fertility