What Explains the Heritability of Completed Fertility? Evidence from Two Large Twin Studies

Abstract

In modern societies, individual differences in completed fertility are linked with genotypic differences between individuals. Explaining the heritability of completed fertility has been inconclusive, with alternative explanations centering on family formation timing, pursuit of education, or other psychological traits. We use the twin subsample from the Midlife Development in the United States study and the TwinsUK study to examine these issues. In total, 2606 adult twin pairs reported on their completed fertility, age at first birth and marriage, level of education, Big Five personality traits, and cognitive ability. Quantitative genetic Cholesky models were used to partition the variance in completed fertility into genetic and environmental variance that is shared with other phenotypes and residual variance. Genetic influences on completed fertility are strongly related to family formation timing and less strongly, but significantly, with psychological traits. Multivariate models indicate that family formation, demographic, and psychological phenotypes leave no residual genetic variance in completed fertility in either dataset. Results are largely consistent across U.S. and U.K. sociocultural contexts.

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Notes

  1. 1.

    We attempted to incorporate ADE models in subsequent analyses, but this resulted in a severe increase in the standard errors for personality-fertility associations. Comparing AE to ADE models, a similar amount of genetically-linked covariation was found for each personality dimension, but in the ADE model, none of the pathways were statistically different from zero. In the AE models we present below, several personality-fertility associations are statistically significant. We primarily interpret this as resulting from low power to differentiate the A pathway from the D pathway..

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Acknowledgments

This research is funded by the European Research Council Consolidator Grant SOCIOGENOME (615603, www.sociogenome.com) and an Economic and Social Research Council UK, National Centre for Research Methods SOCGEN Grant (www.ncrm.ac.uk/research/SOCGEN).

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Correspondence to Daniel A. Briley.

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Daniel A. Briley, Felix C. Tropf, and Melinda C. Mills declare they have no conflict of interest.

Human and Animal Rights and Informed Consent

The MIDUS was approved by the institutional review boards of the University of Wisconsin and Harvard Medical School. The TwinsUK study was approved by the King’s College London Ethics Committee. All participants provided informed consent before taking part in the study.

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Briley, D.A., Tropf, F.C. & Mills, M.C. What Explains the Heritability of Completed Fertility? Evidence from Two Large Twin Studies. Behav Genet 47, 36–51 (2017). https://doi.org/10.1007/s10519-016-9805-3

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Keywords

  • Fertility
  • Family formation
  • Behavior genetics
  • Personality
  • Cognitive ability