Genetic influence on age at first birth of female twins born in the UK, 1919–68

Abstract

Using a sample of monozygotic (945, 42 per cent) and dizygotic (1,329, 58 per cent) twin pairs born 1919–68 in the UK, we applied innovative tobit models to investigate genetic and environmental influences on age at first birth (AFB). We found that a substantial part (40 per cent) of the variation in AFB is caused by latent family characteristics. Genetic dispositions (26 per cent) play a more important role than the shared environment of siblings (14 per cent), with the non-shared environment/measurement error having the strongest influence (60 per cent). Like previous studies, this study reveals marked changes in estimates over time, and supports the idea that environmental constraints (war or economic crisis) suppress and normative freedom (sexual revolution) promotes the activation of genetic predispositions that affect fertility. We show that the exclusion of censored information (i.e., on the childless) by previous studies biased their results.

Supplementary material for this article is available at: http://dx.doi.org/10.1080/00324728.2015.1056823

Keywords:

• age at first birth
• fertility
• UK
• heritability
• heredity
• reproductive age
• gene–environment interaction
• twins

Notes

1 Felix Christian Tropf is at the Department of Sociology, University of Groningen/ICS, email: [email protected]. Nicola Barban and Melinda Mills are at the Department of Sociology, and Nuffield College, University of Oxford. Harold Snieder is at the Department of Epidemiology, University of Groningen and the University Medical Center, Groningen. Jornt J. Mandemakers is at the University of Groningen/ICS and the University of Wageningen.

2 The research leading to these results was funded by the Dutch Science Foundation (VIDI Innovation Grant 452-10-012 to M. Mills) and by the European Research Council Consolidator Grant SOCIOGENOME (to M. Mills, ERC-2013-CoG-615603). The authors gratefully acknowledge Tomas Sobotka for information and advice on age at first birth data for the UK. The TwinsUK study was funded by the Wellcome Trust and the European Community’s Seventh Framework Programme (FP7/2007-2013). The study also received support from the National Institute for Health Research (NIHR)-funded BioResource, Clinical Research Facility and Biomedical Research Centre based at Guy’s and St Thomas’ NHS Foundation Trust in partnership with King’s College London. They also thank the journal’s anonymous reviewers for their helpful comments.