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

Popul Stud (Camb). 2015;69(2):129-45. doi: 10.1080/00324728.2015.1056823. Epub 2015 Aug 3.


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.

Keywords: UK; age at first birth; fertility; gene–environment interaction; heredity; heritability; reproductive age; twins.

Publication types

  • Twin Study

MeSH terms

  • Adult
  • Birth Order
  • Female
  • Fertility / genetics*
  • Gene-Environment Interaction*
  • Humans
  • Maternal Age*
  • Middle Aged
  • Parturition / genetics*
  • Pregnancy
  • Pregnancy, Twin / genetics
  • Registries
  • Twins, Dizygotic / genetics*
  • Twins, Monozygotic / genetics*
  • United Kingdom