Is the Age at First Birth in the Genes? A Study of UK Twins

This study investigates the genetic component in age at first birth (AFB) for twins from United Kingdom. We decompose the observed variance in AFB into that caused by: genes, shared environment, and the unique environment of twins. We first fit continuous structural equation models followed by the innovation to estimate tobit survival models to include censored cases. Our data contain 2 274 monozygotic (42 %) and dizygotic (58 %) female twin pairs born 1919-1968. Results show that up to 40 % of the observed variance in AFB is due to additive genetic differences. In line with previous research, results underscore the relevance of genetic factors in determining fertility outcomes. A historical comparison of cohorts shows that the genetic component in AFB decreases in more recent cohorts. Results emphasize the relevance of environmental conditions for the expression of genetic predisposition (gene-environment interaction).