What Explains the Heritability of Completed Fertility? Evidence from a Large U.S. Twin Study

Daniel A. Briley, University of Texas at Austin

In modern societies, individual differences in completed fertility are linked with genotypic differences between individuals. Explaining the heritability of completed fertility has been somewhat inconclusive, with alternative explanations centering on family formation timing, pursuit of education, beliefs about family importance, or other psychological traits. In this paper, I use the twin subsample from the Midlife Development in the United States study to examine these issues. In total, 933 adult twin pairs reported on their completed fertility, age at first birth, age at first marriage, level of education, family formation beliefs, Big Five personality traits, and cognitive ability. I use a quantitative genetic Cholesky decomposition to partition the variance in completed fertility into genetic and environmental variance that is shared with the predictor variables and unique residual variance. I find that genetic influences on completed fertility are strongly related to family formation timing and less strongly, but significantly, with psychological traits.