Social science genetics and fertility:Essays on the Interplay Between Genes, Social Environment and Human Fertility

To what extent do genes influence the age at which you have your first child and the number of children that you have? Does the social environment influence genetic effects on fertility? Do genes lead to spurious associations between life outcomes such as education and age at first birth? The social sciences have been slow to integrate a genetic approach to the study of fertility choice and behaviour, resulting in theories and findings that are largely socially deterministic. This dissertation investigates genetic and environmental influences on human fertility—as well as their interplay—using both twin data and molecular genetic data of more than 31,000 genotyped individuals from six countries.The central finding from this dissertation is that individual differences in fertility behaviour are associated with genetic differences: this implies ongoing natural selection in humans. However, it is also found that genetic variants associated with fertility can differ across countries and time. Taking the historical and geographic environment into account can increase the explanatory power of genes for fertility up to five times. A twin study shows that the association between education and the age at first birth is largely spurious due to family background effects—not genes. This casts doubt on the idea that the educational expansion in the second half of the 20th century is responsible for the strong fertility postponement all over Europe. Overall, the results demonstrate that an integrative research approach of social sciences and genetics generates important insights into human fertility

What explains the heritability of completed fertility? Evidence from two large twin studies.

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

Problems and consequences of aging societies. Theoretical considerations, methodological issues and empirical analyses

Dieser Band versammelt Beiträge zu verschiedenen grundlegenden Problemen, welche nachhaltig alternde Gesellschaften mit sich bringen. Die einzelnen Artikel entstanden im Rahmen des Forschungspraktikums im Fach Bevölkerungswissenschaft an der Universität Bamberg. Die ersten beiden Beiträge beschäftigen sich mit der selbstberichteten Gesundheit. Der erste Beitrag thematisiert die subjektive Gesundheit in Abhängigkeit vom Übergang in den Ruhestand. Im zweiten Beitrag stehen mit der internationalen Vergleichbarkeit subjektiver Gesundheitsmessung methodische Aspekte im Vordergrund. Der dritte Beitrag beschäftigt sich mit geschlechtsspezifischen Aspekten intergenerationaler Hilfeleistungen. Diese aktuellen sozialwissenschaftlichen Fragestellungen werden anhand der Daten der ersten beiden Wellen des Survey of Health, Ageing and Retirement in Europe (SHARE) empirisch untersucht.This volume brings together selected contributions on fundamental challenges of population ageing. All of the articles evolved from an empirical research training course in population studies at the University of Bamberg. The first two contributions deal with self-perceived health measures. The first article analyses the effect of transition into retirement on self-perceived health. The second contribution focuses on methodological issues concerning the comparability of self-perceived health and discusses different strategies to account for cross-national variation in scaling subjective health. The last contribution deals with gender-related aspects of intergenerational transfers. These up-to-date research questions are empirically investigated using the first two waves of the Survey of Health, Ageing and Retirement in Europe (SHARE)

Hidden heritability due to heterogeneity across seven populations

Meta-analyses of genome-wide association studies, which dominate genetic discovery, are based on data from diverse historical time periods and populations. Genetic scores derived from genome-wide association studies explain only a fraction of the heritability estimates obtained from whole-genome studies on single populations, known as the ‘hidden heritability’ puzzle. Using seven sampling populations (n = 35,062), we test whether hidden heritability is attributed to heterogeneity across sampling populations and time, showing that estimates are substantially smaller across populations compared with within populations. We show that the hidden heritability varies substantially: from zero for height to 20% for body mass index, 37% for education, 40% for age at first birth and up to 75% for number of children. Simulations demonstrate that our results are more likely to reflect heterogeneity in phenotypic measurement or gene–environment interactions than genetic heterogeneity. These findings have substantial implications for genetic discovery, suggesting that large homogenous datasets are required for behavioural phenotypes and that gene–environment interaction may be a central challenge for genetic discovery

Genome-wide analysis identifies genetic effects on reproductive success and ongoing natural selection at the FADS locus

: Identifying genetic determinants of reproductive success may highlight mechanisms underlying fertility and identify alleles under present-day selection. Using data in 785,604 individuals of European ancestry, we identified 43 genomic loci associated with either number of children ever born (NEB) or childlessness. These loci span diverse aspects of reproductive biology, including puberty timing, age at first birth, sex hormone regulation, endometriosis and age at menopause. Missense variants in ARHGAP27 were associated with higher NEB but shorter reproductive lifespan, suggesting a trade-off at this locus between reproductive ageing and intensity. Other genes implicated by coding variants include PIK3IP1, ZFP82 and LRP4, and our results suggest a new role for the melanocortin 1 receptor (MC1R) in reproductive biology. As NEB is one component of evolutionary fitness, our identified associations indicate loci under present-day natural selection. Integration with data from historical selection scans highlighted an allele in the FADS1/2 gene locus that has been under selection for thousands of years and remains so today. Collectively, our findings demonstrate that a broad range of biological mechanisms contribute to reproductive success