The Biodemography of Fertility: A Review and Future Research Frontiers

Abstract

The social sciences have been reticent to integrate a biodemographic approach to the study of fertility choice and behaviour, resulting in theories and findings that are largely socially-deterministic. The aim of this paper is to first reflect on reasons for this lack of integration, provide a review of previous examinations, take stock of what we have learned until now and propose future research frontiers. We review the early foundations of proximate determinants followed by behavioural genetic (family and twin) studies that isolated the extent of genetic influence on fertility traits. We then discuss research that considers gene and environment interaction and the importance of cohort and country-specific estimates, followed by multivariate models that explore motivational precursors to fertility and education. The next section on molecular genetics reviews fertility-related candidate gene studies and their shortcomings and on-going work on genome wide association studies. Work in evolutionary anthropology and biology is then briefly examined, focusing on evidence for natural selection. Biological and genetic factors are relevant in explaining and predicting fertility traits, with socio-environmental factors and their interaction still key in understanding outcomes. Studying the interplay between genes and the environment, new data sources and integration of new methods will be central to understanding and predicting future fertility trends.

Mega-analysis of 31,396 individuals from 6 countries uncovers strong gene-environment interaction for human fertility

“…Our findings are of interest to scientists within the medical, biological and social sciences alike [1,2,36,37]. Research has successfully identified genetic variants associated with reproductive diseases and traits [37].…”

“…Second, they imply that lower heritability estimates from GWAS studies compared to GREML approaches or family studies might be due to the fact that genetic effects are (to some extent) not universal but context specific. In the model considering gene-environment interaction across population and demographic cohort, we report heritability findings of 0.22 for NEB and 0.19 for AFB (see Fig 2 and S4 Table), which are fourfold higher than across all contexts and approach heritability estimates from family models [2,14]. It is therefore central to understand the cultural and environmental factors that interact with human fertility as well as their origins across (family) environments in order, for example, to define missing heritability or validate the findings from twin studies.…”

“…Demonstrating that genetic effects on fertility outcomes differ across environments, our study substantially contributes to the current knowledge on the genetic architecture of human reproduction. Previous twin studies show for several countries and birth cohorts that fertility outcomes such as NEB and AFB are genetically influenced [1,2]. However, it remained unclear whether the same genes are associated with fertility across environments.…”

“…Different levels of heritability have also been reported across countries [2]. Our multi-matrix GREML approach distinguishes between pairs of individuals who are living in the same or in different populations.…”

“…However, concerning fertility, this requires investigation given that environmental upheavals such as the introduction of the pill or educational expansion have substantially changed fertility behavior in the recent past [10,11]. A second and interrelated point is that studies resorting to molecular genetic data to quantify heritability as the variance in a trait explained by genetic variance result in lower estimates than family studies [12] – and this is true also in fertility research [1,2,7,13,14]. This discrepancy might, amongst other reasons, be a consequence of the interaction between environment and genes.…”