Assortative mating and differential fertility by phenotype and genotype across the 20th century

Abstract

This study asks two related questions about the shifting landscape of marriage and reproduction in US society over the course of the last century with respect to a range of health and behavioral phenotypes and their associated genetic architecture: (i) Has assortment on measured genetic factors influencing reproductive and social fitness traits changed over the course of the 20th century? (ii) Has the genetic covariance between fitness (as measured by total fertility) and other traits changed over time? The answers to these questions inform our understanding of how the genetic landscape of American society has changed over the past century and have implications for population trends. We show that husbands and wives carry similar loadings for genetic factors related to education and height. However, the magnitude of this similarity is modest and has been fairly consistent over the course of the 20th century. This consistency is particularly notable in the case of education, for which phenotypic similarity among spouses has increased in recent years. Likewise, changing patterns of the number of children ever born by phenotype are not matched by shifts in genotype-fertility relationships over time. Taken together, these trends provide no evidence that social sorting is becoming increasingly genetic in nature or that dysgenic dynamics have accelerated.assortative mating | fertility | polygenic scores | cohort trends T he traditional view of evolutionary dynamics in humans was that the history of modern humans was too short for the species to have experienced substantive change in its genetic makeup (1, 2). However, findings from recent population genetics studies suggest the possibility that selective fertility, nonrandom mating, drift, and other violations of the Hardy-Weinberg equilibrium accelerated genetic divergence between modern human populations, particularly since humans began farming and civilization developed (3-6). Extending this logic, rapid economic development and the corresponding demographic transition over the past two centuries may have led to a further shift in the dynamics of reproduction and selection. The present paper uses genetic and phenotypic data from a nationally representative sample of US older adults to test whether the societal changes in the United States during the 20th century were reflected in (i) changes in patterns of genetic assortment in marriage, and (ii) changes in genetic influences on fertility.Understanding trends with respect to specific deviations from random mating and differential fertility is critical to both social and evolutionary scientists. For example, recent research in sociology has suggested that taking a prospective view on social stratification that incorporates differential fertility yields disparate results for estimands such as levels of intergenerational educational mobility (7). Likewise, genetic research on human populations often assumes that mating in a population is random with respect to genotypes (8, 9). Recent empirical evidence suggests otherw...

Mortality selection in a genetic sample and implications for association studies

“…One emerging area of study is variation in genetic effects across birth cohorts. [7][8][9][10][11][12][13] A threat to validity in such studies is mortality selection. We considered implications of mortality selection in one longitudinal social survey, the HRS.…”

“…15,16 Recent studies have examined variation in genetic effects on complex phenotypes across historical contexts indexed by birth cohort. [7][8][9][10][11][12][13] Because most genetic data were collected recently, these studies often observe populations from earlier-born birth cohorts at much older ages, which could introduce bias into estimates of gene-by-cohort interaction if individuals sampled at older ages represent a different population from those sampled at younger ages. Suppose that the genetic effect (G) on some outcome (Y) is a function of birth cohort (B) as indexed by year of birth: ðB 0 Þ T may be biased.…”

“…[7][8][9][10][11][12][13] These studies often interpret variation in genetic effects across birth cohorts as reflecting environmental changes occurring across historical periods. However, because DNA has been collected only recently, members of the earliest-born birth cohorts are relatively long-lived.…”

Mortality Selection in a Genetic Sample and Implications for Association Studies

“…15,16 Recent studies have examined variation in genetic effects on complex phenotypes across historical contexts indexed by birth cohort. [7][8][9][10][11][12][13] Because most genetic data were collected recently, these studies often observe populations from earlier-born birth cohorts at much older ages, which could introduce bias into estimates of gene-by-cohort interaction if individuals sampled at older ages represent a different population from those sampled at younger ages. Suppose that the genetic effect (G) on some outcome (Y) is a function of birth cohort (B) as indexed by year of birth: ðB 0 Þ T may be biased.…”

“…[7][8][9][10][11][12][13] These studies often interpret variation in genetic effects across birth cohorts as reflecting environmental changes occurring across historical periods. However, because DNA has been collected only recently, members of the earliest-born birth cohorts are relatively long-lived.…”