Education and Cognitive Ability as Direct, Mediating, or Spurious Influences on Female Age at First Birth: Behavior Genetic Models Fit to Danish Twin Data

Abstract

The authors study education and cognitive ability as predictors of female age at first birth (AFB), using monozygotic and dizygotic female twin pairs from the Middle-Aged Danish Twin survey. Using mediated regression, they replicate findings linking education (and not cognitive ability) to AFB. But in a behavior genetic model, both relationships are absorbed within a latent variable measuring the shared family environment. Two interpretations are relevant. First, variance in AFB emerges from differences between families, not differences between sisters within the same family. Second, even in a natural laboratory sensitive to genetic variance in female fertility-during demographic transition-the variance in AFB was non-genetic, located instead within the shared environment.Biological thinking and genetic models have not often been a part of the sociologist's toolbox in the past. But this is changing (e.g., Udry 1995;Conley and Bennett 2000;Shanahan, Hofer, and Shanahan 2003). 2 Wilson (1998) introduced the concept of consilience, in which methods 1Direct correspondence to Joseph Lee Rodgers, Department of Psychology, 455 West Lindsey Street, Room 809, University of Oklahoma, Norman, Oklahoma 73019. jrodgers@ou.edu. 2 Similar concerns date back further in some of the other social sciences. In economics, e.g., the use of twin data to control for unobserved genetic and other aspects of family background and, in some cases, to present heritability decompositions of phenotypes goes back at least to Behrman and Taubman (1976) and Behrman et al. (1980), with a resurgence a little over a decade ago that focused on controlling for measurement error and using monozygotic (MZ) and dizygotic (DZ) twins together to estimate intrahousehold allocation responses to individual-specific genetic endowments (Ashenfelter and Krueger 1994;Behrman, Rosenzweig, and Taubman 1994). This work in economics overlaps with the interests of sociologists; it has focused, e.g., on estimating the impact of schooling on wages and on mate selection, the impact of maternal schooling on child schooling, and the impact of fertility on happiness (Behrman and Rosenzweig 2002; Kohler, Behrman, and Skytthe 2005 and theories from different disciplines merge into metadisciplines, fringe disciplines, or subdisciplines. Wilson noted an asymmetry between the social and medical sciences, suggesting that the integration of social science methods into the medical sciences has been relatively smooth, whereas medical and biological thinking (and especially genetic reasoning) has not been as easily integrated into many areas of social science, including sociology. The emerging subdiscipline of biodemography (e.g., Vaupel et al. 1998;Rodgers and Kohler 2003;Wachter and Bulatao 2003) illustrates that this asymmetry is beginning to abate, though it bears noting that scholars from outside sociology are providing much of the impetus for this type of consilience.Social science research is filled with apparently causal relationships that, when subjected to modern sc...

Casting doubt on the causal link between intelligence and age at first intercourse: A cross-generational sibling comparison design using the NLSY

“…First, Rodgers et al (2008) used Danish twin data, and found that the link from education/cognitive ability to maternal age at first birth (AFB) was entirely accounted for by between-family variance: "variance in AFB emerges from [IQ and education] differences between families, not differences between sisters within the same family" (Rodgers et al, 2008, p. 202). We have exactly the same type of result in the current study.…”

The Biodemography of Fertility: A Review and Future Research Frontiers

“…1). However, the AFB turns out to be a challenge to study, since other studies in the U.S. (Neiss et al 2002) and Denmark (Rodgers et al 2008) show no significant effect on the AFB of women.…”

“…Opposed to the commonplace practice of including right-censored cases in demography and sociology (e.g., Mills 2011), it is uncommon to deal with censored information in genetic studies and childless women are generally excluded from the sample Neiss et al 2002;Nisén et al 2013;Rodgers et al 2008). Using data from the TwinsUK, Tropf et al (2015a) empirically examined whether the inclusion of censored information in a Tobit model affects heritability estimates compared to the classic models.…”

How Education Drives Demography and Knowledge Informs Projections

“…Still, studies of monozygotic twins represent an important way of identifying the effects of education after controlling for genetic influences. Several twin studies have found a strong positive effect of education on, for instance, mental health (Haworth et al 2008;Rodgers et al 2008). …”