Birth Order Differences in Education Originate in Post-Natal Environments

Abstract

Siblings share many environments and much of their genetics. Yet, siblings turn out different. Intelligence and education are influenced by birth order, where earlier-born outperform later-born. We investigate whether birth order differences in education are caused by genetic differences. Using data that spans two generations, combining registry, survey, and genotype information, this study is based on the Norwegian Mother, Father and Child Cohort Study (MoBa). We show that there are no genetic differences by birth order as captured by polygenic scores (PGSs) for educational attainment. Furthermore, we show that earlier-born have lower birth weight than later-born, indicating worse in utero environments. Educational outcomes are higher for earlier-born children when we control for PGSs and in utero variables. Finally, we consider environmental influences, such as maternal age, parental educational attainment, and sibling genetic nurture. We show that birth order differences are not genetic in origin, but their environmental cause remains elusive.

First Among Equals? How Birth Order Shapes Child Development

“…Full siblings have the same expected genetic endowments. It is possible that there could still be systematic differences if children's genetics affect subsequent fertility, but empirically this does not appear to be the case, as a number of studies have found no systematic genetic differences between siblings by birth order(Domingue et al, 2015;Muslimova et al, 2020;Isungset et al, 2021).2 For a discussion of the self-report method, its advantages and disadvantages, and a comparison to alternative assessment methods, seePaulhus et al (2007).…”