[Twitter; OSF] Birth order is associated with educational outcomes, with earlier-born siblings outperforming later-borns.
Using genetic data and data on education from a large Norwegian cohort, we find that these differences are not due to genetic differences or in utero differences. Later-born children even have higher birth weight, which is positively associated with education. When we take indicators of genetic and in utero differences into account, earlier-born siblings still do better in school.
This establishes that birth order differences result from factors after birth, consistent with explanations that focus on home environments.
Siblings share many environments and much of their genetics. Yet, siblings turn out different. Intelligence and education are influenced by birth order, with earlier-born siblings outperforming later-borns.
We investigate whether birth order differences in education are caused by biological differences present at birth, that is, genetic differences or in utero differences. Using family data that spans 2 generations, combining population 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. Earlier-born have lower birth weight than later-born, indicating worse uterine environments. Educational outcomes are still higher for earlier-born children when we adjust for PGSs and in utero variables, indicating that birth order differences arise postnatally. Finally, we consider potential environmental influences, such as differences according to maternal age, parental educational attainment, and sibling genetic nurture.
We show that birth order differences are not biological in origin, but pinning down their specific causes remains elusive.
[Keywords: birth order, education, genetics, birth weight]
…Consistent with other studies, we find that firstborn siblings have better educational outcomes than their later-born siblings. Figure 1 shows the magnitude of these associations. The top panel shows lower test scores for each successive birth order for all family sizes 2–5 siblings. Most of these differences are present in the first test scores we observe (5th grade), but the gaps do grow modestly from the last scores in our data (9th grade; see Figure S2). The bottom panel shows similar patterns for educational attainment in the adult population. Figure 1 also shows that birth order differences increase when adjusted for maternal age.
(A) and (B) Results from family-fixed effects linear regression models run separately by sibship size, with controls for sex and maternal age and cluster-robust standard errors. Firstborns serve as the reference category. All point estimates presented with 95% CI.
In (A) children generation in the population (n = 301,795), where the outcome is the mean of national test score standardized within test, year of test, and birth cohort. (B) Adult generation in the population (n = 2,067,878), where the outcome is educational attainment at age 30, standardized within birth cohort.
…Interrogating Environmental Origins…Third, we examined the influence of other family member’s PGSs on child achievement, net of the child’s own score. Recent findings of “genetic nurture” have documented relationships between nontransmitted parental alleles and child attainment and achievement47–49. Given that mothers are typically more involved in childrearing than fathers—not infrequently to a substantial degree—the finding of some past research that maternal nontransmitted alleles matter more for attainments than paternal nontransmitted alleles would not be surprising50.
We did not find this pattern in our data (see Table S6).
Also, when we include the PGS for the siblings, the sibling score accounts for more than half of the magnitude of the difference in achievement that had been attributed to parental PGSs. Net of one another, the relationship between sibling PGS and achievement is nearly a quarter as large as that of a child’s own score (Table S5, Supplementary Material). These results point to the importance of considering that genetic nurture may reflect sibling influence to a greater extent than has been previously appreciated.
…As for sibling interactions, these have been most prominently raised in variations of “confluence theory”, which proposes that siblings generally have negative effects on one another’s cognitive development13, 14, 58. Because firstborns have less exposure to being reared with siblings, and may also benefit from the opportunity to teach younger siblings, the negative influence of siblings is proposed to be least for firstborns.
Our finding that the apparent influence of parental alleles shrinks markedly when sibling PGSs are included, and that sibling PGSs are substantially associated with child outcomes net of a child’s own score, indicates that some of what has hitherto been called “genetic nurture” could in fact be “sibling genetic nurture”. Recent enthusiasm for using sibling-based GWAS to purge GWAS studies of indirect genetic effects assumes that these indirect genetic effects are primarily due to parents59. Because the approach does not account for indirect effects via siblings, the possibility that sibling genetic effects on achievement may be larger than commonly supposed60 is important to resolve for evaluating the success of sibling-based GWAS.