Siblings often turn out quite different from one another, despite sharing much of their genetics and childhood environment.
One of the most robust findings in this realm is that earlier-born siblings tend to outperform later-borns in measures of intelligence and educational achievement.
But what causes these birth order differences? Is it nature or nurture?
A new study using genetic data from a large Norwegian cohort sheds important light on this question.
Key Facts:
- In a sample of over 300,000 Norwegian siblings, firstborns scored higher on national standardized tests compared to later-borns. This gap was present by 5th grade and grew modestly through 9th grade.
- Similar patterns were found when looking at completed education level at age 30 for over 2 million Norwegian adults – earlier-borns attained more years of schooling.
- No differences were found in genetic scores predicting educational attainment between siblings of different birth orders.
- Later-borns had higher birth weights than firstborns, suggesting they had better prenatal environments.
- Accounting for genetic scores and prenatal factors like birth weight did not explain away the birth order differences in achievement. The gap remained after adjusting for these biological factors.
- The findings rule out genetic and prenatal explanations for birth order differences and point to postnatal environmental factors as the likely cause. The mechanisms remain unclear but may involve family dynamics like parental resources and sibling interactions.
Source: PNAS Nexus 2022
The Firstborn Advantage: IQ Points & Education
Numerous studies over the past century have shown a slight but consistent educational edge for firstborn children compared to their later-born siblings.
The effects start small in early childhood and grow over time, leading to measurable gaps in achievement by adolescence and attainment by adulthood.
For example, a recent meta-analysis found a 2-4 IQ point advantage for firstborns over secondborns.
Other research shows firstborns attain about a quarter year more education on average.
While small, these differences matter – one study found that just a one month increase in early childhood education leads to 7% higher wages down the road.
The consistency of the firstborn advantage across cultures and eras raises the question of what causes it.
Is it nature – something innate or biological that differs between siblings? Or nurture – postnatal experiences within families that vary by birth order?
Norway Study: Disentangling Genes From Environment
To pin down the mechanisms, researchers need data that includes both genetic information and environmental measures on large samples of siblings.
Norway provides an ideal setting because its extensive population registries allow linking together data on genetics, family structure, school performance, and educational attainment.
The current study utilized data on over 300,000 Norwegian siblings spanning two generations.
For the younger “child” generation, standardized national test scores in 5th, 8th and 9th grade were analyzed.
For the older “adult” generation, the outcome was years of completed education by age 30.
In both generations, earlier-borns scored higher on the achievement measures compared to their later-born siblings, replicating the oft-observed firstborn advantage.
But were genetic differences contributing?
Genes & Polygenic Score: No Difference Between Sibling Birth Orders
The dataset included genetic data that allowed the researchers to calculate a polygenic score for each person predicting their genetic propensity for educational attainment.
This score aggregates across millions of genetic variants to estimate someone’s genetic endowment for education based on genome-wide association studies.
Within families, there was no difference in polygenic scores between siblings of different birth orders – firstborns did not differ genetically from later-borns.
This makes genetic explanations for birth order differences unlikely.
Prenatal Environment: A Later-Born Advantage
Another possibility is that prenatal biological factors might differ systematically between siblings in a way that disadvantages later-borns.
For example, conditions in the womb tend to worsen with maternal age, potentially hampering cognitive development.
However, the study found the opposite pattern – later-borns had higher birth weights than firstborns, which generally indicates a better prenatal environment.
The researchers also accounted for maternal age and gestational length, and found no clear biological disadvantage for later-borns.
If anything, these prenatal factors might be expected to give later-borns an edge, not firstborns.
Yet the firstborn advantage persisted even after controlling for genetic scores and prenatal measures.
Postnatal Environment: The Likely Key
By ruling out genetic and biological explanations, the results underscore that firstborn differences likely originate from postnatal environmental causes – something about being raised with older vs. younger siblings.
Exactly how this works remains unclear, but leading hypotheses focus on family dynamics like differential parental resources and the effects of sibling interactions.
Birth order differences were already present in early childhood and didn’t grow markedly from 5th to 9th grade.
This suggests that whatever family environmental factors are at play exert their influence predominantly during the first decade of life.
The findings highlight that even seemingly biological human traits like intelligence are profoundly shaped by social context.
The lifelong effects of birth order point to early childhood as a critical period for cultivating cognitive abilities through high-quality parenting and education.
Parental Resource Dilution: Attention & Investment Decline
One of the most common explanations for birth order differences involves parental time and resources.
As parents have more children, their energy, attention and investment gets divided, diluting the resources available to later-borns.
Firstborns enjoy a period of being an only child and having parents’ undivided focus.
During this time, parents may provide more intellectual stimulation and be more attentive to early learning.
Having to share parental attention and resources may impair cognitive development in later-borns.
Consistent with this view, the study found that birth order differences declined as the spacing between siblings increased.
With more years between children, parental resources have more time to “replenish” before dividing again.
Nonetheless, the parental resource theory has trouble explaining why maternal and paternal genetics weren’t associated with differences between siblings.
If parental investment is key, maternal effects might be expected to be stronger, since mothers tend to spend more time engaged in child rearing.
Sibling Interactions: Learning With & From
Beyond differential parental inputs, siblings themselves directly influence each other’s development through their interactions.
Confluence theory proposes that siblings generally have detrimental effects on each other’s cognitive growth.
Having to share parents’ attention and being exposed to younger siblings’ less advanced intellectual climate may impede learning.
In contrast, studies show firstborns gain verbal skills through extensively speaking to their younger siblings.
Intriguingly, the study found that after accounting for siblings’ genetic propensities, parental genetic associations with birth order differences declined markedly.
In fact, siblings’ own genetic scores were more predictive of their achievement than their parents’ genetics.
This highlights the possibility that some effects previously attributed to parents’ genetics were actually reflecting genetic differences between siblings.
It points to sibling influences as potentially more impactful on birth order differences than commonly realized.
The origins of psychological traits prove complex, with genes, prenatal environments and postnatal experiences all working together in intricate ways.
Disentangling the interplay between nature and nurture remains challenging.
While the mechanisms behind birth order differences are still not fully resolved, this study elucidates key pieces of the puzzle.
Ruling out genetic and biological causes further implicates family dynamics. It also demonstrates the power of social contexts to shape outcomes above and beyond inborn potential. Nurture matters.
Human Development Shaped By Social Contexts
The persistent firstborn advantage illustrates that siblings growing up in the same home with the same parents still turn out different.
Shared genes and parents do not determine personality or IQ.
Rather, human development is flexible, not fixed.
A key insight from the study is that genetic propensities can manifest differently depending on social environments.
Having genetic potential is not enough – realization depends on environmental affordances.
Birth order likely influences which opportunities and experiences children encounter within families to support or hinder realizing their genetic promise.
Social contexts shape how, and how much, genetic potentials get expressed.
Far from immutable, abilities emerge through ongoing gene-environment interplay.
All children have an innate capacity to learn and thrive.
Providing nurturing environments tailored to their needs allows children to flourish.
Early childhood is a unique period of neuroplasticity and rapid cognitive development.
These findings underscore the need for enriching, stimulating environments during the first decade of life to cultivate learning.
Quality parenting and education build critical foundations that last.
While the specific environmental dynamics behind birth order remain debated, this research confirms postnatal family experiences as the likely cause.
Nature sets the range of possibility, but nurture determines how much potential gets fulfilled.
References
- Study: Birth order differences in education originate in postnatal environment
- Authors: Martin Arstad Isungset et al. (2022)
