Does Birth Order Affect Intelligence? What Large-Scale Studies Reveal

The belief that firstborn children are smarter than their younger siblings is one of the most persistent ideas in folk psychology. Parents joke about it, media repeats it, and surprisingly, the research largely supports it — though the effect is far smaller than most people assume and the reasons behind it are still debated.

What Do the Large Studies Show?

The most compelling evidence comes from studies large enough to detect small effects reliably:

• Norwegian military data (Kristensen & Bjerkedal, 2007): Using IQ scores from over 250,000 male conscripts, this landmark study found that firstborns scored approximately 2.3 IQ points higher than second-borns, who in turn scored about 1 point higher than third-borns. The effect was dose-dependent: each successive birth position was associated with a small but consistent decline.
• Swedish conscription data: Studies using Swedish military records with similarly large samples replicate the Norwegian findings almost exactly — firstborn advantages of approximately 2–3 points over second-borns.
• U.S. National Longitudinal Survey of Youth (NLSY): American data show comparable patterns, with firstborns scoring approximately 1–3 points higher on standardized cognitive tests than later-born siblings.

The consistency across countries, time periods, and measurement instruments gives confidence that the effect is real rather than artifactual. But 2–3 IQ points is a small effect — too small to predict anything meaningful for any individual pair of siblings, but large enough to shift population-level statistics.

Is It Really About Birth Order or Family Size?

This is the critical methodological question. Birth order is confounded with family size: a third-born child necessarily comes from a family of at least three children, while a firstborn can come from any size family. If larger families produce lower-scoring children for reasons unrelated to birth order (less per-child investment, lower SES, etc.), then the apparent birth-order effect could be a family-size artifact.

Research on family size and child development documents the “quantity-quality trade-off” — the finding that children from larger families tend to receive fewer parental resources per child, less individual attention, and lower educational investment. This trade-off could explain why later-borns score lower without invoking birth order itself.

However, the Norwegian study controlled for this elegantly by comparing siblings within the same family. Even within families of the same size, firstborns scored higher than second-borns. This within-family design rules out family-size confounding and confirms that birth order has an independent effect — though a smaller one than between-family comparisons suggest.

What Explains the Firstborn Advantage?

Three main theories have been proposed:

1. The Resource Dilution Model: Parents have finite resources — time, attention, money, energy. The firstborn has exclusive access to these resources before siblings arrive. Each additional child dilutes the resources available to all children. Firstborns also typically spend more time in a “higher-resource” environment (before siblings arrive) during the critical early years of cognitive development.

This model is supported by the finding that the birth-order effect is larger in families with more children and smaller in families with greater resources. It also connects to research on nurturing caregiving and adolescent outcomes, which shows that the quality and quantity of parental attention have measurable effects on cognitive development.

2. The Confluence Model (Zajonc): This model proposes that the intellectual environment of the family matters. The firstborn grows up in an environment of two adults — a high-average intellectual milieu. When a second child arrives, the average intellectual level of the household decreases. Each subsequent child further dilutes this average. Additionally, firstborns benefit from serving as tutors for younger siblings — the “teaching effect” — which reinforces and deepens their own understanding.

3. The Social Role Model: Firstborns may develop different personality traits and cognitive strategies as a function of their family role. They are often given more responsibility, held to higher expectations, and serve as models for younger siblings. These social demands may promote more mature cognitive functioning, particularly in areas like planning, organization, and verbal ability.

Is the Effect Biological or Environmental?

The Norwegian study provided a critical insight: the effect is primarily social, not biological. When a firstborn child died in infancy and the second-born was raised as the functional firstborn, the second-born showed the cognitive profile typical of firstborns. This finding strongly supports environmental explanations (parental investment, family dynamics) over biological ones (prenatal hormonal environment, immune system effects).

If the effect were biological — related to, say, maternal immune responses that increase with each pregnancy — then a second-born raised as a firstborn would still show the biological disadvantage. The fact that social birth order, not biological birth order, predicts IQ supports the resource dilution and confluence models.

How Large Is the Effect Compared to Other Factors?

The birth-order effect is real but modest. It is dwarfed by genetic factors, socioeconomic environment, and major environmental exposures. A later-born child from an enriched environment will dramatically outscore a firstborn from a deprived environment. The genetic and environmental origins of cognitive abilities operate on a much larger scale than sibling position.

Does Birth Order Affect Personality Too?

The popular belief that firstborns are conscientious leaders while later-borns are rebellious free spirits has much weaker empirical support than the IQ findings. Large within-family studies (Rohrer et al., 2015; Damian & Roberts, 2015) using samples of tens of thousands find:

• Essentially no reliable birth-order effects on Big Five personality traits (openness, conscientiousness, extraversion, agreeableness, neuroticism)
• Very small effects on intellect/openness (firstborns slightly higher) that may reflect the IQ difference rather than a distinct personality effect
• No effects on agreeableness, emotional stability, or extraversion

The personality claims — so vivid in popular psychology books — largely reflect stereotypes, retrospective self-reports, and methodologically weak between-family comparisons. The IQ finding, by contrast, is robust.

What About Only Children?

Only children tend to score comparably to firstborns — consistent with the resource dilution model (they never have to share parental resources). Some studies show only children scoring slightly higher than firstborns from multi-child families, which is also predicted by the model: they receive 100% of parental investment throughout childhood, whereas firstborns experience dilution when siblings arrive.

The common cultural stereotype of only children as socially maladjusted or spoiled is not supported by the research literature. On measures of cognitive ability, academic achievement, and social adjustment, only children perform comparably to or slightly better than firstborns.

Does Spacing Between Siblings Matter?

Some evidence suggests that larger age gaps between siblings attenuate the birth-order effect. When siblings are spaced more than 5 years apart, the later-born child may receive more concentrated parental attention during their own critical developmental window, partially compensating for the dilution effect.

This finding is consistent with the resource model: wider spacing gives parents time to “recharge” their investment capacity. It also aligns with research on early cognitive development, which emphasizes the importance of intensive caregiver engagement during the first years of life — a period when parental bandwidth is most constrained by the presence of other young children.

Conclusion

Birth order does affect intelligence — firstborns score approximately 2–3 IQ points higher than second-borns in within-family comparisons, with a consistent gradient for later-born positions. The effect is primarily environmental rather than biological, driven by differences in parental resource allocation and the intellectual environment of the family. However, the effect is small — far smaller than the influence of genetics, socioeconomic status, or education. No parent should worry that their second or third child is meaningfully disadvantaged by birth position. The 2-point difference is a population statistic, not a prediction for any individual child, and it is easily overwhelmed by the many other factors — from early nutrition to educational opportunity — that shape cognitive development.