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Related Experiment Video

Updated: Jul 14, 2026

Using Cholesky Decomposition to Explore Individual Differences in Longitudinal Relations between Reading Skills

Using Cholesky Decomposition to Explore Individual Differences in Longitudinal Relations between Reading Skills

Published on: September 17, 2019

Explaining the relation between birth order and intelligence.

Petter Kristensen¹, Tor Bjerkedal

¹National Institute of Occupational Health, N-0033 Oslo, Norway. petter.kristensen@stami.no

Science (New York, N.Y.)

|June 26, 2007

Summary

Birth order does not impact intelligence; instead, a child's social rank within the family influences their intelligence quotient (IQ) scores. This finding supports family interaction as the key factor, not birth position.

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Area of Science:

Psychology
Human Development
Behavioral Science

Background:

Numerous studies indicate a negative association between birth order and intelligence levels.
The underlying reasons for this correlation remain unclear, with competing hypotheses.
Hypotheses include differential family interaction/stimulation and prenatal factors.

Purpose of the Study:

To investigate the relationship between birth order, family social rank, and intelligence quotient (IQ) scores.
To differentiate between family interaction and prenatal factors as explanations for birth order effects on IQ.
To provide empirical evidence supporting or refuting existing hypotheses.

Main Methods:

Analysis of intelligence quotient (IQ) scores from a large sample of nearly 250,000 military conscripts.
Statistical examination of the correlation between birth order and IQ.
Statistical examination of the correlation between family social rank and IQ.

Main Results:

Intelligence quotient (IQ) score levels were found to be dependent on social rank within the family.
No significant direct relationship was observed between birth order itself and IQ scores.
The findings provide support for family interaction-based explanations.

Conclusions:

Social rank within the family, rather than birth order per se, is a significant factor influencing intelligence.
Family dynamics and interaction patterns appear to play a crucial role in cognitive development.
This study challenges previous assumptions linking birth order directly to intelligence differences.