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Intelligence, birth order, and family size.
1London School of Economics and Political Science, United Kingdom. S.Kanazawa@lse.ac.uk
Personality & Social Psychology Bulletin
|May 15, 2012
Summary
Birth order does not influence general intelligence. This study found no genuine causal link, suggesting the perceived effect is an artifact of family size, not child development.
Area of Science:
- Psychology
- Sociology
- Human Development
Background:
- Previous research suggested a link between birth order and intelligence.
- The confluence and resource dilution models proposed causal mechanisms.
- The role of family size in this relationship remained debated.
Purpose of the Study:
- To investigate the causal relationship between birth order and general intelligence.
- To determine if within-family data are necessary to observe the birth-order effect.
- To test the admixture hypothesis against alternative models.
Main Methods:
- Analysis of the National Child Development Study (n = 17,419) from the United Kingdom.
- Statistical control for the number of siblings in between-family analyses.
- Replication of earlier findings using large-scale longitudinal data.
Main Results:
- The apparent birth-order effect on intelligence disappeared when controlling for family size.
- Birth order was not associated with intelligence in between-family data.
- Support was found for the admixture hypothesis, not confluence or resource dilution models.
Conclusions:
- Birth order has no genuine causal effect on general intelligence.
- The apparent effect is likely an artifact of family size (admixture hypothesis).
- Confluence and resource dilution models are less supported by these findings.
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