What causes birth order-intelligence patterns? The admixture hypothesis, revived
1Department of Psychology, University of Oklahoma, 455 West Lindsey, Room 705, Norman, OK 73019, USA. jrodgers@ou.edu
The American Psychologist
|June 21, 2001
Summary
Birth order and intelligence findings differ between cross-sectional and within-family studies. The admixture hypothesis explains these birth order patterns, invalidating previous theories and guiding future research.
Area of Science:
- Psychology
- Human Development
- Behavioral Genetics
Background:
- Previous research on birth order and intelligence yielded inconsistent results.
- Cross-sectional studies often suggested a link between higher birth order and lower intelligence.
- Within-family studies, however, did not consistently support these findings.
Purpose of the Study:
- To reconcile discrepancies in birth order-intelligence research.
- To evaluate the validity of existing theories (e.g., dilution, confluence model).
- To propose and support the admixture hypothesis as an explanation for observed patterns.
Main Methods:
- Analysis of existing empirical data on birth order and intelligence.
- Comparison of findings from cross-sectional versus within-family study designs.
- Theoretical evaluation of the admixture hypothesis against empirical evidence.
Main Results:
- Empirical evidence contradicts conclusions from traditional cross-sectional birth order studies.
- The dilution and confluence models lack empirical support when examined within families.
- The admixture hypothesis effectively explains the observed patterns in birth order-intelligence research.
Conclusions:
- The admixture hypothesis provides a robust explanation for birth order and intelligence findings.
- Past research relying solely on cross-sectional data is invalidated.
- Reframing the birth order-intelligence argument is crucial for future research.
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