A multilevel approach to the relationship between birth order and intelligence

Area of Science:

• Psychology
• Developmental Psychology
• Behavioral Genetics

Background:

• Previous research suggests a link between birth order and intelligence.
• Many studies have employed cross-sectional designs, posing threats to internal validity.
• Potential confounding variables have not always been adequately controlled.

Purpose of the Study:

• To re-examine the relationship between birth order and intelligence.
• To determine if birth order effects persist after controlling for threats to internal validity.
• To differentiate between within-family and between-family factors influencing intelligence.

Main Methods:

• Multilevel analyses were conducted on data from the National Longitudinal Survey of Youth.
• The study included children aged 7–8 and 13–14 years.
• Hierarchical data structures and age variance were accounted for, distinguishing between-family and within-family variance.

Main Results:

• Birth order was found to have no significant influence on children's intelligence.
• Previously reported birth order effects were attributable to between-family variations.
• Within-family analyses revealed no significant impact of birth order on intelligence.

Conclusions:

• The association between birth order and intelligence is not supported when methodological threats are addressed.
• Socioeconomic status and other between-family factors likely explain prior observed birth order effects.
• Future research should focus on within-family dynamics and robust statistical controls.