Birth Order and Intelligence: Further Tests of the Confluence Model

Abstract

Confluence theory was originally developed to explain the negative effect of birth order on intelligence, as well as some peculiar effects of birth order on the intelligence of last-born children, in a large set of Dutch data. Subsequently the theory was elaborated to explain positive effects and nonlinear relationships between birth order and intelligence in other data sets. We test the mathematicalform of the theory using aggregate data, between-family data, and within-family data from the Wisconsin Longitudinal Study and find no support for the theory. Fundamentalflaws are identified in the statistical methods used tofit the model to the Dutch data. When the analysis is done correctly, the fit is very poor. An alternative analysis based on a within-family study design that uses sibling pairs indexed by birth order in the Wisconsin Longitudinal Studyfinds no evidence of birth order effects on intelligence.

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