Resolving the debate over birth order, family size, and intelligence

Hundreds of research articles have addressed the relationship between birth order and intelligence. Virtually all have used cross-sectional data, whic h are fundamentally flawed in the assessment of within-family (including bi rth order) processes. Although within-family models have been based on patt erns in cross-sectional data, a number of equally plausible between-family explanations also exist. Within-family (preferably intact-family) data are prerequisite for separating within- and between-family causal processes. Th is observation reframes an old issue in a way that can be easily addressed by studying graphical patterns. Sibling data from the National Longitudinal Survey of Youth are evaluated, and the results are compared with those fro m other studies using within-family data. It appears that although low-IQ p arents have been making large families, large families do not make low-IQ c hildren in modern U.S. society. The apparent relation between birth order a nd intelligence has been a methodological illusion.