A Multilevel Analysis of the Relationship Between Birth-Order and Intelligence

Many previous studies have found relationships between birth order and intelligence, but use cross-sectional designs or manifest other threats to internal validity. Using the National Longitudinal Survey of Youth Children (NLSY) data and multilevel analyses with control variables, we show that when these threats are removed, two major results emerge : (a) Birth order has no measurable influence on children’s intelligence; and (b) Control variables provide strong evidence that earlier-reported birth order effects on intelligence are attributable to environmental and genetic factors that vary between, not within families. Identical sets of analyses on 7-8 and 13-14 year-old children from the NLSY support these conclusions. When hierarchical data structures, age-variance of children, and within-family vs. between-family variance sources are taken into account, previous research is seen in a new light