This study helps establish reliable effect size estimates for birth order on self-report data of personality and on intelligence, taking into account several background factors, in a large representative sample of U.S. high school students. In a between-family design, we found very small associations between birth order and personality, with an average absolute partial correlation of .02. The partial correlations between birth order and cognitive abilities were slightly higher, the average being .04, with a maximum overall association of .08 for verbal ability, where positive associations indicate higher scores for firstborns. In sum, although the direction of some of the effects supported the hypothesized relation between birth order and both personality and intelligence, we would conclude that the magnitude of the effects would indicate that birth order is not an important consideration to either of these outcomes.
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