Intelligence and birth order in boys and girls

The relation between intelligence and birth order was shown in a recent publication [Bjerkedal, T., Kristensen, P., Skjeret, G. A. & Brevik, J. I. (2007). Intelligence test scores and birth order among young Norwegian men (conscripts) analyzed within and between families. Intelligence, 35, 503–514] to be negative. Subjects in this and in an influential earlier study [Belmont, L. & Marolla F. A. (1973). Birth order, family size, and intelligence. Science, 182, 1096–1101] were all men. We tested if the association of IQ and birth order is the same in men and women. Longitudinal IQ data were available from 626 Dutch twin pairs at ages 5, 12 and 18 years. The number of older siblings in these twin families was between zero and five, and was recoded into 3 categories (0, 1 and 2, or more). IQ data were analyzed with a model in which age cohort, number of older sibs, sex and all interactions were included as fixed effects. The dependency between twins was modeled as a function of additive genetic effects (A) and common environment (C) shared by children from the same family. Effects of A, C and unique environment (E) were allowed to differ as a function of age. The correlation across time between IQ scores was modeled a function of genetic and environmental factors. st for the effect of N of older sibs was significant [F(2,827) = 6.51 (p = 0.0016)], while the interaction of N of older sibs with sex was not significant [F(2,933) = 1.93, p = 0.15]. Heritability for IQ was estimated at 37% at age 5 (C explained 34% of the variance). At ages 12 and 18 heritability for IQ was 81% and 82%, respectively. At these ages C did not contribute to IQ variation. We conclude that the dependency of IQ scores on birth order does not differ for boys and girls. We discuss these results in the context of the general findings of the absence of common environmental influences on IQ scores in the genetic analyses of adolescent and adult twin data.