Gender differences in IQ - are boys more variable in intelligence?

There is little doubt that the long history of research on racial and gender differences in intelligence has been controversial. Interestingly, there has been a recent upswing in recent studies in both domains (click here, here, here , here, here, here here, here, here, and here, for a sampling of some prior gender/IQ posts on this blog).

Most of the research that has captured the attention of both researchers and the popular press have focused on mean (average) IQ score differences. Less attention has focused on possible differences in the range/variability of intelligence differences by race or sex. Arden and Plomin address this issue in a recently published study of the differences in the variability of intelligence scores (particuarly at the high and low ends of the IQ distributions) in a large British sample. The article, including a link to a pdf copy, is listed below along with the abstract and some select comments by the authors.

• Arden, R. & Plomin, R. (2006). Sex differences in variance of intelligence across childhood. Personality and Individual Differences 41, 39–48. [click here to view article]
  

Abstract

• Why are males over-represented at the upper extremes of intelligence? One possibility for which there is some empirical support is that variance is greater among adult males. There is little published evidence of the development of that variability – is it manifest in early childhood or does it develop later? We explored sex differences in phenotypic variance in scores on a general ability factor extracted from several tests of verbal and non-verbal ability at ages 2, 3, 4, 7, 9 and 10 (Ns from >10,000 to >2000) in a sample of British children. We found greater variance, by Levene’s test of homogeneity of variance, among boys at every age except age two despite the girls’ mean advantage from ages two to seven. Girls are signi?cantly over-represented, as measured by chi-square tests, at the high tail and boys at the low tail at ages 2, 3 and 4. By age 10 the boys have a higher mean, greater variance and are over-represented in the high tail. Sex di?erences in variance emerge early – even before pre-school – suggesting that they are not determined by educational influences. This large sample indicates that boys and girls follow sex-specifc developmental pathways. It is a commonplace within mothers’ groups that girls’ mental abilities develop earlier than do those of boys; we have evidence indicating that such anecdotal observations are well founded. Boys seem to ‘get going’ a little later than do girls; the boys in our sample catch up in middle childhood and have nudged ahead of the girls by age 10. This study offers tantalizing evidence of developmental trends in variance dfferences between the sexes.

Why might boys vary more than girls? The authors offer some general genetic and environmental possibilities.

• Genetic possibilities include not only X-linked genes but alsoautosomal genes that have different ffects in boys and girls. In terms of the sex-limitation model-fitting of quantitative genetics, genetic effects on variance could arise from differences in heritability (the same genes can affect boys and girls differently) or differences in genetic correlation(different genes might affect boys and girls). These genetic differences could be the result of different selective forces having acted ancestrally on g in males and females.

• There are also several environmental possibilities, although social forces such as differential parenting or schooling are a more plausible source of means differences than variance differences. A variance difference in the absence of a means dfference, as we found during the pre-school years of 3 and 4, then again at age 7 is di?cult to explain by socialization though we cannot exclude this possibility. A socialization hypothesis would predict that variability would increase or decrease in males and females under different cultural and temporal contexts.

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