Heritability of academic outcomes with a surprising difference between girls and boys

The heritability of cognitive ability influencing school academic outcomes is well established and at a general population level Nature vs. Nurture influences are roughly split as 50% genetic (parents), 30% shared environment (family and school) and a remaining 20% unshared (uniqueness).

A recently published UK study of 11,000 twins  (Shakeshaft et al. 2013) produced some interesting segmentation of heritability across subject areas: English 52%, mathematics 55%, science 58%, humanities 42%, with the impact of shared environment being 36%, leaving uniqueness between 4-12%.  Overall GCE scores gave 53% heritability, 30% shared and 17% uniqueness.

The more fascinating result that came out of the study however, is that heritabilities are somewhat greater for boys (57%) than for girls (47%) and that shared environmental influences are greater for girls than for boys.  While this result is very interesting, the authors “prefer merely to note these significant sex differences in our sample and to defer speculation about their origins until these results are replicated, for reasons discussed later.”

Difference in heritability could be explained if boys had a greater variance in cognitive ability compared to girls due to sex differences in the heritability of autism and dyslexia etc., however the statistical variance for boys and girls are similar for this study. While the mean academic scores for girls are appreciably higher than for the boys in the study.

This would suggest that the Y chromosome somehow amplifies genes associated with cognitive abilities while at the same time impeding genes associated with social ability, leading to lower shared environment effects.  Social behaviour genes however, could give rise to stronger shared environment effects while at the same time generating significant group learning and problem solving benefits.  Leading to the conclusion that within the larger set of hereditary influences on cognitive ability there is a sub-set of heredity influencing aspects of sociality associated with the development of cognitive ability through group learning.  This hypothesis would be consistent with observations of differences in learning styles between girls and boys.

It is important to note that there is a fair amount of indirect heritability occurring through the ‘shared environment’.  A large part of the family influences can be attributed to the cognitive ability of the parents affecting their socio-economic status and thereby their ability to invest in their children’s education.

The significance of this indirect heritability can be seen in circumstances where children are adopted into a family with biological offspring.  In a study of African American children adopted into Caucasian American families by Scarr & Weinberg (1976) it was found that ‘Black’ children from parents of average IQ and low socio-economic status adopted into ‘White’ families of high IQ and high socio-economic status performed better than the average for ‘White’ children.  Biological children of the ‘White’ parents scored even higher on the tests.

Adoptive twin studies have shown that environmental & social impacts are greatest when adoption occurs when very young and where the children come from very low socio-economic backgrounds (Plug & Vijverberg 2003). This underscores the importance of government funded early childhood education in preference to school education when trying remediate socio-economic impacts on developing a child’s cognitive ability.

In the UK study, socio-economic background was not analysed so we don’t know whether there was any heterogeneity based on the ability of parents to invest in there children education.  It would have been interesting to see whether there was any reversal of heritability of cognitive ability for the lowest socio-economic groups  indicated by other studies (Turkheimer et al. 2003).

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Shakeshaft, N. G., Trzaskowski, M., McMillan, A., Rimfeld, K., Krapohl, E., Haworth, C. M., … & Plomin, R. (2013). Strong genetic influence on a UK nationwide test of educational achievement at the end of compulsory education at age 16. PLoS One, 8(12), e80341.

Scarr, S., & Weinberg, R. A. (1976). IQ test performance of Black children adopted by White families. American Psychologist, 31(10), 726.

Plug, E & Vijverberg, W 2003, ‘Schooling, family background, and adoption: Is it nature or is it nurture?’, Journal of Political Economy, 111(3), 611-641.

Turkheimer, E., Haley, A., Waldron, M., D’Onofrio, B., & Gottesman, I. I. (2003). Socioeconomic status modifies heritability of IQ in young children. Psychological science, 14(6), 623-628.

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