The influence of genes on exceptional mental ability

We know a great deal about the relative genetic and environmental influences on average intelligence and on learning disabilities, but far less about the role of genes in exceptional cognitive ability – in lay terms, what we might call genius or innate talent.

A new "mega-analysis" of 11,000 twin pairs, aged between 6 and 71, has helped to plug that gap. The results suggest that genes exert a significant influence on exceptional cognitive ability, similar in magnitude to their influence on the normal range of intelligence. The findings challenge versions of the "discontinuity hypothesis" – the idea that the relative contribution of nature and nurture changes for exceptional ability.

Claire Haworth and colleagues, of the newly-established Genetics of High Cognitive Abilities (GHCA) consortium, combined data from six studies, involving twins from four countries – the UK, Netherlands, Australia and United States. Combining so much data altogether allowed them to restrict their analyses to participants in the top 15 per cent for intelligence performance, whilst still maintaining enough power for statistical tests.

By comparing intelligence differences between pairs of identical twins (who share all their genes) and non-identical twins (who share half their genes like normal siblings), the study showed that genetic differences explained approximately half the variation found in high intelligence, whilst shared environmental factors - those experienced by both twins in a pair, such as education and parenting style - explained just 28 per cent of the variation. The remaining influence is down to unique environmental influences (experienced by one twin but not the other) and other unknown factors.

The observed level of genetic influence on exceptionally high intelligence is similar to that found by the researchers for the normal range of intelligence in the same sample of twin pairs, and supports the idea that exceptional cognitive ability is on a continuum with the normal range of intelligence, and is likely subject to the same genetic and environmental influences. However, final proof that the same genes affect high intelligence and the normal distribution won’t be found until specific genes are identified through DNA testing of gifted and control participants.

It should be noted that the cited contributions of genes and the environment aren’t necessarily fixed. Rather these estimates reflect the amount of variation explained by genetic and environmental factors for this particular group of twin participants at one particular time. The generalisability of the findings are, however, enhanced by the large size and cross-national nature of the sample. Another caveat is that investigating the top 15 per cent of intelligence test performers may not be high enough to capture any influences that uniquely affect exceptional cognitive ability.

"We hope that our study, the many interesting and unanswered questions about high cognitive ability, and the importance of studying the high end of the distribution of ability as well as the low end, will stimulate much-needed research on the genetics of high cognitive ability," the researchers said.
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Haworth, C., Wright, M., Martin, N., Martin, N., Boomsma, D., Bartels, M., Posthuma, D., Davis, O., Brant, A., Corley, R., Hewitt, J., Iacono, W., McGue, M., Thompson, L., Hart, S., Petrill, S., Lubinski, D., & Plomin, R. (2009). A Twin Study of the Genetics of High Cognitive Ability Selected from 11,000 Twin Pairs in Six Studies from Four Countries. Behavior Genetics, 39 (4), 359-370 DOI: 10.1007/s10519-009-9262-3

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