Genetic study reveals that a significant proportion of intelligence is inherited

By Robert Hoskin
University of Sheffield, UK

This summary was highly commended by the judges for Access to Understanding 2013

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To what extent do biological and environmental factors influence how an organism develops? This question, often framed as the ‘nature-nurture debate’, is one of the most fundamental problems that science has to address. Within this debate it is of particular importance to understand how biological and environmental factors contribute to the development of human intelligence, as intelligence plays a substantial role in determining life outcomes. A 2011 study, published in the Molecular Psychiatry journal, has demonstrated that differences in adult intelligence can, to a large extent, be explained by genetic variations between people. The study therefore provides clear evidence that a significant proportion of adult intelligence is hereditary.

The research was the result of wide-scale collaboration between scientists at 8 different universities, across 3 different countries. They analysed DNA collected from over 3000 unrelated volunteers. Each individual’s DNA was scanned for the presence of a large number of common genetic variations (known as single-nucleotide polymorphisms or ‘SNPs’). These genetic variations effectively encode the biological differences between people. Their study can therefore reveal the genetic basis of individual differences. For example, if a particular gene has a causal effect on a trait (in this case intelligence) then individuals with an SNP within that gene should have noticeably different scores on the trait compared to those who do not. Measures of each individual’s intelligence were ascertained from various psychometric test scores collected from each volunteer during middle to late adulthood. Two separate measures of adult intelligence were computed: ‘crystallised intelligence’, which is a measure of the ability to acquire and recall knowledge, and ‘fluid intelligence’, which is a measure of cognitive skills such as abstract reasoning, logical thinking and problem solving.

The research team were not able to establish a firm relationship between any individual SNP and either measure of intelligence. One SNP (named FNBP1L) was found to predict fluid intelligence, but this relationship could not be replicated using an independent sample of data from Norway, suggesting that the effect may be spurious. The identification of SNPs also however enables a genetic profile of each individual to be created, a process which in effect allows the cumulative genetic variation between individuals to be quantified. These profiles therefore allow an assessment to be made of the extent to which more general variations in genetic makeup contribute to individual differences. Using this method the researchers found that 40% of the variance in crystallised intelligence and 51% of the variance in fluid intelligence between individuals could be explained by genetic differences. Furthermore the predictive information contained within the SNP profile was also able to predict intelligence scores in the independent Norwegian dataset, confirming the validity of the original finding. These results strongly suggest that around half the variation in intelligence between people can be attributed to inherited abilities. Indeed as only common SNPs were analysed, rather than every single genetic variation, the actual proportion of intelligence that is heritable may be much higher than this.

The estimates regarding the heritability of intelligence provided by the study are in broad agreement with those obtained from twin and family studies. This study however strengthens our understanding of the heritability of intelligence because it side-steps some methodological issues that exist with twin and family studies, such as the difficultly in parsing genetic from environmental influences when closely related individuals are studied. The study therefore represents the first direct demonstration that genetic differences explain a significant amount of the variance in intelligence between individuals.

As no individual SNPs were found that strongly predicted intelligence, an additional conclusion that can be drawn from the study is that a very large number of different genes are likely to interact to determine our intelligence, rather than there being a small number of ‘intelligence genes’. This is perhaps not surprising as intelligence is a complex trait that is reliant on a number of different cognitive abilities, which are in turn likely to be reliant on a number of different biological processes. The likely influence of a large number of genes does not suggest that associations between individual SNPs and intelligence cannot be uncovered in the future. Instead it suggests that studies with far larger samples, and perhaps looking at more specific cognitive abilities, will be required in order to identify such associations. Nevertheless the study represents an important step in the battle to understand the genetic basis of intelligence. By improving our understanding of the biological mechanisms that support intelligence we may be able in the future to identify ways in which the development and maintenance of human mental functioning can be improved. This may lead to interventions that can help to both promote cognitive abilities in the general population, and preserve these abilities to a greater extent in old age.

This article describes the research published in:

Genome-wide association studies establish that human intelligence is highly heritable and polygenic (2011) G. Davies, A.Tenesa, A. Payton, J. Yang, S. E. Harris, D. Liewald, X. Ke, S. Le Hellard, A. Christoforou, M. Luciano, K. McGhee, L. Lopez, A. J. Gow, J. Corley, P. Redmond, H. C. Fox, P.Haggarty, L. J. Whalley, G. McNeill, M. E. Goddard, T. Espeseth, A. J. Lundervold, I. Reinvang, A.Pickles, V. M. Steen, W. Ollier, D. J. Porteous, M. Horan, J. M. Starr, N.Pendleton, P.M. Visscher, I. J. Deary Mol. Psychiatry 16(10), 996-1005
http://EuropePMC.org/articles/PMC3182557

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