Myriad genes in concert (Image: Patrick Gaillardin/Picturetank)

YOUR eye colour is a product of your DNA, but what about your IQ? The biggest-ever search for genes that affect intelligence, and the first to give reproducible results, has found 10 variations in DNA that seem to influence intelligence – but not by much.

Studies of families show intelligence is 40 to 50 per cent inherited, and otherwise depends on environment. Since mass-analysis of DNA variations became possible, a number of studies have sought the genes involved in this inheritance, and some papers have claimed strong associations between particular genes and IQ. Yet results have varied widely and none have been replicated.

“Many of the published findings of the last decade are wrong,” says John Hewitt of the University of Colorado in Boulder, who was not involved in the new study.


So if intelligence is inherited, where are the genes hiding? The research may have hit problems because each gene linked with IQ has only a tiny effect on overall intelligence. This means you need data on a large number of people to reliably distinguish such effects from measurement error. Most studies have involved between 100 and 2000 subjects.

“Research has hit problems because each gene linked with IQ has a tiny effect on overall intelligence”

Now, some 200 researchers have assembled 54 sets of data on more than 126,000 people who have had their genomes analysed for 2.5 million common, small mutations called SNPs. Information was also available for how long they spent in education and the level they reached.

Educational achievement is only a rough proxy for intelligence, says Philipp Koellinger of Erasmus University in Rotterdam, the Netherlands, an organiser of the study. But this information is available for the requisite large number of people.

The team began by studying 100,000 of the people and found three SNPs that correlated significantly with educational attainment. They then used data from the other 26,000 people to test whether the same correlations held. They did, replicating the first analysis.

But when the team did a statistical analysis for the strength of the correlations for each SNP, they found that even the strongest accounted for just 0.02 per cent of the total variation in educational attainment (Science, doi.org/mqw).

Seven other SNPs correlated detectably with education, but many more SNPs had tiny but, on their own, immeasurable effects. When the team applied the correlation analysis to all 2.5 million SNPs, they accounted for only 2 to 3 per cent of all the educational variation. Yet these 2.5 million SNPs account for half of the heritability of other complex traits such as height. If intelligence is 40 to 50 per cent heritable, the SNPs should have accounted for at least 20 per cent of the variation in educational attainment. Why the discrepancy?

“Probably thousands of SNPs are involved, each with an effect so small we need a much bigger sample to see it,” says Koellinger.

Either that, or intelligence is affected to a greater degree than other heritable traits by genetic variations beyond these SNPs – perhaps rarer gene mutations or interactions between genes.

Amassing data from many studies to detect the small effects of SNPs makes sense for now, says Robert Plomin of King’s College London, who was not involved in the study. “When whole genome sequencing is cheap enough, we can look for sequence variations of every kind.” Then, the missing genes for intelligence may finally be found.

This article appeared in print under the headline “Finding the players in the symphony of IQ genes”