In at least one population, about half of intelligence differences between individuals can be attributed to genetics—specifically, the sum of many small effects from hundreds or even thousands of genes. The study, published today (August 9) in Molecular Psychiatry, is the first to pin down the genetic influence on cognitive abilities.
“The value of this paper is that it is the first clear and empirical demonstration that part of intelligence comes down to something which is writ in DNA,” said Patrick Sullivan, a psychiatric geneticist at University of North Carolina Chapel Hill, who was not involved in the study.
For decades, scientists have fiercely debated how much of the variation in individual intelligence can be attributed to genes. Studies of identical twins have suggested that 60-80 percent of intelligence comes down to genes, but “the controversy in the past has been, ‘well, maybe there’s just no separating out nature and nurture,’” said study co-author Peter Visscher, a quantitative geneticist at the Queensland Institute of Medical Research in Australia.
In addition, aside from the Alzheimer’s-linked APOE4 gene, researchers who went looking for “intelligence genes” have been unable to find them. Furthermore, many studies purporting to look at the heritability of intelligence have been accused of using faulty methodology or tweaking the data to justify racist beliefs.
To tease out the genetic differences directly, Visscher and his colleagues analyzed roughly 500,000 single nucleotide polymorphisms (SNPs) in about 3,500 adults aged 18 to 90 from the United Kingdom and Norway. The subjects took an array of vocabulary, speed of processing, and reasoning tests designed to measure intelligence.
Not surprisingly, they didn’t find any specific genes that were associated with higher cognitive abilities. Given the complexity of the trait, a single gene is unlikely to have a large effect on intelligence, meaning researchers would need a much larger sample size to detect those minute effects.
Instead, they were able to use a statistical technique to analyze the overall effect of genetics on smarts. Specifically, they used all 500,000 SNP locations to determine how genetically similar each subject was to every other individual in the study. They found that people who tested higher on intelligence were more genetically similar to each other than to those who scored lower in intelligence, and that roughly half of the variation in intelligence between individuals could be attributed to underlying genes. The results suggest that hundreds or even thousands of genes may each contribute a small amount to intelligence. But what those genes are remains a mystery.
One way to find those genes would be to increase the sample size by pooling genetic data from all studies on intelligence, Visscher said.
The findings also have a number of other limitations, researchers said. First, the study participants were mostly older adults, born as early as 1921, and thus represented a biased population—those healthy enough to still be alive, said geneticist Dorret Boomsma, who leads the Netherlands Twins Register at the VU University in Amsterdam, and was not involved in the study. The study population is also inherently biased because the selection criteria make it a nonrandom sample.
In addition, some researchers have questioned the statistical methods used, said Greg Gibson, director of the Center for Integrative Genomics at Georgia Institute of Technology who was not involved in the study. Because the researchers sampled only a small subset of the SNPs in the human genome, most of which are not in the actual genes that may affect intelligence, its quantitative estimate is not predictive of the larger population, he said. So even though genetics predicted about half of the variation in intelligence within their sample population, what they found would only predict about 1 percent of the variance in intelligence across the wider population in Northern Europe, Gibson said.
Finally, the study is only applicable to intelligence in populations they studied, Gibson added. “This study says absolutely nothing about differences between groups,” such as people of different ethnicities.
Davies, G., et. al, "Genome-wide association studies establish that human intelligence is highly heritable and polygenic," Molecular Psychiatry, doi: 10.1038/mp.2011.85, 2011.