Something as subtle and complex as intelligence was never going to be pinned on just a handful of genes, as a huge trawl across the human genome seems to confirm. Although it did turn up hundreds of genes that make a contribution, their individual effects are so small that for the most part they are barely detectable. This does not mean, however, that intelligence is not inherited.
The research, led by Robert Plomin of the Institute of Psychiatry in London, identified six genes that were strongly associated with high or low intelligence, but even the most powerful of these accounted for just 0.4 per cent of the variation in intelligence between individuals. The six together accounted for about 1 per cent of the variation in intelligence. Dozens of previous studies on twins and adopted children have established that about half of the variation in intelligence is down to environment, but almost all of the genetic component has yet to be uncovered.
鈥淒ozens of studies have established that about half of the variation in intelligence is down to environment鈥�
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鈥淚f we鈥檝e creamed off the biggest ones, and they only account for 1 per cent of the variance, then there鈥檚 a long way to go,鈥� Plomin says. 鈥淭he most striking result is that there are no large effects.鈥�
In their experiment, Plomin and his colleagues obtained intelligence scores for 7000 7-year-olds, based on verbal and non-verbal reasoning tests. They also took DNA samples from the children. To find regions of DNA that correlated with high or low intelligence, they screened DNA from the children who scored highest or lowest in the tests against a gene chip coated with 500,000 single nucleotide polymorphisms (SNPs) 鈥� regions of DNA that differ between individuals. This enabled the team to identify hundreds of SNPs that differed between the high and low scorers.
Next, they whittled the significant SNPs down to just 37 that correlated most strongly with intelligence, then looked at those SNPs in the DNA from children across the whole intelligence range. This process revealed the six highly significant gene regions (, ).
Most researchers contacted by New Scientist were not the least bit surprised that the genetic aspects of a complicated trait like intelligence are down to the cumulative effects of a wide combination of genes. 鈥淚ntelligence is a function of the way the brain is put together, and at least half of our genome contributes in some way or another to brain function, which means that in order to build a human brain, you need thousands of genes to work together,鈥� says Gary Marcus, a psychologist at New York University.
鈥淚ntelligence is a function of the way the brain is put together, and at least half of our genome contributes in some way鈥�
Neuropsychologist Stephen Pinker at Harvard University warns that the study should not be interpreted as evidence that intelligence is not heritable. 鈥淭here are many ways in which genes could affect intelligence other than single genes with large effects,鈥� he says.
Pinker is confident researchers will soon build on Plomin鈥檚 findings. 鈥淲hole-genome scans are in their infancy, and a variety of molecular and statistical improvements will soon tell us how the heritability of intelligence 鈥� which is not in reasonable doubt except by politically motivated critics 鈥� is related to variations in the genome.鈥�
Indeed, Katherine Burdick of the Zucker Hillside Hospital in Glen Oaks, New York, claims to have found genes with a greater impact on intelligence than those identified by Plomin, even though she screened the same 500,000 SNPs. 鈥淥ur top three SNPs have [cumulative] effects in the range of 9 to 10 per cent [of variance],鈥� she says. Burdick is due to present her research in Boca Raton, Florida, at the annual meeting of the American College of Neuropsychopharmacology.
Plomin is sceptical: 鈥淯ntil people publish, you must take things with a large pinch of salt.鈥�
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