A single gene has a measurable impact on a person's brain size as well as intelligence, say researchers.
There is little debate about genetics being behind considerable variation in people's intelligence, but studies have constantly failed to zero in on any single gene that has a significant impact. Instead, researchers usually find that hundreds of genes contribute.
A large-scale genetics study depending on fMRI brain scans and DNA samples from more than 20,000 people has revealed what is claimed as the biggest effect yet of a single gene on intelligence - although the effect is small as the gene alters IQ by only 1.29 points.
According to some researchers, that basically proves that intelligence depends on the action of a number of genes after all.
"It seems like the biggest single-gene impact we know of that affects IQ," the New Scientist quoted Paul Thompson of the University of California, Los Angeles, who led the collaboration of 207 researchers as saying.
"But it's not a massive effect on IQ overall," he said.
The variant is in a gene called HMGA2, which has earlier been tied to people's height. At the site of the relevant mutation, the IQ difference relies on a change of a single DNA "letter" from C, standing for cytosine, to T, standing for thymine.
"C is the good one," said Thompson.
Apart from raising IQ by 1.29, it boosts the overall volume of the brain - but only by 0.58 per cent of average brain size, adding around 9 cubic centimetres of tissue.
"It's a loss or gain of about 2 teaspoons," insisted Thompson.
The brain-size-altering effect of the gene is what guided the researchers to evaluate the impact on IQ.
In their study, comprising 21,151 adult subjects, not only did they take DNA samples but also scanned each volunteer's brain, particularly seeking size differences either in the brain overall or in specific parts of it, like the hippocampus, considered to be the seat of memory and learning.
After the researchers had affirmed that HMGA2 influenced overall brain size, they looked deeper at a subset of 1642 volunteers from a twin study in Brisbane, Australia, who had all taken standard IQ tests.
The analysis helped them measure the effect of the C on IQ.
When people inherit C-variants from both parents they enjoy double the effect: a jump in IQ to nearly 2.6.
"It's important they've found this gene, but it took a sample of 20,000 people to find it, precisely because the effect is so small," said Robert Plomin at the Institute of Psychiatry in London, and lead author of a groundbreaking study in 2007 which failed to find any single genes of disproportionate importance in intelligence.
"If it's this hard to find an effect of just 1 per cent, what you're really showing is that the 'cup' is 99-per-cent empty," he said.
"It's an important finding, assuming it holds up," Steven Pinker, an author and professor of neuropsychology at Harvard University, said.
Pinker asserted that the findings are a first step in showing that intelligence depends on numerous genes, each with a tiny effect, rather than on single genes that have moderate or large effects, but which are so rare that none has been identified yet.