Scientists can try combining smaller studies, but they often have merge different test together, potentially masking the effects of genes.
As a result, the first generation of genome-wide association studies on intelligence failed to find any genes. Later studies managed to turn up promising results, but when researchers turned to other groups of people, the effect of the genes again disappeared.
But in the past couple of years, larger studies relying on new statistical methods finally have produced compelling evidence that particular genes really are involved in shaping human intelligence.
“There's a huge amount of real innovation going on,” said Stuart J. Ritchie,a geneticist at the University of Edinburgh who was not involved in the new study.
Dr. Posthuma and other experts decided to merge data from 13 earlier studies, forming a vast database of genetic markers and intelligence test scores. After so many years of frustration, Dr. Posthuma was pessimistic it would work.
“I thought, ‘Of course we're not going to find anything,’” she said.
She was wrong. To her surprise, 52 genes emerged with firm links to intelligence. A dozen had turned up in earlier studies, but 40 were entirely new.
But all of these genes together account for just a small percentage of the variation in intelligence test scores, the researchers found；each variant raises or lowers I.Q. by only a small fraction of a point.
“It means there's a long way to go, and there are going to be a lot of other genes that are going to be important,” Dr. Posthuma said.
Christopher F. Chabris, a co-author of the new study at Geisinger Health System in Danville, Pa., was optimistic that many of those missing genes would come to light, thanks to even larger studies involving hundreds of thousands, perhaps millions, of people.
“It's just like astronomy getting better with bigger telescopes,” he said.
In the new study, Dr. Posthuma and her colleagues limited their research to people of European desent because that raised the odds of finding common genetic variants linked to intelligence.
But other gene studies have shown that variants in one population can fail to predict what people are like in other populations. Different variants turn out to be important in different groups, and this may well be the case with intelligence.
“If you try to predict height using the genes we've identified in Europeans in Africans, you'd predict all Africans are five inches shorter than Europeans, which isn't true,” Dr. Posthuma said.
Studies like the one published this week don't mean that intelligence is fixed by our genes, experts noted. “If we understand the biology of something, that doesn't mean we're putting down to determine,” Dr. Ritchie said.
As an analogy, he noted that nearsightedness is strongly influenced by genes. But we can change the environment - in the form of eyeglasses - to improve people's eyesight.
Dr. Harden predicted that an emerging understanding of the genetics of intelligence would make it possible to find better ways to help children develop intellectually. Knowing people's genetic variations would help scientists measure how effective different strategies are.
Still, Dr. Harden said, we don't have to wait for such studies to change people's environments for the better. “We know that lead harms children's intellectual abilities,” she said. “There's low-hanging policy fruit here.”
For her part, Dr. Posthuma wants to make sense of the 52 genes she and her colleagues discovered.
The genetic variants that raise intelligence also tend to pop up more frequently in people who have never smoked. Some of them also are found more often in people who take up smoking but quit successfully.
