In a significant advance in the study of mental ability, a team of European and American scientists announced this week that they had identified 52 genes linked to intelligence in nearly 80,000 people.
However, these genes do not determine intelligence. Their combined influence is minuscule, the researchers said, suggesting that thousands more are likely to be involved and still await discovery. Just as important, intelligence is profoundly shaped by the environment.
Still, the findings could make it possible to begin new experiments into the biological basis of reasoning and problem-solving, experts said. They could even help researchers determine which interventions would be most effective for children struggling to learn.
"This represents an enormous success," said Dr Paige Harden, a psychologist at the University of Texas who was not involved in the study.
For over a century, psychologists have studied intelligence by asking people questions. Their exams have evolved into batteries of tests, each probing a different mental ability, such as verbal reasoning.
In a typical test, the tasks might include imagining an object rotating, picking out a shape to complete a figure, and then pressing a button as fast as possible whenever a particular type of word appears.
Each test-taker may get varying scores for different abilities. But overall, these scores tend to hang together - people who score low on one measure tend to score low on the others, and vice versa. Psychologists sometimes refer to this similarity as general intelligence.
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.
Professor Danielle Posthuma, a geneticist at Vrije University Amsterdam and senior author of the new paper, first became interested in the study of intelligence in the 1990s. She wanted to find the genes that influence intelligence.
She started by studying identical twins who share the same DNA. Identical twins tend to have more similar intelligence test scores than fraternal twins, she and her colleagues found.
Hundreds of other studies have come to the same conclusion, showing a clear genetic influence on intelligence. But that doesn't mean that intelligence is determined by genes alone.
Our environment exerts its own effects, only some of which scientists understand well. Lead in drinking water, for instance, can drag down test scores. In places where food doesn't contain iodine, giving supplements to children can raise scores.
Advances in DNA sequencing technology raised the possibility that researchers could find individual genes underlying differences in intelligence test scores. Some candidates were identified in small populations, but their effects did not reappear in studies on larger groups.
So scientists turned to what's called the genome-wide association study: They sequenced bits of genetic material scattered across the DNA of many unrelated people, then looked to see whether people who shared a particular condition - say, a high intelligence test score - also shared the same genetic marker.
In 2014, Prof Posthuma was part of a large-scale study of more than 150,000 people that revealed 108 genes linked to schizophrenia. But she and her colleagues had less luck with intelligence, which has proved a hard nut to crack for a few reasons.
Standard intelligence tests can take a long time to complete, making it hard to gather results on huge numbers of people. Scientists can try combining smaller studies, but they often have to merge different tests 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 have finally produced compelling evidence that particular genes are really involved in shaping human intelligence.
"There's a huge amount of real innovation going on," said Dr Stuart J. Ritchie, a geneticist at the University of Edinburgh who was not involved in the new study.
Prof 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, she was pessimistic it would work. 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 accounted for just a small percentage of the variation in intelligence test scores, the researchers found; each variant raised or lowered IQ 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," Prof Posthuma said.
Professor Christopher F. Chabris, a co-author of the new study at Geisinger Health System in Danville, Pennsylvania, 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.
In the new study, Prof Posthuma and her colleagues limited their research to people of European descent 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 5 inches shorter than Europeans, which isn't true," Prof 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 it down to determinism," Dr Ritchie said.
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, Prof Posthuma wants to make sense of the 52 genes she and her colleagues discovered. There are intriguing overlaps between their influence on intelligence and on other traits.
The genetic variants that raise intelligence also tend to pop up more frequently in people who have never smoked. Some of them are also found more often in people who take up smoking but quit successfully.