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 or memorization.
In a typical test, the tasks might include imagining an object rotating, picking out a shape to complete a figure, as well as 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. although over all, these scores tend to hang together — people who score low on one measure tend to score low on the others, as well as vice versa. Psychologists sometimes refer to This specific similarity as general intelligence.
This specific’s still not clear what inside brain accounts for intelligence. Neuroscientists have compared the brains of people with high as well as low test scores for clues, as well as they’ve found a few.
Brain size explains a tiny part of the variation, for example, although there are plenty of people with tiny brains who score higher than others with bigger brains.
different studies hint of which intelligence has something to do with how efficiently a brain can send signals via one region to another.
Danielle Posthuma, a geneticist at Vrije University Amsterdam as well as senior author of the brand new paper, first became interested inside study of intelligence inside 1990s. “I’ve always been intrigued by how This specific works,” she said. “is usually This specific a matter of connections inside brain, or neurotransmitters of which aren’t sufficient?”
Dr. Posthuma wanted to find the genes of which influence intelligence. She commenced by studying identical twins who share the same DNA. Identical twins tended to have more similar intelligence test scores than fraternal twins, she as well as her colleagues found.
Hundreds of different studies have come to the same conclusion, showing a clear genetic influence on intelligence. although of which doesn’t mean of which intelligence is usually 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 of which researchers could find individual genes underlying differences in intelligence test scores. Some candidates were identified in tiny populations, although their effects did not reappear in studies on larger groups.
So scientists turned to what’s right now called the genome-wide association study: They sequence bits of genetic material scattered across the DNA of many unrelated people, then look to see whether people who share a particular condition — say, a high intelligence test score — also share the same genetic marker.
In 2014, Dr. Posthuma was part of a large-scale study of over 150,000 people of which revealed 108 genes linked to schizophrenia. although she as well as 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, generating This specific hard to gather results on huge numbers of people. Scientists can try combining smaller studies, although 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, although when researchers turned to different groups of people, the effect of the genes again disappeared.
although inside past couple of years, larger studies relying on brand new statistical methods finally have produced compelling evidence of which particular genes truly 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 inside brand new study.
Dr. Posthuma as well as different experts decided to merge data via 13 earlier studies, forming a vast database of genetic markers as well as intelligence test scores. After so many years of frustration, Dr. Posthuma was pessimistic This specific could 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, although 40 were entirely brand new.
although all of these genes together account for just a tiny percentage of the variation in intelligence test scores, the researchers found; each variant raises or lowers I.Q. by only a tiny fraction of a point.
“This specific means there’s a long way to go, as well as there are going to be a lot of different genes of which are going to be important,” Dr. Posthuma said.
Christopher F. Chabris, a co-author of the brand new study at Geisinger Health System in Danville, Pa., was optimistic of which many of those missing genes could come to light, thanks to even larger studies involving hundreds of thousands, perhaps millions, of people.
“This specific’s just like astronomy getting better with bigger telescopes,” he said.
inside brand new study, Dr. Posthuma as well as her colleagues limited their research to people of European descent because of which raised the odds of finding common genetic variants linked to intelligence.
although different gene studies have shown of which variants in one population can fail to predict what people are like in different populations. Different variants turn out to be important in different groups, as well as This specific 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 several inches shorter than Europeans, which isn’t true,” Dr. Posthuma said.
Studies like the one published today don’t mean of which intelligence is usually fixed by our genes, experts noted. “If we understand the biology of something, of which doesn’t mean we’re putting This specific down to determinism,” Dr. Ritchie said.
As an analogy, he noted of which nearsightedness is usually strongly influenced by genes. although we can change the environment — inside form of eyeglasses — to improve people’s eyesight.
Dr. Harden predicted of which an emerging understanding of the genetics of intelligence could make This specific possible to find better ways to help children develop intellectually. Knowing people’s genetic variations could 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 of which 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 as well as her colleagues discovered. There are intriguing overlaps between their influence on intelligence as well as on different traits.
The genetic variants of which 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 although quit successfully.
As for what the genes actually do, Dr. Posthuma can’t say. Four of them are known to control the development of cells, for example, as well as three do an assortment of things inside neurons.
To understand what makes these genes special, scientists may need to run experiments on brain cells. One possibility could be to take cells via people with variants of which predict high as well as low intelligence.
She as well as her colleagues might coax them to develop into neurons, which could then grow into “mini-brains” — clusters of neurons of which exchange signals inside laboratory. Researchers could then see if their genetic differences made them behave differently.
“We can’t do This specific overnight,” Dr. Posthuma said, “although This specific’s something I desire to be able to do inside future.”
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