Carl Zimmer in The New York Times:
It’s still not clear what in the brain accounts for intelligence. Neuroscientists have compared the brains of people with high and low test scores for clues, and they’ve found a few. Brain size explains a small part of the variation, for example, although there are plenty of people with small brains who score higher than others with bigger brains. Other studies hint that intelligence has something to do with how efficiently a brain can send signals from one region to another. 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. “I’ve always been intrigued by how it works,” she said. “Is it a matter of connections in the brain, or neurotransmitters that aren’t sufficient?” Dr. Posthuma wanted to find the genes that influence intelligence. She started by studying identical twins who share the same DNA. Identical twins tended 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 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 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 finally have produced compelling evidence that particular genes really are involved in shaping human intelligence.