According to Ars Technica, initial studies clearly indicated that the effect of any individual gene on intelligence is small. As a result, the first genetics studies found very little, since you needed to look at a large number of people in order to see these small effects. Now, a new study has combined much of the previous work and has turned up 40 new genetic regions associated with intelligence test scores. But again, the effect of any individual gene is pretty minor.
Hunting for genes
The team behind the new work took advantage of open data to pull together information from 13 different studies, which cumulatively looked through the genomes of over 78,000 individuals. While those individuals had been given a variety of tests, the authors focused on measures of general intelligence or fluid intelligence (the two seem to measure similar things). The genomes of these individuals had been scanned for single base pair differences, allowing the authors to look for correlations between regions of the genome and test scores.
Two separate analyses were done. The first simply looked at each base difference individually. That turned up 336 individual bases, which clustered into 22 different genes. Half of these had not been associated with intelligence previously. To provide a separate validation of these results, the authors did a similar analysis with educational achievement. They found that nearly all of the sites they identified also correlated with that.
In a second analysis, the authors tracked base differences that cluster in a single gene. Since there are more markers for each gene, this tends to be a more sensitive way of looking for effects. And in fact, it produced 47 genes associated with the intelligence test scores. Seventeen of those had been identified in the earlier analysis, which brought the total genes identified to 52, only 12 of which had been previously associated with intelligence test scores.
What we’ve found
What do all these genes tell us? Once again, the genetics of intelligence are really complicated, and no single gene has a large impact. The team calculated that the genes identified in the first analysis, which should have a stronger effect, account for less than five percent of the variation in intelligence.
The other point of interest is that many of these areas of the genome had previously been implicated in other aspects of human biology. One of those, as mentioned above, is educational achievement. But there were weaker associations with things like autism spectrum, successful smoking cessation, and height. These genes also seem to be somewhat protective against Alzheimer's, depression, schizophrenia, and neuroticism, as well as a low body mass. So these clearly aren't "genes for intelligence," per se; they're genes that influence a broad range of biology, some of which influences how we perform on intelligence tests.
As for the genes themselves, there's no strong, consistent theme to their function. A couple seem to be involved in the controlled death of neurons that takes place as the brain is developing. But many others were more generally involved in development, either of the brain or other organs. And we're a long way from figuring out how they contribute to intelligence that's often measured decades after the genes may be active.
It would be nice to think that testing the genetics of nearly 80,000 people might give us a clearer picture of the biology of intelligence. But as the authors point out, the work was meant to be a foundation rather than the last word: "These findings provide starting points for understanding the molecular neurobiological mechanisms underlying intelligence."
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