IQ appears to be about 50% genetic, but how many genes contribute is a mystery. Now, two teams report finding associations between two genes and intelligence in healthy individuals. This kind of research could help scientists understand what goes wrong in brain disorders, such as Alzheimer's disease, that erode thinking skills.
Researchers had associated only one gene, IGF2R, with intelligence, and that observation has yet to be repeated. Investigators are pinning their hopes on the possibility that at least some genes will have large enough effects to detect.
New findings from two research groups buoy those hopes. Tony Payton of Manchester University, United Kingdom, and his colleagues tested 767 healthy adults, age 50 and up, for their cognitive skills over a period of 15 years, looking for genes that contribute to the mental decline associated with Alzheimer's disease. Those people carrying a mutation in a gene called cathepsin D (CTSD) scored 3% lower on the intelligence tests right from the start (their scores did not go down more over time than did those without the mutation). CTSD might increase intelligence in early brain development, the researchers suggest, by helping kill off unnecessary neurons. The gene also seems promising, Payton says, because CTSD relies on IGF2R for its function.
The second group, led by David Comings of City of Hope Medical Center in Duarte, California, tested 828 adults for a change in a gene called CHRM2, which is associated with Alzheimer's disease. People with one copy of the mutation had lower IQs than did those who had none, and individuals with two copies had the lowest scores of all. Although the spread was only about three to four IQ points, it's a significant effect for just one gene. Both teams report their work in the February issue of Molecular Psychiatry.
"It gives us hope that we're going to find [more] genes" with a measurable effect on intelligence, says behavioral geneticist Robert Plomin of Kings College, London. "That's why we find these studies exciting." Because many people were tested in the two studies, the results should be reliable, says Plomin. Still, he cautions, "until these findings have been replicated, we don't want to get too excited."