New Insight Into Aging Brains
Study Links 24% of Intelligence Changes Over a Person’s Life to Genetic Factors

Nearly a quarter of the changes often seen in a person’s intelligence level over the course of a lifetime may be due to genes, a proportion never before estimated, new research shows.

The study suggests that genes may partly explain why some people’s brains age better than others, even though environmental factors likely play a greater role over a lifetime.

Understanding the factors behind healthy mental aging has become an increasingly vital one for societies with large elderly populations. However, it isn’t an easy task.

Traditional methods of estimating the influence of genes and the environment on intelligence have largely been limited to comparisons between people who are related, such as identical or fraternal twins. The shortcoming of such studies is they didn’t clearly apportion the effects of each factor on intelligence.

Modern DNA-based techniques are now helping to refine the search.

The new study, published in the journal Nature, offers one of the first estimates of how much genes and the environment contribute to fluctuations in a person’s intelligence between adolescence and old age. It found that genetic differences account for 24% of the variation.

However, the paper didn’t identify any of the myriad genes or environmental factors that might be involved.

“The nature-nurture controversy is never more contentious than when it concerns the genetics of intelligence,” wrote Robert Plomin, a psychologist at King’s College in London, in a commentary accompanying the study, in which Dr. Plomin wasn’t involved.

The Nature paper, he said, “may mark the beginning of the end of this controversy” because it relies on DNA data from unrelated people, which is harder to dispute.

The scientists behind the Nature paper were able to do their analysis thanks to an unusual database maintained in Scotland: records from 1,940 unrelated individuals whose intelligence was measured first at age 11 and then again at age 65, 70 or 79. It is rare for researchers to have access to intelligence data for a group of people from both childhood and old age. The participants also provided blood for DNA analysis.

With these separate pieces of information, the researchers used a new statistical technique to seek out any associations between genes and how intelligence levels might have shifted over the years.

As a first step, the scientists examined half a million genetic markers in the participants’ DNA. These markers, known as single nucleotide polymorphisms, or SNPs, can together reveal how genetically similar people are, even though they are not conventionally related.

The similarity of the SNPs was then compared with other aspects of the individuals’ similarity—in this case, with their cognitive ability in both youth and old age, and how much that had changed over the years.

One finding of this technique, which is known as genome-wide complex-trait analysis, is that many of the same genetic factors seem to explain why people differ in intelligence in childhood and old age.

Another finding is that people tend to retain a similar “rank order” in intelligence level between childhood and old age. In other words, those who had above-average cognitive ability at age 11 also tended to be above average when much older. Still, not everyone showed this kind of stability.

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