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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.

Hazel Soares, center, at Mills College in Oakland, Calif., in 2010. At 94, she was believed to be the second-oldest person to graduate from college. ENLARGE

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.

DNA and IQ

How scientists discovered that almost one-quarter of the change in intelligence over a lifetime is due to genes:

  • Scientists found decades-old intelligence tests done on 1,940 11-year-olds in Scotland, and then retested the participants at age 65 and above.
  • DNA from blood samples was collected from all 1,940 people.
  • An analysis of half a million genetic markers in the participants revealed various genetic similarities.
  • Scientists compared these genetic similarities with the participants' intelligence levels in both youth and old age.
  • The study concluded 24% of the life-span changes in intelligence could be linked to genes, the rest to environmental factors.

Source: Nature

While the study lacks statistical power in some crucial aspects, it is valuable because "it is very rare to have an estimate of the genetic contribution to lifetime cognitive change," said psychologist Ian Deary of the University of Edinburgh and lead author of the study, in an emailed response to questions.

As to which genes may account for the changes, Dr. Deary said, "Most of us think that we shall be looking for many genes, each contributing very small effects."

Dr. Deary noted that several other studies have indicated that people's differences in a gene called Apolipoprotein E can contribute a small amount to cognitive aging. But links to other such genes have yet to be properly established.

Scientists elsewhere have determined that a person's intelligence level as measured by an IQ test isn't fixed at birth. Instead, a person's IQ can rise or fall as the person ages. One recent brain-scanning study found that a teenager's IQ can increase or decline by as many as 20 points in just a few years.

There also has been progress in figuring out which environmental factors may affect intelligence. For example, experiments have shown that IQ scores can change after just a few weeks of cognitive training, though the increases are small and tend to fade after a few months.

Meantime, a three-decade study done at the U.S. National Institute of Mental Health found that people whose jobs involved setting up elaborate systems or dealing with tough or complex relationships tended to do better over time on cognitive tests.

Schooling also has been shown to boost IQ, while music lessons have been associated with higher IQ throughout life. And brain-imaging studies have found that circus jugglers and London cab drivers who use maps can show brain changes linked to practice.

The study described in Nature was done by scientists at the University of Edinburgh, the University of Aberdeen and the University of Queensland in Australia.

Dr. Deary said the team's next step would be to examine whether there are "links between genetic factors and brain structure (which we have from brain imaging) and whether this in turn links to cognitive change."

Write to Gautam Naik at

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