A Scottish study that began 80 years ago has shown that genes may play a role in maintaining intelligence till old age. The study participants joined when they were 11 years old. Researchers have long been interested in understanding how cognition changes with age, and why these changes are more rapid in some people than in others. But, in the past, studies of age-related intelligence changes were often performed when the subjects were already elderly.
Then in the late 1990s, research psychologist Ian Deary of the University of Edinburgh, UK, and his colleagues realized that Scotland had two data sets that would allow them to take such studies a step further. In 1932 and 1947, officials had conducted a sweeping study of intelligence among thousands of 11-year-old Scottish children. The data, Deary learned, had been kept confidential for decades.
The team then tracked down the original participants, many of whom did not remember taking the original tests. The team collected DNA samples and performed fresh intelligence tests in nearly 2,000 of the original participants, then aged 65 or older.
Previous analyses of the team's data had shown that childhood intelligence correlated well with intelligence in old age. “But it’s not a perfect relationship,” says Deary. “Some people move up the list and some move down.” In short, some people’s intelligence 'ages' better than others. So Deary and his colleagues set out to discover why.
They looked at the DNA samples they had collected for the presence of more than half a million common genetic variants, each affecting only a single letter in the DNA sequence of the genome. The team then calculated whether these variants were associated with cognitive stability — how well intelligence had been maintained over time.
Although the sample size of 2,000 people was too small to grant the statistical power needed to track down individual genetic signatures associated with cognitive stability. But it was enough to estimate how much genetics contributes to cognitive ageing. Researchers found genetic factors account for 24% of changes in intelligence over a person's lifetime, meaning that the biggest effect was environmental. The findings, published in the journal Nature, also suggest that many of the genes that affect intelligence in childhood also influence intelligence in old age.
Professor Deary, said, “Until now, we have not had an estimate of how much genetic differences affect how intelligence changes across a lifetime. These new findings were possible because our research teams were able to combine a range of valuable resources. The results partly explain why some people's brains age better than others. We are careful to suggest that our estimates do not have conventional statistical significance, but they are nevertheless useful because such estimates have been unavailable to date.”
Australian co-author Professor Peter Visscher, from the University of Queensland, said, “Unique data and new genome technologies combined with novel analysis methods allowed us to tackle questions that were not answerable before. The results also strongly suggest how important the environment is helping us to stay sharp as we age. Neither the specific genetic nor environmental factors were identified in this research. Our results provide the warrant for others and ourselves to search for those.”
Some of the individual genetic variants that affect life-long intelligence may turn up as researchers search for the genetic underpinnings of degenerative diseases, says David Goldstein, a geneticist at Duke University Medical Center in Durham, North Carolina. Meanwhile, Stern notes that Deary and his team may soon have access to more data, as participants in other large, long-term studies, such as the UK National Survey of Health and Development, come of age. “We don’t know what is going to be required to find these genetic influences,” says Goldstein, “but the point is knowing that they really exist motivates us to search for them.”
Professor James Goodwin, from the charity Age UK, which funded the study, said, “This research is extremely exciting as it provides a greater understanding about why mental abilities change throughout our lifetime. It is also incredibly positive as it suggests that we can have a real influence on how our brain ages through our lifestyle and other external factors. The key now is to establish which lifestyle and environmental factors are most important so that we are able to do all we can to maximize our chances of ageing well.”
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