By Heidi Ledford of Nature magazine

A Scottish intelligence study that began 80 years ago has borne new fruit. Researchers have tracked down the study's surviving participants--who joined the study when they were 11 years old--to estimate the role that our genes have in maintaining intelligence through to old age.

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.

He and his colleagues set about tracking 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.

Stable inheritance

Deary and his team tested 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.

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 aging.

The team found that these variants accounted for nearly one-quarter of the differences in cognitive stability. The results were published online January 18 in Nature.

Identifying the DNA sequence changes that underlie that trait will be the next step. "Exactly what does genetic influence mean here?" asks Yaakov Stern, a neuropsychologist at Columbia University Medical Center in New York, who was not involved in the study. "How does that play out biologically?"

Stern, who has studied the impact of environmental influences such as education, job type and leisure activities on cognitive ageing, says that the "fascinating" Scottish data have provided an unusual glimpse into the heritability of age-related cognitive decline. Still, he notes, the genetic factors could also be interacting with the environment, making a person more or less susceptible to environmental effects on intelligence. "There is still a lot of room for environmental influence," he argues.

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 (see "Epidemiology: Study of a lifetime"), 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."

This article is reproduced with permission from the magazine Nature. The article was first published on January 18, 2012.