WEDNESDAY, Jan. 18, 2012 (HealthDay News) -- Most people have met some 65-year-olds who aren't as "with it" as they once were, and some 90-year-olds who are still sharp as a whip.
What accounts for the differences? Using a genetic analysis and intelligence tests taken in a group of people in childhood and old age, researchers concluded both genes and environmental factors play a role in whether you'll maintain your level of intelligence throughout your lifespan.
"We estimated that about a quarter of the lifetime changes in intelligence test scores might be due to genetic factors," said lead study author Ian Deary, a professor of differential psychology and director of the University of Edinburgh Centre for Cognitive Ageing and Cognitive Epidemiology, in Scotland.
The study appears in the Jan. 18 online edition of Nature.
Maintaining brain health into old age is a key to aging well, including the ability to do everyday tasks and stay independent, according to background information in the report. Plenty of prior research has found that how smart you are in adolescence generally carries over into adulthood and old age.
And yet, "some people's intelligence ages better than others," Deary and colleagues noted.
In the study, investigators used genome-wide association data on 1,940 unrelated people in Scotland, along with information from intelligence tests participants took when they were about age 11, and then again between 54 and 68 years later, when they were either 65, 70 or 79.
Genome-wide association research "involves rapidly scanning markers across the complete sets of DNA, or genomes, of many people to find genetic variations," the U.S. National Human Genome Research Institute explains.
Deary's team looked specifically for differences in bits of DNA called SNPs (single nucleotide polymorphisms) associated with people whose intelligence either declined or stayed stable.
"We were able to make estimates of the genetic contribution to intelligence differences in childhood and old age, and the change between these times, in the same people," Deary said. "What was novel about these estimates was that they were made from actual testing of DNA, not from twin or adoption studies."
S. Duke Han, an assistant professor in the department of behavioral sciences and a clinical neuropsychologist at Rush University Medical Center, in Chicago, said the study is unique in that it was able to measure intelligence in the same group of people over such a long period of time.
"What this is saying is something many researchers have accepted for a long time, that intelligence seems to be very much influenced by genetic makeup but also environmental factors," Han said.
Those environmental factors may include things that influence cardiovascular health, exposures to toxins and education, among other factors.
In analyzing the data, the researchers were able to make broad inferences about how much genetics played a role in maintaining intelligence over the lifespan, but they weren't able to identify specific genes or gene variants that might contribute.
In general, there's a "paucity of data about genetic influences on lifetime cognitive change," with a few exceptions, the study authors noted. The APOE4 mutation, for example, is a risk factor for severe cognitive decline associated with Alzheimer's disease.
The U.S. National Institute on Aging has more on aging issues.