Whether we become dotards or quick-witted retirees appears to have more to do with our genes than years of schooling or experience. That startling conclusion, reported in tomorrow's issue of Science,* upsets the widely held view that life's lessons play a dominant role in determining an elderly person's cognitive ability.
Excluding the ravages of Alzheimer's disease, gerontologists have assumed "that environmental factors play the larger role" in intelligence in old age, says geneticist Gerald McClearn of Pennsylvania State University. To put this theory to the test, McClearn and his colleagues measured cognitive ability in 240 pairs of Swedish fraternal and identical twins in their 80s. Identical twins share identical copies of genes, while fraternal twins have half their genes in common. Thus, for traits in which "genetic influence is important, one can predict that the identical twins would be more similar than fraternal twins," says co-author Robert Plomin, a behavioral geneticist at the Institute of Psychiatry in London. On the other hand, he says, fraternal twins would resemble each other more if a trait were largely shaped by environmental factors.
The researchers put the twins through a battery of intelligence tests in which the subjects had to offer synonyms of words, match color cubes with patterns on cards, and recognize pictures shown earlier. Pairs of identical twins scored much more similarly than did fraternal pairs, suggesting a strong genetic influence. Even after age 80, the scientists estimated, about 62% of a person's general cognitive ability is inherited.
This is a "landmark" study, says Irving Gottesman, a behavioral geneticist at the University of Virginia, Charlottesville, in that it contradicts the conventional wisdom that nurture overrides nature over time. It appears, he says, that "genetic factors are at least as important in 80-year-olds as they are in populations aged 15 to 65."
The next step is to try to identify genes involved in intelligence. "If we knew some of the specific genes, we could study whether the same genes relate to cognitive abilities throughout the life-span," Plomin says. Such knowledge someday may allow physicians to better identify those at risk for cognitive decline late in life.