Elderly intellect may owe a lot to genes.
Even in people age 80 and over, genes strongly influence individual differences in intelligence, as measured by tests of cognitive abilities, contend psychologist Gerald E. McClearn of Pennsylvania State University in State College and, his colleagues.
The impact of DNA on mental acumen in octogenarians adds to evidence that genes affect intelligence more in adulthood than during childhood.
Studies of twins conducted within the research discipline known as behavioral genetics have spurred scientists' interest in picking apart DNA to find specific genes that contribute to individual variations in intellect, McClearn's group asserts in the June 6 Science.
The team's comparison of elderly, well-functioning identical and fraternal twins represents a "landmark study" of cognitive ability, writes Irving I. Gottesman, a psychologist at the University of Virginia in Charlottesville, in an accompanying comment. The data challenge a widespread assumption that genetic influences on intelligence decline as adults accumulate experience, he says.
However, some researchers question the assumption of behavioral geneticists that genetic and environmental influences on mental traits can be statistically cordoned off (SN: 12/7/91, p. 376). Critics also note that twin studies shed no light on the genetic mechanisms that underlie behavior.
The new investigation considered 110 pairs of identical twins, who possess matching sets of genes, and 130 pairs of fraternal, same-sex twins, who on average share half their genes. Most of the volunteers were between 80 and 89 years old; nine pairs were in their nineties. Each person successfully completed all or most of a 1-1/2-hour battery of tests that emphasized verbal, spatial, and memory skills and tasks calling for speedy mental manipulations.
Estimates of heritability--the proportion of individual differences in intelligence caused by genes--reached 62 percent for general cognitive ability (which corresponds to IQ), 55 percent for verbal ability, 32 percent for spatial ability, 52 percent for memory, and 62 percent for mental processing speed.
Previous twin studies had found that heritability for general cognitive ability rises from 40 percent in childhood to about 60 percent in young adulthood and around 80 percent at age 60.
The increased heritability for intelligence during adulthood suggests that inherited thinking capacities nudge people into environments that accentuate their particular genetic strengths, proposes psychologist Robert Plomin of the Institute of Psychiatry in London. Plomin, a coauthor of the Swedish twin study, is currently conducting a search for genes linked to performance on intelligence tests.
Heritability is difficult to interpret for intelligence or any other trait, remarks psychologist Elizabeth A. Bates of the University of California, San Diego. For instance, height has a high heritability, but average heights in the United States and Japan have risen dramatically in a few generations because of changes in nutrition--an environmental influence. The last 60 years have also witnessed large, poorly explained boosts in average IQ scores of people living in many nations.
"It's very hard to separate genetic from environmental influences on behavior with any precision," asserts psychologist Thomas R. Zentall of the University of Kentucky in Lexington. "There's a lot of uncertainty involved in behavior genetics."
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||wisdom may come from life experiences, but smartness comes from heredity|
|Article Type:||Brief Article|
|Date:||Jun 7, 1997|
|Previous Article:||Patients savor this brain disorder.|
|Next Article:||No bones about it: gene vital to skeleton.|