A new study of Swedish twins who are 80 years of age or older shows that individual differences in how they acquire and process knowledge (cognition) relies as much on genetic inheritance patterns as on environmental factors. The study, which was funded by the National Institute on Aging (NIA), is the first to look at the genetic influence (heritability) on many different aspects of cognition in older people, and confirms patterns that have emerged from similar studies in younger and middle-aged people. Because cognitive function plays a crucial role in determining the quality of life for older people, understanding how cognition develops as people age could lead to beneficial interventions that might slow or reverse cognitive decline.
The study, the cover story of the June 6,1997 issue of Science*, was headed by NIA grantee and National Advisory Council on Aging member Gerald E. McClearn, Ph.D., of the Center for Developmental and Health Genetics at Pennsylvania State University. Dr. McClearn states that "in the group of Swedes that we studied, even the effects of over 80 years of environmental influence didn't eliminate the impact of heredity on cognitive ability."
A wide range of environmental variables such as geography, education, socioeconomic status, nutritional habits, occupation, disease and stress exposure might be expected to have substantial influences on cognition. Over the course of a lifetime, twins exposed to differing environments might be expected to display wide variances in cognition. Yet given the cumulative impact of a lifetime of environmental disparities, this study shows that the effects of environment on cognition are barely equal to the effects that genetic heritability has on cognition.
Dr. McClearn and colleagues' research is unique in that it looks, in people age 80 and older, at general (intellectual ability) and specific (spatial, verbal, and memory) cognition, and examines in detail each of the 3 separate areas of specific cognition. Previous twin studies have shown that general cognitive abilities are among the most heritable behavior traits, with heritable influence increasing from 20 percent at infancy to 60 percent in adulthood. This finding contradicts the commonly held assumption that environmental influence increases throughout the lifespan with a corresponding decrease in genetic influence. The present study shows that the relative contributions of genetics and environment -- about half and half -- extends into very advanced age.
Investigators in this study were able to utilize the Swedish Twin Registry, which has tracked 96 percent of all twins in Sweden. The study utilized 240 sets of these twins born before the start of World War I. They were an average age of 83 years old. To assess cognitive abilities, twins were tested by licensed nurses using tests for verbal meaning, figure logic, block design, and picture memory. Analysis of combined scores of cognitive ability showed that heritability accounted for 55 percent of the individual differences in ability, a result similar to that seen in people who are middle-aged. The heritable impact on specific cognitive abilities, something little studied previously, was somewhat less than 50 percent but still highly significant. For both general and specific cognitive abilities, identical twins, as would be expected, showed much stronger similarities than did fraternal twins. Additionally, living together or sharing the same environment in later life did not account for any significant similarity or dissimilarity of environmental impact on cognition.
According to Dr. McClearn, "it is now becoming possible to identify specific genes which may be responsible for some of the differences in cognitive abilities." For example, certain forms of the ApoE gene have been associated with cognitive decline in older people, particularly in those with Alzheimer's disease.
Dr. Jared B. Jobe, Chief of the Adult Psychological Development Branch of the NIA, suggests that "the next step to better understanding genetic influences on cognition in older people is to conduct additional long-term studies on twins as well as studies using siblings and population-based samples. Discovering how we learn in old age could lead to a better understanding of how people can remain active and involved in society up to the very end of their lives."
Co-authors on this study with Dr. McClearn were Boo Johansson, Ph.D. and Stig Berg, Ph.D., of the Institute for Gerontology at the University College of Health Sciences in Jonkoping, Sweden, Nancy L. Pedersen, Ph.D., at the Institute for Environmental Medicine of the Karolinska Institute in Stockholm, Sweden, Frank Ahern, Ph.D., at Pennsylvania State University, and Stephen A. Petrill, Ph.D. and Robert Plomin, Ph.D., of the Social, Genetic and Developmental Psychiatry Research Centre at the Institute of Psychiatry in London, England.
*GE McClearn, B Johansson, et al., Substantial genetic influence on cognitive abilities in twins 80+ years old, Science, 276, 1560-1565 (1997).
The National Institute on Aging, one of the 18 Institutes that make up the National Institutes of Health, leads the Federal effort supporting basic, clinical, epidemiological and social research on aging and the special needs of older people.
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