Substantial Genetic Influence on Cognitive Abilities in Twins 80 or More Years Old, Gerald E. McClearn, Boo Johansson, Stig Berg, Nancy L. Pedersen, Frank Ahern, Stephen A. Petrill, Robert Plomin, 1997.
General and specific cognitive abilities were studied in intact Swedish same-sex twin pairs 80 or more years old for whom neither twin had major cognitive, sensory, or motor impairment. Resemblance for 110 identical twin pairs significantly exceeded resemblance for 130 fraternal same-sex twin pairs for all abilities. Maximum-likelihood model-fitting estimates of heritability were 62 percent for general cognitive ability, 55 percent for verbal ability, 32 percent for spatial ability, 62 percent for speed of processing, and 52 percent for memory. There was also evidence for the significant influence of idiosyncratic experience as the environmental component that most determines individual differences in cognitive abilities late in life.
Table 1 presents means, standard deviations, and correlations with age and gender for the seven tests. The means and standard deviations indicate a wide range of variability for each of the tests. Age accounts for a significant amount of variance in performance on six of the seven cognitive tests for these individuals from 80 to 97 years of age. Gender is significantly related to only one of the tests. Because age and gender effects inflate twin resemblance for same-sex twins, all scores were adjusted for age and gender with a multiple regression procedure (29).
Factor analyses of these diverse tests yielded a strong general cognitive ability factor that accounted for 50% of the variance, consistent with other studies in middle age (16). All tests correlated above 0.52 with the first principal component used as an index of general cognitive ability. Because such principal component scores can be assigned only for subjects with complete data on all tests (52 identical and 65 fraternal twin pairs), we also indexed general cognitive ability with a short form of the WAIS (30), which adds scores on Information and Block Design (90 identical and 104 fraternal twin pairs). We also examined specific cognitive abilities constructed by standardizing and adding tests to represent verbal ability (Verbal Meaning and Information; 78 and 93 pairs of identical and fraternal twins, respectively), spatial ability (Figure Logic and Block Design; 86 and 89 twin pairs, respectively), speed of processing (Symbol Digit; 73 and 92 twin pairs); and memory (Digit Span and Picture Memory; 63 and 82 twin pairs). In addition to conducting model-fitting twin analyses using only pairs for whom data were available for both twins, we also employed pedigree analysis (31), which utilizes all available information, including data in which one twin is missing. We also analyzed pairs with complete data for comparison purposes.
Distributions of scores for general cognitive ability as indexed by the first principal component (PC; Fig. 1A) and the WAIS short form score (Fig. 1B) were nearly normal, especially for the WAIS short form. Similar distributions were found for the scales of specific cognitive abilities. For general and specific cognitive abilities, identical twin correlations were significantly greater than fraternal twin correlations, indicating genetic influence (Fig. 2). Twin correlations were similar when only those twin pairs with complete data on all tests were included (32). The data were submitted to standard maximum likelihood model-fitting analysis for twin data (33) to estimate genetic and environmental components of variance. We report model-fitting results using data for pairs for whom data were available for both twins, although similar results were obtained when we employed pedigree analysis (34).
Estimates of heritability — the proportion of total phenotypic variance attributable to genetic variance — and their 95% confidence intervals were 62% (29 to 73%) for general cognitive ability as indexed by the PC and 53% (19 to 76%) as indexed by the WAIS short form, 55% (24 to 81%) for verbal ability, 32% (0 to 58%) for spatial ability, 62% (29 to 73%) for speed of processing, and 52% (7 to 67%) for memory ability (Fig. 3). In all cases, dropping the genetic parameter from the model resulted in a significant reduction in the fit of the model, demonstrating the significance of the heritability estimates. If variance due to error of measurement (about 10%) was removed from the total phenotypic variance, heritability estimates would account for a larger proportion of the remaining phenotypic variance.
The shared environment parameter, or twin resemblance not explained by heritability, accounted for 11% (0 to 47%) of the variance for PC and 15% (0 to 43%) for WAIS general cognitive ability, 20% (0 to 47%) for verbal ability, 13% (0 to 48%) for spatial ability, 0% (0 to 27%) for speed of processing, and 0% (0 to 32%) for memory ability. Dropping the shared environment parameter from the model did not result in a significant reduction of fit for any of the cognitive abilities, indicating that shared environment does not account for significant variance. However, the classical twin method of comparing identical and fraternal twin correlations is not a powerful design for detecting shared environment influence (35). Most of the nongenetic variance is due to nonshared environment and error of measurement. If the measures are about 90% reliable, then the amount of variance resulting from nonshared environment would be 17% for PC and 22% for WAIS general cognitive ability, 15% for verbal ability, 45% for spatial ability, 28% for speed of processing, and 38% for memory.
Although genetic influence on cognitive functioning late in life appears to be substantial, these data also demonstrate considerable environmental influence. About 40% of the variance for general cognitive ability and even more of the variance for specific cognitive abilities is environmental in origin. Moreover, consistent with results from studies of younger adults, our results indicate that, for the most part, these environmental influences are not shared by twins growing up in the same family nor are they due to adult experiences shared by twins. In other words, environmental influences that contribute to individual differences in cognitive abilities are those that make family members, in this case twins, different (36). The most direct evidence for this conclusion is that identical twin correlations are considerably less than the reliability of the measures (usually given as 0.80 to 0.95), even though identical twins are genetically identical. Differences within pairs of identical twins provide a tool with great potential for identifying these non-shared environmental factors.