Substantial genetic influence on cognitive abilities in twins 80 or more years old

Science

Volumn 276, Issue 5318, 1997, Pages 1560-1563

  McClearn, Gerald E ^a   Johansson, Boo ^a   Berg, Stig ^a   Pedersen, Nancy L ^a   Ahern, Frank ^a   Petrill, Stephen A ^a   Plomin, Robert ^a  

^a NONE

Author keywords

[No Author keywords available]

Indexed keywords

ARTICLE; COGNITION; DIZYGOTIC TWINS; GENETICS; HERITABILITY; MEMORY; PRIORITY JOURNAL; VERBAL BEHAVIOR;

AGED; AGED, 80 AND OVER; AGING; COGNITION; ENVIRONMENT; FEMALE; HUMANS; INTELLIGENCE; INTELLIGENCE TESTS; LIKELIHOOD FUNCTIONS; MALE; REGISTRIES; SWEDEN; TWINS, DIZYGOTIC; TWINS, MONOZYGOTIC;

EID: 0030976148     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.276.5318.1560     Document Type: Article

References (39)

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22. The Swedish Twin Registry was used to identify 947 same-sex twin pairs who were 80 years old or older and both alive during the 3-year period of testing that began in 1991. Research funds permitted us to contact 737 of these pairs. Of these 737 pairs, 188 pairs could not participate because one or both twins had died before the onset of testing and 198 pairs were excluded because one or both twins declined to participate, usually for reasons of frailty or incapacity. In-person testing was conducted for 351 intact pairs, although for 111 pairs one or both members of the pair were unable to complete the tests for reasons of suspected dementia (88 pairs) or major sensory or motor handicaps (23 pairs). Suspected dementia was diagnosed as described [American Psychiatric Association Diagnostic and Statistical Manual of Mental Disorders (American Psychiatric Association, Washington, DC, ed. 3 revised, 1987)]. The preliminary diagnosis was made on the basis of performance on a battery of tests for dementia and cognitive impairment, including the Mini-Mental State Examination [M. F. Folstein, S. E. Folstein, P. R. McHugh, J. Psychiatr. Res. 17, 189 (1975)]. A diagnostic work-up of all suspected dementia cases is under way. The final sample for these analyses of individual differences in normal cognitive functioning includes 240 pairs (110 identical pairs and 130 fraternal pairs) who were able to complete most of the tests in the 1.5-hour battery; 51% of these individuals 80 years old and older completed all of the tests. Zygosity was diagnosed from physical similarity information, with the exception of nine pairs for whom zygosity was uncertain; for these, DNA fingerprinting revealed one pair to be identical and eight pairs to be fraternal.
23. S. F. Simmons et al., J. Aging Health, in press. For example, although there is only a weak genetic contribution to longevity (2), it is possible that fraternal twins who both survive past 80 years of age are more similar in their cognitive functioning than fraternal pairs in which only one twin survives. This could lead to a lower estimate of heritability in analyses of intact pairs 80 or more years old, a hypothesis that could be tested in a longitudinal twin study that began before 80 years of age. The sample is also limited to the cohort born between 1894 and 1913 in Sweden.
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32. For twins with complete data on all tests, correlations were similar to those shown in Fig. 2. For example, for general cognitive ability as indexed by the WAIS short form, the monozygotic (MZ) and dizygotic (DZ) correlations in Fig. 2 were 0.68 and 0.41, respectively, and the correlations for twins with complete data on all tests were 0.73 and 0.41. The largest discrepancy, still far from statistically significant, emerged for speed of processing. In Fig. 2, the MZ and DZ correlations were 0.49 and 0.24, and the correlations for twins with complete data on all tests were 0.58 and 0.27.
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34. Pedigree analysis, which takes into account data from pairs in which only one member of a twin pair had a score on a given scale (31), yielded similar results to those shown in Fig. 3, which were based on twin variance-covariance matrices for cases in which both members of the pair had a score on a given scale. For example, the largest discrepancy in heritability estimates emerged for memory in which the Fig. 3 data yielded an estimate of 56%, whereas the estimate from pedigree analysis was 52%.
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39. We thank the twins and the research team at the Institute for Gerontology (B. Andersson, L. Ahlbäck, A. Carlholt, I. Cronholm, E. Georgsson, G. Hjalmarsson, B. Ljungquist, and A.-L. Wetterholm) for making the study possible. We are also grateful to E. Pyle at the Center for Developmental and Health Genetics for administering the research program. Supported by the National Institute on Aging (grant AG08861) of NIH.