Heritability of cognitive performance in aging twins. The National Heart, Lung, and Blood Institute Twin Study
G. E. Swan, D. Carmelli, T. Reed, G. A. Harshfield, R. R. Fabsitz and P. J. Eslinger
SRI International, Menlo Park, CA 94025.
The genetic contribution to performance on scales designed to measure mild
to moderate decrements in cognitive functioning in a population at risk is
unknown. In the present analysis, 134 monozygotic and 133 dizygotic male
twin pairs (mean age, 63 years) were given three cognitive tests: the
Mini-Mental State examination, the Iowa Screening Battery for Mental
Decline, and, for comparison, the Digit Symbol Substitution Test from the
Wechsler Adult Intelligence Scale. The primary objective of the analysis
was to test for a significant heritable component to performance on these
measures. A secondary objective was to determine the extent to which shared
variance with significant confounders such as education, age, and
depression affects the outcome of the heritability analysis. Results
indicate that performance on tests intended to measure cognitive decline in
the elderly does have a significant genetic component and that these
estimates tend to increase after adjustment for covariates. Heritability
estimates adjusted for covariates were 30% for the Iowa Screening score,
60% for the Mini-Mental State score, and 67% for the Digit Symbol
Substitution score.
