Objective: To investigate the relative proportion of genetic and environmental contributions to verbal memory in community-dwelling World War II veteran twins. Design: The California Verbal Learning Test (CVLT) was administered to 94 monozygotic (MZ) and 89 dizygotic (DZ) elderly male twin pair participants in the fourth examination of the National Heart, Lung, and Blood Institute Twin Study. Setting: Subjects voluntarily participated on an outpatient basis at a research or medical center facility in 1 of 4 sites in the United states. Participants: Subjects had a mean age of 71.8 years (SD, 2.9 years), a mean educational level of 13.6 years (SD, 2.8 years), and no history of stroke and/or a Mini-Mental State Examination score of 23 or greater. Main Outcome Measures: Twin pair similarity in performance on 4 factor analytically derived components of the CVLT measuring verbal learning and memory, response discrimination, learning strategy, and recognition memory. Results: The MZ intraclass correlation was significantly larger than the DZ correlation for verbal learning and memory (I<.001) but not for the other 3 components of memory. Using maximum likelihood methods, the best- fitting genetic model indicated that verbal learning and memory has a substantial genetic component (56% of total variance), whereas response discrimination has a much smaller, although still detectable, genetic component (24% of total variance). There is no evidence of genetic influence on learning strategy or recognition memory. Conclusion: Differential contribution of genetic and environmental influences to specific components of memory suggest that, in this group of elderly male twin pairs, some components may be more amenable to intervention than others.
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