Stephen A. Petrill1, Boo Johansson2, Stig Berg2Robert Plomin3, & Gerald E. McClearn4
Numerous studies have examined the genetic and environmental influences on cognitive ability in the elderly. Many of these studies have attempted to explain either the individual differences across the range of ability, or the genetic and environmental etiology of groups selected for dementia. The current study examines intermediate groups of non-demented twins selected for low general cognitive ability. The current study is based upon the more than 350 80+ year-old twin pairs participating in the OctoTwin Study. Low-g groups were formed at or below the 10%tile, 20%tile, 30%tile, 40%tile. DeFries-Fulker DF (J. C. DeFries, and D. W. Fulker. 1985. Behav. Genet. 15, 467-473; 1988. Acta Genetica, Med Gemellol, Twin Res. 37, 205-216.) analyses suggest that genetic influence is nonsignficant in groups at or below the 40%tile, dropping from a group heritability of .51 at the 40%tile, to .22 at the 30%tile, to .00 at or below the 20%tile. As a comparison, groups at or above the 60%tile, 70%tile, 80%tile, and 90%tile were also ascertained. In contrast to the low-g results, DF analyses above the population mean suggest statistically significant genetic influences, with group heritability exceeding .74 in all groups selected at or above the 60%tile. These results suggest that general cognitive ability may be genetically influenced in the oldest-old above the population mean, but may be more environmentally influenced below the population mean. Implications are discussed.
Address: Department of Psychology, Wesleyan University, 207 High Street, Middletown, CT 06459-0408, (860) 685-2602 phone, (860) 685-2761 fax, spetrill@wesleyan.edu
1Department of Psychology, Wesleyan University 2Institute of Gerontology, Jonkoping, Sweden 3Social Genetic and Developmental Psychiatry Research Centre, Institute of Psychiatry, London 4Center for Developmental and Health Genetics, The Pennsylvania State University 1Supported by NICHD Postdoctoral Fellowship (HD-27088) at the Pennsylvania State University The OCTO Twin Study is supported by a grant form the National Institute on Aging (NIA: AG 08861).