Low cognitive functioning in nondemented 80+-year-old twins is not heritable

Late-onset dementia is moderately heritable, but little is otherwise known about the origins of low cognitive functioning very late in life. We investigated genetic influence of low cognitive functioning in 200 pairs of 80+-year-old twins identified as nondemented. Twin analyses of groups selected below the 40th percentile of cognitive functioning showed no genetic influence, suggesting that low cognitive functioning is due to nongenetic factors. Although the entire range of cognitive functioning shows moderate genetic influence, this masks very high heritability for the high end of the distribution and very low heritability at the low end. Preclinical dementia and/or terminal decline may account for nonsignificant heritability at the low end of ability. These results have implications for molecular genetic attempts to identify genes associated with late-onset dementia. The results should also stimulate the search for nongenetic factors responsible for low cognitive functioning in late life, a critical aspect for quality of life.