A twin study of the genetic contribution to age-related functional impairment

A key element in the quality of later life is the prevalence of age-related functional impairments. The objective of this study was to quantify the genetic and environmental influences on age-related functional impairment in a population of white male twin elders who were fit in young adulthood when entering military service. The extent of genetic influence on functioning in later life affects the role of public health, personal initiative, and service interventions.

Indicators of functional impairment were determined by telephone survey and by twin pair responses to 10 indicators of basic, instrumental, and social activities, and mobility. Responses were analyzed using structural equation modeling. Prevalence and concordances were determined by zygosity status. Covariance was partitioned between twins in a pair into components attributable to additive genetics, common environment, and unique environment.

Data from 2721 twin pairs (1384 monozygotic and 1337 dizygotic) were analyzed for the 10 dichotomous indicators of functional impairment and for a subscale of 8 of these indicators. For the subscale, additive genes accounted for approximately 21% of covariance in liability for a higher score, whereas unique environment accounted for approximately 78% of variance, with age accounting for a very small proportion. In two indicators there were nontrivial effects of common environment.

Within the expressed limits on generalization, the study findings suggest a major potential role for interventions aimed at a person's unique environment to maintain good functioning in aging and to lengthen the period of active life. Genetic effects play a modest but also important role in age-related functional impairment.