Genetic influences on cognitive functions in the elderly: a selective review of twin studies

In this review, we examined the published reports on the heritability of cognitive functioning in old age. Twenty-four papers from five study centers, comprising of participants with a mean age of 65 years and above were examined. The comparability of findings from different studies was compromised by the use of different measures for the same cognitive domain, and with large scale twin studies in cognitive aging limited to a few Scandinavian countries. While the results from cross-sectional samples appear to lend support for the notion that heritability of cognitive functions decreases in the elderly, the findings are best considered inconclusive. Longitudinal reports show little evidence for genetic effects, but an increase in unique environmental influences on the rate of cognitive change as age increases. In relation to the two prominent theories of cognitive aging, the genetic influence on processing speed as a major contributor to cognitive aging has been indicated in three reports, whereas the genetic relationship between executive functions and other cognitive functions has not been explored. Only two studies have focused on sex difference and did not find sex-specific genetic influence in cognitive abilities. This review indicates that there are complex relationships between heritability, environmental influence, and cognitive functions in the elderly. It highlights the need for more research, with consistent and appropriate cognitive measures, with data obtained from larger and more geographically and culturally diverse twin samples.