Sources of influence on rate of cognitive change over time in Swedish twins: an application of latent growth models.

The use of latent growth models to examine influence on individual differences on ability level versus rate of change were examined for measures of fluid ability, memory, and perceptual speed in a sample of twins from the Swedish Adoption/Twin Study of Aging. Results indicated a larger amount of individual variation for average ability level (i.e., intercept) than rate of change (i.e., slope) for all three traits: Block Design, Thurstone's Picture Memory, and Symbol Digit. Generally, genetic influences were of greater importance to individual variation in ability level whereas variation for rate of change exhibited a larger environmental component. These findings support theories of increasing environmental influences with age. When genetic and environmental sources of covariation between educational attainment and pulmonary function with latent growth parameters were considered, the sources of covariation between the latent cognitive growth model parameters (i.e., intercept and slope) and both covariates were primarily genetic for ability level (intercepts) but environmental for rate of change (slopes). Such findings suggest that the forces important to timing or entry into cognitive decline may reflect stochastic processes or external environmental factors, primarily nonshared, that may differentially hasten cognitive decline in twins. These same forces may overlap with those that influence higher or lower educational attainment or those leading to better or worse pulmonary functioning.