In this longitudinal study we examined the stability of general cognitive ability (GCA), as well as heterogeneity and genetic and environmental influences underlying individual differences in change. We investigated GCA from young adulthood through late midlife in 1,288 Vietnam Era Twin Study of Aging participants at ages ~20, ~56, and ~62 years. The correlations among the 3 occasions ranged from 0.73 to 0.85, reflecting substantial stability. The heritability was significant on each of the 3 occasions and ranged from 0.59 to 0.66. The influence of the shared environment was not significant at any of the ages. The genetic correlations across the 3 occasions ranged from 0.95 to 0.99 and did not differ significantly from 1.0. The nonshared environmental correlations ranged from 0.21 to 0.47. Latent growth curve analysis was applied to characterize trajectories over the 42-year period. Slope was significantly different from 0 and indicated that there was modest change over time. There was a significant genetic influence on initial level of GCA (h[superscript 2] = 0.67), but not change (h[superscript 2] = 0.23). Genetic factors primarily contribute to stability, while change reflects the influence of nonshared environmental influences. There was a significant negative correlation between initial level of GCA and change (r = -0.31). Latent class growth analysis identified 4 trajectories. In general, the 4 groups followed parallel trajectories and were differentiated mainly by differences in AFQT performance level at the time of military induction.