Individual differences in patterns of thoughts, feelings, and behavior tend to stabilize over development. Along with increases in phenotypic stability, genetic and environmental influences both increase in stability with age. Near age 30, genetic stability approaches unity, and true environmental stability slowly increases across the majority of the lifespan to reach similar levels of stability in old age. The genetic and environmental decomposition of phenotypic stability is likely the most surprising and informative finding of the present study. Genetic influences exert a relatively constant influence on stability across the lifespan and fully explain phenotypic stability at young ages. Environmental contributions to stability, on the other hand, are almost nonexistent in early childhood, but by midlife the environment contributes only slightly less to phenotypic stability than do genetic influences. This result indicates that the trend of increasing phenotypic stability can largely be explained by increasing environmental contributions. Previous research has identified instances when the influence of the environment on personality depends on genotype and instances of genetic predispositions leading to nonrandom exposure to the environment. A crucial next step for personality theory and research will be to document how the effects of such gene-environment interplay carries forward in time.
