We detailed hypotheses from intrinsic and social maturation perspectives, some of which our results confirmed and some of which they did not. The FFT correctly predicted that phenotypic and genetic stability would peak near age 30 and that genes would represent the primary contribution to stability across the lifespan. Discrepancies with the FFT were found in the increasing stability of, and variance accounted for by, environmental effects. As originally reported in a classic cross-sectional meta-analysis of twin studies (McCartney et al., 1990), heritability of personality decreased with age, and environmental influences increased. Moreover, environmental influences increased in stability across development, evincing impressive levels in adulthood. Further, the increase in phenotypic stability could largely be explained by increasing environmental contributions. These results of increasing environmentality, increasing environmental stability, and increasing contributions of environmental factors to phenotypic stability, were predicted by the SPMs. Deviations from SPMs were observed in the near perfect stability of genetic effects after age 30. This suggests that the environmental changes typically experienced in adulthood do not meaningfully influence gene expression relevant to personality.
