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The maturity principle and cumulative continuity principle have been fundamental in understanding adult development of personality traits. We tested new predictions derived from both principles for Honesty-Propriety, a factor from the newer Big Six model, as well as for the original Big Five domains. We also tested hypotheses about lower-order Big Five aspects, gender differences in change, and a plateau in rank-order stability in early and middle adulthood. Data came from a national sample of U.S. adults (N = 858) who were assessed via both self- and informant reports in an accelerated longitudinal design with four annual waves. Consistent with the maturity principle, self-reports of Honesty-Propriety increased with age; changes in informant reports were directionally similar but nonsignificant. We also replicated previous findings supporting the maturity principle: Agreeableness, Conscientiousness, and Emotional Stability all increased with age. Analyses of lower-order aspects showed largely similar patterns of change at a different level of the personality taxonomy. Women increased in Emotional Stability more than men did. In tests of the cumulative continuity principle, we did not find evidence of increasing rank-order stability from age 18 to 58. This suggests a modification to the cumulative continuity principle in which rank-order stability is relatively unchanging across this period of life, what we term the working years plateau.