The emergence of new behaviors and the reorganization of psychological structures are often attributed to critical events and crises in the life course. A fundamentally different perspective is offered: Potentially disruptive transitions produce personality continuity, not change. The behavioral responses of adolescent girls to the onset of menarche was studied in a longitudinal study of an unselected birth cohort. Predictions from 3 rival hypotheses about the relation between pubertal change and social psychological change were first tested: the stressful change, off time, and early-timing hypotheses. The results supported the early-timing hypothesis. Whether stressful, early menarche generated new behavioral problems or accentuated premenarcheal dispositions was then tested. The results supported an accentuation model: Stressful transitions accentuated behavioral problems among girls who were predisposed to behavioral problems earlier in childhood. Speculations are offered for a broader theory about the role of individual differences in the life course.