This longitudinal study examined consistency and change in human personality. Rather than studying the consistency of individual differences over time, an approach which considers only the stability of a variable at the group level and ignores differences in stability at an individual level, this study used a person-centered approach to study consistency over time of an individual's personality profile. Subjects were 100 individuals who participated in an experimental study in 1975, at 1 year of age. Personality descriptions were obtained at age 7, 10, 12 and 20 years. Measures included the California Child Q Set (California Q Set for age 20), the Symptom Checklist, the National Youth Survey, and a peer acceptance measure using sociometric procedures or a social network interview (age 20). Findings indicated that consistency of personality is related to ego resiliency and to supportive relationships with peers. Resiliency and peer acceptance predicted later consistency of personality, and personality consistency predicted later resiliency and perceived peer support. Personality consistency as perceived by peers predicts later externalizing problems, especially violent behavior and excessive use of alcohol. Self-reported personality consistency predicted later internalizing problems, especially in the areas of sensitivity and distrust in personal relations and in feelings of hostility towards others. From a transactional perspective, results suggested that personality consistency from childhood to young adulthood is accompanied by social competence and by adequate social relationships. (Contains 19 references.) (KDFB)