Cohort-sequential latent growth modeling was used to analyze longitudinal data for children's externalizing behavior from four overlapping age cohorts (4, 5, 6, and 7 years at first assessment) measured at three annual time points. The data included mother and father ratings on the Child Behavior Checklist and the Five-Factor Personality Inventory and teacher ratings on the Hierarchical Personality Inventory for Children of 674 children of a proportional stratified general population sample. Results indicated a significant nonlinear decline in problem behavior from ages 4 to 9. Child benevolence was negatively related to initial levels of problem behavior. Higher scores on emotional stability in children corresponded to larger reductions of problem behavior over time. Parents' conscientiousness and emotional stability were negatively related to initial levels of externalizing problem behavior. At the age of 4 years, boys had higher initial scores than girls, but boys and girls followed a similar developmental trajectory.