Longitudinal data from a large sample of twins participating in the Netherlands Twin Register (n = 42,827, age range 3-16) were analyzed to investigate the genetic and environmental contributions to childhood aggression. Genetic auto-regressive (simplex) models were used to assess whether the same genes are involved or whether new genes come into play as children grow up. The authors compared 2 different simplex models to disentangle potentially changing behavioral expressions from changes in genetic and environmental effects. One model provided estimates of genetic and environmental effects at the level of individual aggression questionnaire items, and the other model assessed the effects at the level of an aggression sum score computed from the individual items. The results from both models provided evidence for largely stable genetic effects throughout childhood. The results also highlighted the differential heritability of the different indicators of aggression measured with the Childhood Behavior Checklist, with destruction of property showing a very high genetic component during early childhood and fighting behaviors being more heritable in early adolescence.