In addition to modeling trajectories of externalizing problems, we also examined many risk factors as predictors of individuals’ trajectories. Risk factors tend to co-occur, so considering them together may provide a more accurate estimate of the unique contributions of individual risk factors. It is also important to consider the timing of risk factors to clarify the developmental process. Some risk factors such as child maltreatment appear to have stronger effects on externalizing problems in later than early childhood (Keiley, Howe, Dodge, Bates, & Pettit, 2001), whereas many other risk factors appear to have stronger effects earlier in childhood (Appleyard, Egeland, Van Dulmen, & Sroufe, 2005). In their review, Dodge, Coie, and Lynam (2006) highlighted the importance of genetics, child temperament, language ability, pregnancy complications, poverty, non-family child care, family processes, and peer processes in the etiology of externalizing behavior problems. They also observed, however, that these risk factors may be markers of other causal processes or may mediate the effects of each other (e.g., poverty may influence externalizing behavior in part via its effect on parenting). To clarify the independent
