We opted to model growth curves by treating individual differences in externalizing behavior trajectories as dimensional rather than categorical because we find the dimensional models richer. Moreover, there is evidence that externalizing problems are dimensional, not categorical (Coghill & Sonuga-Barke, 2012; Krueger, Markon, Patrick, & Iacono, 2005; Markon & Krueger, 2005; Walton, Ormel, & Krueger, 2011). Researchers have argued that theory provides stronger support for modeling the heterogeneity of developmental trajectories by allowing trajectories to differ continuously rather than categorically in order to describe individual trajectories more parsimoniously and accurately, both conceptually and empirically (Bauer, 2007; Little, Card, Preacher, & McConnell, 2009). Three recent studies found that externalizing problem trajectories are more accurately modeled dimensionally than with Moffitt’s (1993) subgroups (Walters, 2011, 2012; Walters & Ruscio, 2013). Moreover, Burt, Donnellan, Iacono, and McGue (2011) found that subdimensions of externalizing problems were more strongly predictive of later antisocial behavior than was the age of antisocial behavior onset, which has often been used in determining subtypes. These findings suggest that the identified subtypes differ quantitatively in degree of severity rather than qualitatively in kind.
