There was a significant change in fit when the CPM was compared against the IPM (Δχ2=17.47, df=8, p=.03). However, we proceeded with fitting the CPM because: (1) we aimed to be consistent with the modeling approach taken for the age of substance use initiation, and (2) this was necessary to facilitate model-fitting for the primary goal of this analysis: to examine the extent of overlap in the genetic and environmental influences on initiation and disorder. Furthermore, additional fit indices indicated that the CPM fit the data very well (RMSEA=.01, CFI=.994, TLI=.995), and the AIC value was slightly lower for the CPM than the IPM (CPM: AIC=46211.44, IPM: AIC=46212.21). Therefore, we fit a CPM to the data. Full model estimates for the IPM are presented in Table S8 in supplemental materials.
