Our finding that those with internalizing personality problems are more likely to be chronic users of both tobacco and marijuana (data not shown) supports the relationship between ��distress�� and chronic polysubstance use. The externalizing factor appears to be more influential than the internalizing factor (Table 4). This is in accord with previous research on the effect Ixazomib proteasome of such personality factors on trajectories of use of only one drug (Windle & Wiesner, 2004). The externalizing personality attributes partially reduced the comorbidity of pairs of trajectories of tobacco and marijuana use, despite our small sample size. In future research using larger samples, the relationship of a variety of biopsychosocial factors to pairs of comorbid trajectories of tobacco and marijuana use may be even more apparent.

Peer factors partly reduced the ORs of the comorbidity of pairs of trajectories of tobacco and marijuana use (i.e., chronic tobacco use and maturing-out marijuana use and chronic tobacco use and chronic marijuana use). One possibility for this effect is that deviant peers have an adverse effect on Peer Substance Use through socialization processes (Hoffman et al., 2007). Moreover, substance-using individuals seek out peers with similar behaviors (Ennett & Bauman, 2006). In general, these findings are in accord with FIT, which emphasizes the influence of peers through such processes. There were no major differences in Ethnicity or Gender in the risk and protective factors related to pairs of comorbid trajectories of use, in accord with Jackson et al. (2008).

The results have considerable generalizability for male and female and for Black and Puerto Rican individuals. Clinical programs designed to deal with the comorbidity of tobacco and marijuana use might be similar for both Blacks and Puerto Ricans. Nevertheless, as Compton, Grant, Colliver, Glantz, and Stinson (2004) have noted, interventions need to be linguistically appropriate and culturally relevant. Limitations First, it remains possible that the associations between the predictors and pairs of comorbid trajectories may arise from genetic risk factors and other environmental variables (e.g., school influences) that were not examined in this study. Second, our data are based on self-reports rather than on external measurements from official records, such as police records, though studies have shown that use of this type of self-report data yields reliable results (Harrison, Martin, Enev, & Harrington, 2007).

Third, the sample sizes for some of the comorbid trajectory pairs are limited in size. Given the relatively small N, and the restriction of our sample to New York City, the study of additional samples is warranted. Despite these Cilengitide limitations, the study supports and extends the literature in a number of important ways. We assess psychosocial variables over a span of 12 years.