Testing an expanded theory of planned behavior model to explain marijuana use among emerging adults in a promarijuana...

Opinions about marijuana use in the United States are becoming increasingly favorable, making it important to understand how psychosocial influences impact individuals' use in this context. Here, we used the theory of planned behavior to examine the influence of initial attitudes, norms, and efficacy to resist use on initial intentions and then to examine the effect of initial intentions on actual marijuana use measured 1 year later using data drawn from a community with relatively high use. We expanded the traditional theory of planned behavior model by investigating 2 types of normative influence (descriptive and injunctive) and 2 types of intentions (use intentions and proximity intentions), reasoning that exposure to high use in the population may produce high descriptive norms and proximity intentions overall, but not necessarily increase actual use. By contrast, we expected greater variability in injunctive norms and use intentions and that only use intentions would predict actual use. Consistent with hypotheses, intentions to use marijuana were predicted by injunctive norms (and attitudes) and in turn predicted marijuana use 1 year later. By contrast, descriptive norms were relatively high among all participants and did not predict intentions. Moreover, proximity intentions were not predictive of actual use. We also found that increasing intentions to use over a 1-year period predicted greater use. Given the greater efficacy of theory-based as compared with non-theory-based interventions, these findings provide critical information for the design of successful interventions to decrease marijuana-associated harms.