The theory of planned behavior: Precursors of marijuana use in early adolescence?☆
Introduction
The Netherlands has a tolerant policy with respect to marijuana use, which influences the Dutch social norm on marijuana use (Erikson et al., 2005). Not surprisingly, marijuana use among Dutch adolescents is highly prevalent. Twenty-five percent of all 16-year old Dutch adolescents have tried marijuana at least once (Monshouwer et al., 2008). Compared to other European countries, the Netherlands is among the top 10 countries for adolescent marijuana use (Hibell et al., 2009). Marijuana use at an early age can lead to detrimental consequences, such as distortion of brain development due to long lasting neurobiological changes (e.g., Pistis et al., 2004), and to an elevated risk for later dependence, misuse, and psychosis (Grant and Dawson, 1998, Moore et al., 2007). Given these adverse health effects, it is pivotal to identify the youngsters who are at the highest risk for an early initiation of marijuana use.
Affective–cognitive factors appear to affect adolescents’ substance use behaviors (e.g., Van Zundert et al., 2009). The theory of planned behavior (TPB: Ajzen, 1991) provides an explanation for marijuana use and describes the relationship between people's cognitive characteristics and the development and maintenance of behavioral patterns. According to the TPB, cognitive determinants of behavior are attitudes, normative beliefs, self-efficacy, and behavioral intentions. Attitudes can be differentiated according to expected consequences of marijuana use (i.e., positive and negative expectancies) and affective or evaluative aspects of marijuana use (Petraitis et al., 1995). Although attitudes can be differentiated in outcome expectancies and evaluative attitude, these constructs are seen as conceptually distinct, playing equally important roles in substance use behaviors (Leigh, 1989, Stacy et al., 1990, Wall et al., 1998). Normative beliefs concern adolescent perceptions of social approval of marijuana use (i.e., perceptions of others’ approval of their use of marijuana). Self-efficacy is often defined as the belief in one's ability to refrain from marijuana use in tempting situations. Lastly, behavioral intention is the motivation or readiness to start using marijuana in the future (e.g., Kam et al., 2009). The TPB presumes that attitudes, social approval, and self-efficacy precede one's intention and that one's intention in turn precedes actual behavior (see Fig. 1).
Research has demonstrated that TPB factors predict marijuana use behaviors in (young) adulthood (e.g., Armitage et al., 1999, Bandura, 1999, Connor and McMillan, 1999, McMillan and Connor, 2003, Morrison et al., 2002, O’Callaghan and Joyce, 2006). Generally, in young adulthood, having more positive attitudes, experiencing more approval from the social environment, as well as having less confidence in one's own abilities to refrain from marijuana is indicative of stronger intentions to use and subsequent marijuana use. However, given the fact that marijuana use at an early age has many detrimental health consequences (e.g., Grant and Dawson, 1998), it is of crucial importance to focus on the early adolescent years. Most studies on the TPB factors in early adolescence have been conducted in the alcohol and tobacco field (e.g., Aas et al., 1995, Cameron et al., 2003, Conrad et al., 1992, De Vries et al., 1995, Jones et al., 2001, Kam et al., 2009, Leigh and Stacy, 2004, Marcoux and Shope, 1997, Patrick et al., 2010, Randolph et al., 2006, Scheier et al., 1999, Tyas and Pederson, 1998, Van De Ven et al., 2007). In adolescence, more favorable attitudes (both rational and evaluative), more approval, and lower self-efficacy are predictive of stronger intentions and increased alcohol and tobacco use similar to the findings on marijuana use in young adulthood.
In contrast to the extensive work on alcohol and tobacco use, only a few studies investigated the prospective role of cognitive–affective aspects on marijuana use in early adolescence (Ellickson et al., 2004, Kam et al., 2009, Skenderian et al., 2008, Stephens et al., 2009) and only one of these actually investigated the mediating role of intention in the relationship between the TPB factors and marijuana use (Stephens et al., 2009). This later study simultaneously included both adolescent users and non-users without controlling for prior marijuana use. Marijuana users have more favorable attitudes compared to their non-using peers (e.g., O’Callaghan and Joyce, 2006, Skenderian et al., 2008), and it is possible that they have lower self-efficacy compared to non-users (Aas et al., 1995, Epstein et al., 2001). Further, it is plausible that intentions to initiate marijuana use are different from intentions to maintain the behavior. Including users and non-users in the same analyses without controlling for prior marijuana use is thus problematic, since behavioral feedback only informs the group of users which might lead to differences in determinants. To gain more insight into the role of the TPB in marijuana use initiation, it is necessary to examine this prospectively in a non-using sample of early adolescents.
The purpose of the present study was to examine the predictive validity of attitudes, social approval, self-efficacy, and the intention to initiate marijuana use on actual marijuana use among 1023 early Dutch adolescents who never used marijuana, by applying a 3-wave 20-months prospective design. We postulated that more positive attitudes, more approval from the social environment, and lower self-efficacy at Time 1 was directly relate to stronger intentions to start using marijuana at Time 2 and indirectly to actual marijuana use initiation at Time 3. Furthermore, we hypothesized that having stronger intentions at Time 2 was predictive of actual marijuana use at Time 3.
Section snippets
Procedure and sample
Data were collected as a part of national school prevention program “the healthy school and drugs,” a broader effectiveness study (Malmberg et al., 2010). Overall 23, schools from seven regions in The Netherlands were included in this randomized clustered effectiveness trial. Seven schools participated in the control condition, and 16 schools participated in one of the two experimental conditions. We visited participating schools and during these visits, we provided further information about
Descriptive analyses
Table 1 presents the means and standard deviations of the rational and evaluative attitudes, social approval, self-efficacy, intention, and the percentages of lifetime and monthly marijuana use by sex and education. As can be seen from Table 1, at T3 7.1% of the participants (n = 63) reported trying marijuana at least once, and 3.4% reported marijuana use in the prior month (n = 30). Table 2 shows the correlations among the model variables. The study variables are a mix of interval (the six
Discussion
We found relationships of evaluative attitude, social approval, and self-efficacy with actual marijuana use via intention to marijuana use. In particular, adolescents who thought more positively about being under the influence of marijuana, those who experienced more approval from the social environment, and those who had less confidence in their ability to refrain from marijuana use showed stronger intentions to start using marijuana. Consequently, they were more likely to initiate marijuana
Conflict of interest
The authors declare that they have no conflict of interest.
Funding
This study was funded by a grant from The Dutch Ministry of Health, Welfare, and Sport.
Contributions
Malmberg M. was responsible for data collection and writing the manuscript. Together with Vermulst A. she was also responsible for data analysis. All other authors are supervisors and grant applicators. All authors read and approved the final manuscript.
Acknowledgements
The authors wish to thank the schools and students who participated in the survey.
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Supplementary information for this article is available. Please see Appendix A for more information.