Our study aimed to examine the effects of internalizing and externalizing behavior problems and prospective self on adolescent alcohol and marijuana use occurrences in the past 12 months. To control for potentially confounding factors, we also covaried race, ethnicity, sex, school age, and parental education in the first model of all regressions. Finally, we assessed the effects of the interactions of internalizing problems with prospective self and externalizing problems with prospective self to determine whether there were associations beyond their individual relationships with substance use.
4.1. Alcohol
For alcohol use, we hypothesized that externalizing behavior problems would positively correlate with alcohol use, while a higher prospective self would be linked to decreased alcohol use. We also hypothesized that internalizing behavior problems would not be associated with alcohol use. Consistent with our hypothesis, a higher level of externalizing behavior problems was significantly associated with lower alcohol use and a higher level of prospective self is associated with less alcohol use. For internalizing behavior problems, at the clinical range, lower levels of internalizing behavior problems were significantly associated with higher alcohol use. We also tested whether higher levels of prospective self may have served as a protective factor against alcohol use among adolescents with higher externalizing and internalizing behavior problem scores. None of the interactions were significant, indicating no differential susceptibility to alcohol use among adolescents with higher levels of internalizing or externalizing behavior problems.
Our results regarding the positive relationship between externalizing behavior problems and alcohol use occasions are consistent with other research findings [
26,
27,
28]. An explanation for this relationship could be the positive association between increased externalizing behavior problems and increased socialization with delinquent peers [
26]. Though only the more severe range of internalizing behavior problems was associated with increased alcohol use, our results follow the direction of other studies [
26,
28,
29]. A potential explanation for the negative association is that internalizing behavior problems serve as a protective factor against delinquent peer association and, therefore, early alcohol use [
26,
29]. For instance, internalizing behavior problems could lead to difficulty interacting with peers early on, which, in turn, is associated with less alcohol use early on as early alcohol use is associated with peer interaction [
29].
4.2. Marijuana
For marijuana use, we hypothesized that higher levels of externalizing and internalizing behavior problems would be associated with increased marijuana use, while a higher prospective self would be linked to lower marijuana use. Our results partially supported these hypotheses. Higher levels of externalizing behavior problems were significantly associated with greater marijuana use. However, findings for internalizing behavior problems were not significant. Consistent with our hypothesis, an increase in prospective self was significantly associated with lower marijuana use.
We also examined whether higher levels of prospective self may have served as a protective factor against marijuana use for adolescents with higher scores on externalizing and internalizing behavior problems. None of the interactions between externalizing problems and prospective self were significant, indicating no differential susceptibility to marijuana use among adolescents with higher levels of externalizing behavior problems. However, the interaction between (clinical range) internalizing behavior problems and prospective self was significant and positive, suggesting that adolescents with both higher internalizing problems and higher prospective self engage in greater marijuana use, beyond their individual predictions.
Similar to alcohol use, our findings regarding the positive relationship between a higher level of externalizing behavior problems and a higher level of marijuana use occasions are consistent with some research [
30,
31]. However, other research has suggested that while externalizing behavior problems relate to marijuana use, other factors are also powerful predictors of marijuana use [
34]. For example, according to Korhonen et al. (2010), early-onset smoking is associated with cannabis use.
Our nonsignificant association between internalizing problems and marijuana use adds to the mixed research findings on this question [
8,
30,
32,
33,
35]. While some findings suggest greater internalizing behavior problems are associated with less marijuana use [
8,
30], others suggest internalizing behavior problems to be positively associated with marijuana use [
33,
36], and others found no association [
32,
35]. A potential explanation for not having found a significant association is that we looked at more severe levels of internalizing problems, and at more severe levels, internalizing behaviors are not a significant risk factor. To further this point, in the
T-score models, internalizing behavior problems were significantly negative across all three models, meaning that an increase in internalizing problems is associated with decreased marijuana use. The negative association in our
T-score models may be explained by the fact we control for externalizing problems. Research suggests that internalizing behavior problems serve as a protective factor when controlling for externalizing problems; when looking at the co-occurrence of internalizing and externalizing problems, there seems to be a weak positive association [
8]. Another potential explanation for our findings could be that substance use in adolescence is primarily used in social contexts, and since social withdrawal often accompanies internalizing symptoms, these symptoms can serve as a protective factor [
8]. Additionally, fear and worry are also two characteristics of internalizing symptoms that may deter adolescents from taking risks, such as using marijuana [
8].
Interestingly, we found a significant positive contribution from the interaction between clinical range internalizing behavior and prospective self, which suggests that adolescents with both high internalizing problems and high prospective self engage in higher levels of marijuana use. Our findings might underscore a potentially higher-risk population of adolescents that exhibit future orientation, academic aspiration, and resistance to peer influence but also have high internalizing symptoms. A potential explanation for our finding could be the changed perceptions of marijuana use that may stem from the changes in legalization. The perception that marijuana is not harmful, in turn, may influence adolescents’ decision to use it as self-medication to relieve internalizing symptoms, such as depression. Though studies have found weak effects on the use of cannabis for self-medication [
12,
37], our study may unveil a change in marijuana use and perception. In 2008, medical use of marijuana was legalized in Michigan, and then in 2018, recreational use was legalized. Recent studies examining the impact of legalization on youth marijuana use and perceptions offer mixed results. Two studies found little to no differences in perceptions of marijuana use among adolescents after the legalization of recreational use of marijuana in Washington state [
38,
39]; however, one study found increases in problems and use disorder symptoms [
38]. Data collected from secondary school students generally suggests decreases in perceived harmlessness and increases in marijuana use, yet not consistently. For example, researchers examining the effects of marijuana legalization on youth in Washington and Colorado found that among 8th and 10th graders in Washington, perceived harmfulness of marijuana use decreased and marijuana use increased after the legalization of recreational use [
40]; however, no significant difference was found among 12th graders, suggesting that younger adolescents might be more vulnerable to the policy changes [
40]. However, they found no significant differences in either perceived harmfulness or marijuana use among Colorado students. Nationally, Monitoring the Future data does indicate a sharp increase in adolescent perception of marijuana as not harmful, with one-fifth of 12th graders perceiving people who use marijuana regularly as posing no risk or harm to their health [
41]. Additional research is needed to fully understand the long term effect of policy changes on adolescent perception and use of marijuana. Our study contributes to the ongoing discourse by highlighting the potential impact of changing marijuana perceptions.
4.3. Prospective Self
Prospective self is composed of future orientation, academic aspiration, and resistance to peer influence. Our results indicate that at more severe levels of internalizing and externalizing behavior problems, prospective self serves as a protective factor against alcohol and marijuana use, as a higher level of prospective self leads to a lower level of alcohol and marijuana use occasions. Our findings are consistent with the only other study examining the relationship between prospective self and externalizing behavior problems [
13]. While prospective self is a new construct introduced by Zinn et al., 2020, research demonstrates that future orientation, academic aspiration, and resistance to peer influence individually work as protective factors against substance use. For example, adolescents with positive future orientation were less likely to engage in risky behaviors, such as alcohol during sex, and marijuana, drug, and alcohol use were less likely to experience alcohol problems [
42,
43]. On the contrary, less positive future orientation was significantly associated with alcohol during sex, more alcohol problems, and increased substance use [
43].
Moreover, research has shown consistent results that compared to adolescent substance users, non-users tend to have higher academic aspirations [
44]. It is also important to note, however, that there is a bidirectional relationship between academic motivation and aspiration and substance use. On the one hand, the lower one’s academic motivation and aspiration are, the more likely they are to use substances. On the other hand, the more an adolescent uses substances, the more likely their academic motivation and aspiration will decrease [
45].
Finally, peer influence is important to consider when thinking about adolescent substance use because peers have an influence on whether or not one uses substances [
16,
46]. While parents and peers both play influential roles in the decisions that adolescents make, parents tend to have more influence on their child’s plans for the future, while peers have a greater influence on whether the adolescent will use substances or not [
16,
47].
4.5. Future Research
Our results open a new avenue for research on where to better understand the internalizing and externalizing pathways to alcohol and marijuana use and the influence of prospective self in protecting against alcohol and marijuana use. Our study provided insight into the relationship between internalizing and externalizing behavior problems separately with alcohol and marijuana use, but it did not focus on the co-occurrence of internalizing and externalizing behaviors as a risk factor. Future research should examine the impact of co-occurring conditions on substance use.
Finally, since prospective self is a new latent construct, additional research is needed on its relation to adolescent substance use. Zinn et al. (2020) suggest that the prospective self plays a role in supporting resilience against externalizing problems. In our study, it seems that prospective self can serve as a protective factor against having a higher level of alcohol and marijuana use occasions. Clearly, prospective self seems to be a significant protective factor for adolescents. Therefore, further research could identify what prospective self may be a protective factor for, and how it may be incorporated into prevention and treatment programs.