The present study found evidence for a bi-directional relationship between adolescent depression and smoking. Peer smoking helped account for the comorbidity. Depression symptoms measured at mid adolescence (age 14) predicted smoking progression across mid to late adolescence (ages 14-18 years old). Peer smoking mediated these developmental influences such that higher depression symptoms predicted an increase in the number of smoking peers, which in turn predicted smoking progression. The assessment of the alternative directional path indicated that smoking progression predicted a deceleration of depression symptoms from mid to late adolescence. A significant indirect effect indicated that greater smoking at baseline predicted a deceleration in the number of smoking peers across time, which predicted a deceleration in depression symptoms from mid adolescence to late adolescence.
The presence of a bi-directional relationship is partially consistent with previous research (17
). In contrast to those studies that found smoking to be a risk factor for depression and depression to be a risk factor for smoking, the bi-directional influences observed in the present study support a self-medication hypothesis for both directional paths. That is, depression contributed to smoking uptake and smoking progression contributed to a dampening or leveling off of depression symptoms. A recent cross-sectional study of smoking associated mood variation found that adolescent smokers reported higher positive affect and lower negative affect after smoking, but these mood modulation effects diminished with high levels of smoking experience (74
). Variability in negative mood or unstable affect regulation appears to be a risk factor for smoking escalation among adolescents and smoking serves to stabilize or decrease the variability in negative mood states (75
). This study extends this research by showing prospectively that depression contributes to smoking uptake and that smoking modulates depression symptoms. These reciprocal relationships may reinforce and maintain smoking behavior into adulthood.
These findings also highlight the notion that there may be overlap in the neural substrates modified by smoking and anti-depressant medication (76
). For example, nicotinic acetylcholine receptors are thought to be involved in modulating the release of several neurotransmitters implicated in mood, such as serotonin, norepinephrine, dopamine, GABA and glutamate (76
). Nicotine receptor inhibition appears to mitigate mood instability and may reduce depression (78
). It is possible that long term (or even shorter term) smoking may be followed by adaptations or tolerance to nicotine, which may promote negative affective states such as depression (77
). The mood modulating or affective regulating functions of nicotine may decline with a greater smoking history (74
). As the epidemiological studies of the depression-smoking relationship extend into young adulthood, capturing longer smoking histories and higher smoking rates, findings that smoking predicts depression are consistent with this explanation (14
Our bi-directional findings may have differed, in part, from previous research (7
) because we used repeated measures of depression and smoking to account for baseline levels and changes across time. This may have allowed us to better capture how these two variables impact each other from mid adolescence to late adolescence (80
). We also controlled for some confounding variables (17
) and considered a more complex relationship between smoking and depression, which included an evaluation of indirect effects.
The findings of this study also indicate that depression contributes to smoking acquisition indirectly by increasing adolescent vulnerability to peers who smoke. The impact of peer smoking on adolescent smoking acquisition is well documented (43
). Adolescents with higher levels of depression may be more sensitive to peer behavior, more likely to select nonconventional peers, or both (45
). A greater number of smoking peers provides better access to cigarettes, promotes a normative perception of smoking, and may be a source of peer approval (38
). These issues may be especially salient for youth with higher levels of depression symptoms.
Of note, the alternative indirect path showed that smoking at mid adolescence (age 14) predicted a leveling off in the number of smoking peers across time, which in turn, contributed to a deceleration of depression symptoms. This finding is contrary to what we would have expected, although cross-sectional research has shown that adolescent smokers tend to have higher depression symptoms, if they attend a school with a lower smoking prevalence (83
). This may reflect the importance of peer group belonging and perceptions of normative behavior on depression. Alternatively, this finding may simply reflect a positive relationship between greater adolescent smoking and a greater number of peers who smoke at baseline, such that fewer smoking peers are acquired across time. The leveling off in the number of peer smokers is then positively linked to the leveling off in depression symptoms. The positive trend to trend relationship between smoking progression, greater number of peers smoking, and an increase in depression symptoms across time was suppressed by a significant and negative direct effect between smoking progression and deceleration in depression symptoms (73
). This may simply exemplify the complexity of the relationship between adolescent smoking and depression, reflect that the peer smoking variable measured the quantity of peers who smoke across time but did not account for the quality and types of peer relationships that may impact mood, and/or highlights the importance of other common and unique mechanisms.
Research indicates that almost 25% of adolescents are regular smokers (37
), about 25% of adolescents have had at least one major depressive episode by 18 years of age (2
), and 20-30% of adolescents experience depressive symptoms (64
). The high rates of comorbidity between depression and smoking emphasize the importance of targeting smoking prevention efforts to this high-risk group. Interventions that have components on depression prevention and management could have an important impact on smoking uptake as well as subsequent depression. Social influence-based models of smoking prevention or intervention address peer influences to smoke (e.g., cigarette offer refusal skills). However, it may also be important to address these issues from the standpoint of adolescent depression. For example, programs that address coping and negative mood management skills, limited social networks, need for peer approval, and accessing nonsmoking peer groups may be especially beneficial for adolescents with elevated depression symptoms. Preliminary findings indicate that social learning-based approaches to smoking prevention tend to be especially helpful for depressed adolescent boys (86
The strengths of the present study include the collection of data from a large group of adolescents, repeated measurements of smoking, depression and peer smoking across mid to late adolescence, an analytic plan that is consistent with the longitudinal nature of the data and the concept of comorbidity, inclusion of potential confounding variables, and a good retention rate. One limitation is that self-reports of depression symptoms were not confirmed by formal clinical interview. Thus, we cannot determine whether these adolescents had a history of major depression or if they met criteria for a current diagnosis. Similarly, our baseline measure of depression (depression level factor) does not encompass depression history. However, our measure of smoking at baseline (smoking level factor) does consider smoking history. Future prospective research should include a baseline variable that encompasses lifetime and present depression. Also, we did not measure co-morbid and potentially confounding psychiatric symptoms, such as anxiety (17
). Although it may be considered a limitation that our model did not include nicotine dependence, our study shows important relationships between depression symptoms and regular smoking, which may or may not reflect nicotine dependent smoking. Many of these adolescent smokers may continue to smoke into adulthood, potentially solidifying a link between smoking, nicotine dependence, and depression. Indeed, data suggests a bi-directional relationship between smoking, nicotine dependence and major depression in young adults ages 18-31 years old (23
In addition, although our sample was 34% minority, we did not have enough adolescents in any one racial group to evaluate racial differences in the link between depression and smoking. Finally, a common limitation of protocols requiring active consent is the rate of nonresponse (88
). Although 75% of those parents who responded provided consent and the differences between those who provided consent and those who declined were relatively small and few (47
), caution is warranted in generalizing the results of this study. However, our sample was nationally and locally representative on basic demographic characteristics (91
), and the smoking rates are fairly comparable to those found in national surveys for the geographical area of our sample (91
), and the region and the population of high school students in the county from which the sample was drawn.
In conclusion, the present study provides the first evidence of bi-directional self-medication processes in the relationship between adolescent smoking and depression and suggests that peer smoking behavior may help account for the comorbidity. Further research on mechanisms may provide novel behavioral and pharmacological intervention targets for adolescent smoking and/or depression. Based on these findings, targeting depression could have an important impact on smoking uptake as well as subsequent depression. The public health implications of further research could have a significant bearing on the psychosocial, economic, and medical morbidity and premature mortality that are associated with these conditions.