Discussion
In this study, we examined the trajectories of AP and DS and their relationship in a longitudinal sample of adolescents in Northern Ireland. The results indicated that while AP remained relatively stable throughout the study, DS decreased overall. Analyses of possible associations of the trajectories indicated a comorbid relationship of AP and DS through association of both initial levels of AP and DS as well as their change over time. Moreover, in females, increased initial levels of DS were associated with a decrease of AP over time. However, this finding was accounted for by externalising symptoms.
Our results support previous studies suggesting that AP and DS peak in late adolescence and decrease somewhat in early adulthood [
36]. Prevalence of AP and DS differed between genders. In line with the literature, DS were more severe among females, while AP were more severe among males in early adulthood (but not in middle adolescence where no gender differences were observed) [
9,
32,
37]. It is interesting to note that individuals reporting initially higher levels of AP also reported a steeper decrease as they entered early adulthood. Individuals may simply decrease their level of drinking as they get older, possibly due to other responsibilities such as further education or family. Alternatively, this observation may be partly explained by the high dropout rates after the initial wave, suggesting AP to be a risk factor of dropping out of longitudinal studies/school. However, the individuals who did drop out did not report any difference in levels of DS. It is plausible that dropouts did not show comorbid symptoms but only AP; if so, our observed comorbid levels are less than the actual ones in early adulthood. Another possibility is that individuals with high levels of AP did develop DS which increased the risk of not participating/dropping out of school; if so, our results are neither representative of actual levels of comorbidity and will cause a bias in our interpretation of our results.
Our results indicated a comorbid relationship of AP and DS, both at one particular time point and over time, in line with previous studies. Furthermore, the present results indicate that higher initial levels of DS are associated with a steeper decrease of AP; higher levels of DS acted as a protective factor of AP over time. Majority of the literature does not support this finding [
10,
56] though there are some which do [
14]. While the present study does not investigate the underlying mechanisms behind this, it is possible that the protective effect observed of DS on AP is due to characteristics of DS such as social withdrawal, which results in less interaction with peers and, therefore, less exposure to alcohol use by peers [
27] (alcohol use in early adolescence is strongly predicted by peer use [
28]). This is further supported by genetic evidence suggesting that internalising traits decrease the risk of heavy alcohol use [
15]. Alternatively, the association could be explained by the strong correlation between AP intercept and slope in females where higher initial levels of DS are associated with higher initial levels of AP which in turn are associated with a faster decrease of AP over time. Post hoc analyses investigated direct effects of DS intercept on AP slope and indicated no significant relationship (
p = 0.42). In light of these results, we can exclude the possibility that initial levels of DS directly impact change in AP.
The protective effects of DS on AP were, however, accounted for by externalising symptoms, indicating that the relationship of DS and AP was conditional on externalising symptoms. The result may simply be due to the fact that the association between DS and AP is small and, therefore, goes unnoticed when externalising disorders are considered as its association with DS and with AP is larger. Furthermore, as internalising and externalising symptoms commonly are comorbid and externalising symptoms frequently are associated with an increased risk of alcohol use and problems, it is possible that externalising symptoms are more prominent even in the presence of internalising symptoms, which would explain why the protective effect of internalising is non-observable when accounting for externalising. Another possible thought but unfortunately out of scope for this paper though nonetheless important, is that AP, DS/internalising symptoms and externalising symptoms may just be different expressions of the same underlying problem. What is expressed may be a function of environment (such as parents, peers, schooling, and traumatic events), learnt coping mechanisms and genes.
Strengths, limitations and directions for future research
The present study has both limitations and strengths. Three main strengths should be noted. First, the sample is taken from Northern Ireland which is of interest in several aspects, such as heightened mental health issues and post-conflict issues. There are few longitudinal studies of Northern Irish adolescents. We had a relatively large sample and provided a fair representation of participants from deprived backgrounds (participants that were likely to be affected by the discussed issues). Second, the present study accounted for change over time, a factor rarely accounted for in the present context, as well as externalising symptoms and gender. Third, adapting questionnaires to the appropriate age is an important practice in longitudinal studies of youth, but does cause issues regarding analyses, an issue presently addressed.
However, several limitations still need to be addressed. Multiple measures are needed to allow for changes in the way problems present at different stages of development. For example, irritability is a common symptom of depression in adolescence but less so in adulthood. The fact that different measures should be used at different ages was considered when BYDS was designed regarding DS. However, this creates an issue when the intentions are to measure its change over time; different measures were used at different time points, which was the case in the present study and alternative measures had to be created which brings with them further limitations and lack of external validity. This is a common problem in longitudinal studies, but we have in the present paper proposed a method of dealing with this issue.
Moreover, while our findings may not be generalisable to other geographical areas as of the observable higher levels of AP and DS in NI comparatively to both the UK and the world, it does not necessarily imply that the relationships observed are not. Further research is encouraged in other countries as to confirm/reject our findings.
Missing data were a major limitation. Participation rates dropped significantly after the initial wave of the study. This is a common issue with longitudinal data and does cause biased results; as observed in our analyses, dropouts do report higher levels of AP. In the present study, we cannot therefore with certainty say that AP does decrease in early adulthood.
The study only includes three time points of data. The means of AP and DS suggest a curvilinear change; unfortunately, this cannot be modelled with only three time points. Additionally, it would be advantageous to have yearly assessments to better understand the change of the problems and their relationship throughout late adolescence. It would therefore, for both statistical (with more than three time points a curvilinear model can be accounted for) and developmental reasons (investigate change from early to late adolescence), be valuable to include more time points of data collection.
Conclusions
These findings provide additional support for a positive association between AP and DS in youth. The present result also indicates protective effects of DS on AP in females, though this is accounted for by externalising symptoms. Our study indicates the importance of accounting for trajectories of variables, accounting for externalising symptoms, as well as only looking at a narrow age range at each time point as AP and DS vary with age. However, to understand the development and relationship of two or more phenomena, it would ideally involve a birth cohort where details from gestation, school, family, environment, and genes are considered. However, this is costly and time consuming and it is difficult to account for all the possible variables which may have an impact. In the current study, we are aware of these shortcomings and wish the reader to be so as well when interpreting the results. While our results shed some understanding on these problems’ relationship in this particular population, it is far from the full picture.