The findings of this prospective population-based study were twofold. First, internalizing behavior problems as early as at ages 8–9 years and up to emerging adulthood were associated with future DP. Second, externalizing behavior problems implied an increased risk for SA in young adulthood despite the age of assessment.
Our findings of significant associations between behavior problems during the childhood/adolescence and work disability in early adulthood are partly in line with previous research. Significant associations were found between mental health problems in adolescence and medical benefits (including vocational rehabilitation) received in young adulthood due to reduced work capacity . Another study also reported a higher risk for receipt of medical benefits in young adulthood among those who experienced high levels of anxiety and depression symptoms in adolescence . Our results further demonstrated that the risk for future work disability tends to be elevated after experiencing behavior problems at ages of 8–9 years old. On the other hand, in a follow-up study of young adults that were former patients of child psychiatric clinics, the associations between emotional or conduct disorders and DP in mid-adulthood lacked statistical significance . The discrepancy in findings may be due to the sample characteristics (clinic vs. non-clinic), differences in age span and sample size as well as on how long into adulthood participants were followed.
Internalizing behavior problems in childhood/adolescence were associated with a higher risk for DP, but not with SA in early adulthood. Although CBCL is a screening instrument of behavior problems in a nonclinical sample, some of the participants may have had a more serious psychopathology. We had no knowledge on whether participants were diagnosed with, for example, Autism Spectrum Disorders, a diagnosis that often leads to DP at young ages [25, 26]. Also, internalizing behavior problems in childhood were previously reported to imply higher risk for future mood disorders , including depression and anxiety, two of the main DP diagnoses in Sweden as well as in several other Western countries . Interestingly, a previous study reported a significant association between lifetime internalizing disorders (measured in adulthood) and sick leave due to mental diagnoses . In the present study, the associations were estimated for SA due to any diagnosis which may contribute to differences in findings.
Rule-breaking behavior, a subscale of externalizing behavior problems, was shown to be associated with future SA. This finding may sound unexpected as rule-breaking behavior can hardly be a diagnosis behind SA. However, this association becomes rather anticipated in the light of the previous studies. First, externalizing and internalizing behavior problems have repeatedly been shown to be comorbid and partly share the same etiology . Thus, possible diagnoses behind SA among people with high levels of externalizing behavior problems may include those related to internalizing problems, including depression or anxiety. Second, externalizing behavior problems in adolescence have previously been shown to be a risk factor for a wide range of mental disorders in adulthood, including mood and disruptive disorders, as well as physical health outcomes [10, 11, 29]. Previous research has also highlighted the different etiology of aggressive and rule-breaking behavior, also referred as to physically aggressive (e.g., fighting, bullying) and non-aggressive rule-breaking behavior (e.g., stealing, lying), respectively . Those with aggressive behavior problems are usually early starters and were linked to antisocial personality disorder in adulthood, whereas rule-breaking starts usually in adolescence and was linked to higher risk for substance abuse . A few studies have reported that adolescent rule-breaking, but not aggressive, behavior tend to increase the risk for mental health problems in adulthood [10, 29], whereas another study showed significant association between adolescent aggressive behavior (or conduct disorder) and future psychopathology . Our findings suggest that rule-breaking, and not aggressive, behavior tends to increase risk for SA in adulthood.
The results showed a tendency that internalizing behavior problems were associated to DP, whereas externalizing behavior problems were associated to SA. These findings suggest that there might be different pathways leading to SA and DP. Both SA and DP are related to work incapacity, temporary or permanent, due to disease or injury. A process leading to DP is usually several-years long and is often preceded by long-term SA. Thus, one could expect that pathways leading to SA and DP could partly overlap. However, in Sweden, young adults up to age 29 years who are diagnosed with severe diagnoses can be granted DP without having any preceding SA spell. Thus, our findings of the association between internalizing behavior problems and DP may be influenced by the severity of diagnosis which we had no possibility to adjust for. Future studies are needed to shed more light on the underlying mechanisms of the studied associations.
The results of the present study suggest that behavior problems experienced during adolescence may increase risk for work incapacity in adulthood. Before the results are replicated in future studies, we can only speculate on how our findings could be interpreted in terms of prevention and intervention of work incapacity. On one hand, paying efforts to prevent or reduce behavior problems at early ages could help to reduce work incapacity in adulthood. On the other hand, the intervention strategies could also target the consequences of the behavior problems and include, for example, adjustment of working conditions to young adults experiencing behavior problems.
The analyses of discordant twins showed slightly attenuated estimates, suggesting that familial factors played a minor role for the studied associations. That is, factors that are unique to each individual and not shared with a co-twin seem to primarily influence the reported associations. However, the results should be interpreted with caution due to the low number of discordant twin pairs, especially in the analyses of DP, and the relatively low HRs.
The estimates of significant associations between internalizing behavior problems and DP were approximately of the same size irrespective of the age of assessment. Bearing in mind that the follow-up time started directly after the assessment of behavior problems at Waves 3 and 4, one would perhaps expect that the HRs should be higher the closer the assessed behavior problems was to adulthood. However, our findings are consistent with previous studies reporting moderate stability of behavior problems during childhood and adolescence, e.g., .
The estimates of the associations may seem consistently small and the clinical significance of our results may be questioned. However, the estimates imply an increase in hazard following each one-unit increase in the behavior problems. Since the total scores of CBCL scales varied between 10 and 22 scores, even a low HR would suggest a noteworthy risk for those individuals with high scores.
Strengths and limitations
The study has several strengths, including the longitudinal population-based design and nationwide register data with no loss to follow-up. The individuals were followed for up to 20 years since the first assessment of behavior problems was conducted when the twins were 8–9 years old. This gave us a unique opportunity to investigate long-term effects of behavior problems in childhood and adolescence on both SA and DP. Some limitations should also be addressed. First, the number of DP cases observed during the follow up was low. Thus, the results should be interpreted with caution and need to be replicated using larger samples. Second, due to the few people that were granted DP, the analyses were adjusted for, instead of stratified by sex. Previous research has consistently shown sex differences in frequency of occurrence of internalizing and externalizing behavior problems in adolescence (e.g., , as well as in being grated DP . Thus, it is possible that the associations in the present study would be different and/or significant if estimated separately among women and men. Third, only parent-reports of behavior problems were available at Wave 1 and were used in the analyses, whereas self-reported data were used at Waves 2–4. Agreement between parental and child ratings for CBCL symptoms was previously reported to be rather low . Thus, differences in the estimates between those at Wave 1 and those at later Waves might be due to different reporters rather than due to changes in the levels of problem behavior at different ages. Fourth, only SA spells longer than 14 days could be included, which can be seen both as strength and a limitation. Fifth, SA benefits can be granted only the individuals having income from work or unemployment benefits. In the present study, the participants were followed from the years they turned 16 and up to 28–29 years old. Approximately 40% of all individuals born in Sweden 1985–1986 began their higher education studies when they were 24 years old at the latest (Higher Education in Sweden, 2016). Thus, a selection bias may be present in our study as participants that were students and did not work during the follow-up were not at risk for SA. However, at ages of 25–26 years, 75% of the respondents were registered as having income from work (>10,700 Swedish crowns/year) and hence eligible for sickness absence benefits. Sixth, the estimates are shown for one-point change in behavior scores. When comparing between different ages, the one-point change in behavior score at one wave may be different from the change at another wave due to different variability at different waves. However, the variability for externalizing scores show only minor changes between the waves and thus the one-point change in behavior scores is reasonable to compare between the waves. As only twins born in Sweden were included, the results might not be generalizable to immigrants. Lastly, the response rate reached only 52% at Wave 4 and, therefore, the significance of associations could be affected by that.