Internalizing and externalizing problems in childhood and adolescence as predictors of work incapacity in young adulthood

A population-based prospective cohort study including all twins from the Swedish Twin Registry who were born in Sweden 1985–1986 (N = 2960) [19] was conducted. These twins were invited to participate in a longitudinal Twin study of CHild and Adolescent Development (TCHAD), see [20]. Twins and/or their parents were contacted on four occasions (i.e., Waves); when the twins were 8–9, 13–14, 16–17, and 19–20 years old. The participants were mailed a questionnaire including extensive batteries of questions on physical health as well as emotional and behavior problems. The response rates were 75% (n = 1339 for parents reporting information on their twins), 78% (n = 2263 for twin self-reports), 82% (n = 2369 twins), and 59% (n = 1698 twins) at Waves 1–4, respectively.

In Sweden, all residents who have income from work or unemployment benefits are from the age of 16 years entitled to sickness benefits from the Swedish National Social Insurance Agency, if unable to work due to disease or injury. DP can be granted to those aged 16 (and from 2003 to those aged 19), who, due to disease or injury, have long-term or permanently reduced work capacity, even if not having had any previous income. Since 2003, DP can also be granted to young adults for prolonged schooling in order to finish elementary or secondary school. For all twins, data on date of SA and DP were obtained from the Swedish National Social Insurance Agency for the years 2001–2013. For those working, the employer in most cases provides sick pay for the first 14 days of a SA spell, why the register data from the Agency do not include information about SA spells ≤14 days. Data on diagnoses for SA were available for years 2005–2013 and for DP 2001–2013. All register data were linked to the twins, using the unique ten-digit personal identification number assigned to all residents in Sweden.

After excluding individuals with missing questionnaire data at all waves or with missing information on zygosity (n = 246), or who were later granted DP due to mental retardation diagnoses (ICD 10: F70–F79, n = 18), or who died before they turned 16 years (n = 2); the final sample comprised of 2570 twins, whereof 1029 were monozygotic and 1541 dizygotic twins.

The presence of internalizing and externalizing behavior problems was assessed through the Child Behavior Checklist (CBCL) [21]. The CBCL is a reliable and valid instrument for assessment of behavioral and emotional problems in children and adolescents [21]. The CBCL includes 102 items with a three-point Likert-scale response-format that can be summarized into three scales: Internalizing, Externalizing, and Total problems scales. Internalizing and Externalizing scales include syndrome scales, or subscales, referring to the sets of co-occurring problems [21]. In the internalizing scale, the syndromes are grouped as Depressed/Anxiety (score range 0–28), Withdrawal/Depressed (score range 0–14), and Somatic complaints (score range 0–18). The externalizing scale includes Aggressive Behavior (score range 0–38) and Rule-Breaking (Delinquent) Behavior (score range 0–22) syndromes. The CBCL was administered to the twins’ parents in Wave 1 and an adapted self-report version of CBCL, Youth-Self Report [21], was mailed to twins in Waves 2–4.

The following two outcomes were used: having a SA spell (yes/no) or being granted DP (yes/no) during follow-up though 2013.

In Sweden, all residents aged 16–65 years who have income from work or unemployment benefits are entitled to sickness benefits from the Social Insurance Agency, if unable to work due to disease or injury. Among employed individuals, sick pay is in most cases paid by the employer during the first 14 days of a sick-leave spell, which means that we do not have data on most of the short sick-leave spells. DP can be granted to those who, due to disease or injury, have permanently reduced work capacity, even if not having previous income from work and may be granted for both full- and part-time absence.

All individuals were followed until the date of the first SA-spell or the date of granted DP, respectively, until emigration, death, or the last date of follow-up, 2013-12-31. In the analyses of associations with behavior problems at Wave 1 and 2, the individuals were followed regarding SA or DP, respectively, from the year they turned 16 years (i.e., year 2001 or 2002 depending on the birth year of the participants). For analyses of behavior problems at Waves 3 and 4, the follow-up began the year after the assessment of behavior problems. That is, for the analyses of behavior problems measured at Wave 3, the follow-up period began in 2003, whereas for analyses of behavior problems measured at Wave 4, the follow-up began in 2006. Individuals that were granted DP before the measurements of behavior problems at Wave 3 (n = 8) or 4 were excluded from the data analyses (n = 29). The follow-up times depending on the wave of assessment are outlined in Fig. 1.

Descriptive statistics of the sample were computed including frequencies, percentages, and means with standard deviations. Cox proportional hazard regression models were applied to estimate the association between the subscales of internalizing and externalizing behavior problems at different ages and occurrence of SA and DP in adulthood. The analyses were clustered on twin pair identity to adjust for twin dependency within pairs. The subscales were treated as continuous variables and hazard ratios (HR) were calculated of being on SA or DP during the follow-up. The analyses were also adjusted for sex and parental education (measured by a question to both parents: “What is mother’s/father’s education?”). Influence of familial factors was tested by running conditional Cox proportional hazards analyses that only included twin pairs discordant for SA or DP, respectively. Twin pairs were treated as discordant if one twin in a pair was granted SA or DP during the follow-up period, respectively, whereas the co-twin was not. In the analyses of SA, those discordant twin pairs where one twin was on SA and the other one on DP, were removed from the analyses. Familial factors are suggested to influence the association if HRs computed in conditional analyses differ from HRs computed in the whole sample [22].

All statistical analyses were performed with SAS 9.4 [23].

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., [33], as well as in being grated DP [34]. 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 [35]. 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.