The European Journal of Public Health Advance Access originally published online on November 12, 2008
The European Journal of Public Health 2009 19(1):111-116; doi:10.1093/eurpub/ckn101
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Exposure to bullying at school and depression in adulthood: A study of Danish men born in 1953
Rikke Lund1, Karoline Kragelund Nielsen1, Ditte Hjorth Hansen1, Margit Kriegbaum1, Drude Molbo1, Pernille Due2 and Ulla Christensen1
1 Department of Public Health, University of Copenhagen, Copenhagen K, Denmark.
2 National Institute of Public Health, University of Southern Denmark.
Correspondence: Rikke Lund, Section of Social Medicine, Department of Public Health, University of Copenhagen, Øster Farimagsgade 5, 1014 Copenhagen K, Denmark, tel: +45 35 32 79 92 or +45 35 32 79 62, e-mail: r.lund{at}socmed.ku.dk
Received March 3, 2008, accepted October 3, 2008
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Abstract |
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Background: Bullying among children is associated with high symptom load and depression. There is little knowledge about long-term consequences of bullying. The aim of the present study is to examine the association between recall of bullying at school and depression in midlife controlling for adult social class and parents’ mental health. Methods: The analyses were based on the 2004 survey among men from the Metropolit 1953 Danish Male Birth Cohort (n = 6094). Information on depression was retrieved by the Major Depression Inventory (MDI) (prevalent depression) and by a measure of first doctor diagnosed depression between the ages 31–51 years (DD). Information on bullying was based on a recall measure of bullying in school categorized into three dimensions: occurrence; intensity; and duration. Results: Compared to subjects who had never been bullied, those exposed to bullying in school were at a significantly increased risk of having been diagnosed with depression between the ages 31–51 years. Long duration and high intensity of bullying were risk factors for both MDI and DD. Inclusion of the possible confounders (SES, parental mental illness) attenuated the associations somewhat, but the associations remained statistically significant. Conclusion: The present comparisons of the long-term incidence of depression among middle-aged men who experienced high and low levels of bullying at school might indicate that being bullied at school is a contributing factor in the development of depression. Prospective longitudinal studies are needed to confirm the effect of bullying in school on adult depression.
Keywords: bullying, depression, men
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Introduction |
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The rate of major depression is
3–4% in the Danish population.1 Presumably, a still larger proportion experiences mild or moderate depression, and depression is therefore considered a highly prevalent disease. The socio-economic consequences of depression are considerable. In 2002, DALY measures estimated depression as the fourth leading contributor to the global burden of disease and projected that it would rank second by 2030.2 In addition, depression has profound social and personal consequences for the patient, as well as for the next of kin. Men, in particular, experience loss of working capacity due to depression.3 Moreover, people who suffer from depression have a high excess mortality, primarily due to suicide. There are presumably both biological and psychosocial causes of depression. Exposure to bullying is an example of a preventable psychosocial factor that might induce depression. Bullying among children and adolescents occurs frequently. A study found that 25% of Danish schoolchildren were bullied often.4 Bullying is characterized by repeated physical or verbal interactions, with hostile intent, that cause distress and involve a power differential between bully and victim. Bullying is most frequent among younger students, with the youngest and weakest being exposed the most. Bullying, however, does slowly decrease with age.5–7 Studies have shown that bullying has severe consequences for the victims, both physically and psychologically both immediately and later in adolescence.8–13 Moreover, victims of bullying have difficulties in establishing social relationships with peers.14 Furthermore, some children react to bullying by bullying others, whereby a vicious circle is established and more children are bullied.15
Most of the studies examining the relationship between bullying during school life and depression find that victimization is associated with depressive symptoms.16–19 We identified a couple of longitudinal studies examining the relationship between bullying during school life and depression in adolescence.20,21 Both show that children who are bullied develop more depressive symptoms than children who are not bullied. Moreover, Sourander et al. found that boys who were victims of bullying had significantly increased risk of developing anxiety disorders at the ages 18–23 years.22 Likewise, Roth et al. found that young adults (mean age 19 years) who recalled being bullied showed increased risk of both depression and anxiety.23 We have not been able to find studies that analyse whether bullying during school life is associated with depression in middle-aged adults.
Several risk factors for being a bully victim have been suggested. Among these are individual factors such as personality and social skills and factors related to external circumstances (e.g. school policies on bullying and social circumstances of the bully victim). Earlier studies have shown that parent's mental health problems are associated with development of depression among both young and adult off-spring.24 Possibly, having parents with mental health problems increases the vulnerability of the child and thereby increases risk of bullying. Depression and depressive symptoms is in many countries unequally distributed across social classes25,26 and there is some evidence that bullying is also unequally distributed across classes with children in the lower classes more exposed to bullying.27
The aim of this study was to investigate the association between recall of occurrence, intensity and duration of bullying in school and onset of depression in adulthood, in a Danish population of middle-aged men and to study whether this association is confounded by parental mental illness and adult social class.
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Study population
The cohort was defined as all males born in 1953 in the larger metropolitan area of Copenhagen, Denmark. The birth cohort totalled 12 270 subjects and has been described in detail elsewhere.28 Of these 11 532 were eligible in the Civil Registration System in 1968. The present study was based upon the questionnaire-based follow-up study in 2004 of the surviving 9507 participants with a total population of 6292 (response rate 66%). For the present analyses we excluded 195 individuals who had a first diagnose of depression before the age 31 years (n = 6097).
Depression
The survey contains two measures of depression. We employed the Major Depression Inventory (MDI) as a measure of prevalent major depression based on 12 items on depressive symptoms The scale has been shown to hold good validity and it reflects the criteria for diagnosis listed in the ICD-10 and DSM-IV.29
The other measure of depression was measured by the question: Have a doctor ever told you that you have a depression? (Yes/No) (DD). Furthermore, subjects were asked to state their age at first diagnosis and an age variable divided into 4 age intervals was created: 20 years or younger; 21–30 years; 31–40 years; 41 years or older; or never having been diagnosed.
Bullying
The question on bullying was: were you bullied at school? With the following five responses: No; a little for a short period of time; a little for a long period of time; a lot for a short period of time; or, a lot for a long period of time. This information was used to create three variables: (i) occurrence of bullying versus none; (ii) intensity of bullying, expressed as none, little, a lot; and (iii) duration of bullying, expressed as none, short time or long time. The question thus concerns if the respondent has experienced bullying at any point during his school time. The question was developed for the present study based on a measure used in the Health Behaviour in School-aged Children study and the work of Olweus.30,31
Other variables
Parental mental illness was measured by two questions: ‘Did your biological mother suffer from mental illness?’ (yes/no/do not know), ‘Did your biological father suffer from mental illness?’ (yes/no/do not know). A combined measure was constructed in four categories: (i) At least one parent has suffered from mental illness (ii) Neither father nor mother had suffered from mental illness (iii) One parent has not suffered from mental illness and do not know about the other parent and (iv) Do not know about either parent.
Socio-economic position was measured by occupation and coded into social classes I–V in accordance with the standards of the Danish National Institute of Social Research, which is similar to the British Registrar General's Classifications I–V. We added social class VI representing people on transfer income, including sickness benefits and disability pension.
Statistical analysis
In total, 5019 individuals had full records on the included variables for the analyses. We imputed the missing data for the 20% with missing data using multiple imputation (SAS version 9.1 PROC MI and PROC MIANALYZE). We created five imputed datasets. In total the imputed dataset included 6289 individuals (smoking habits was the variable with fewest missing included in the multiple imputation and had three missing values, this variable was not imputed). In the procedures for multiple imputation in SAS the pattern of missing data should be monotone. Thereby the variables can be ordered with the variable with the fewest missing as the first, the variable with second fewest missing next and so on. Once one variable is missing for a subject all following observed values are deleted. Then the imputation is performed and as a supplement to the SAS procedure we afterwards inserted the original observed values where available. After the imputation procedure the 3.11% who had been diagnosed with a depression before age 31 years were excluded to yield a final dataset of n = 6094 observations. This final dataset contained 8.61% imputed values for the variable parental mental illness, 4.05% for adult social class, 1.16% for bullying, 0.83% for MDI and 0.51% for DD. Logistic regression analyses with bullying as determinant and depression—either MDI or DD—as outcomes were performed in PROC MIANALYZE and one OR (95% CIs) was calculated for each model. The bi-variate associations between occurrence, intensity and duration of bullying as well as the potential confounders and depression were analysed by chi-square statistics. Multivariate logistic regression analyses were then performed for the association between each of the bullying variables (occurrence, duration and intensity) and each of the outcome measures of depression adjusted for the selected potential confounders. Furthermore, the possible interactions between bullying and parental mental illness and adult social class respectively were tested by inclusion of an interaction term in the multivariate models.
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Three percent of the population had prevalent symptoms of a MDI at follow-up, whereas 9% reported to have had a first doctor diagnosed depression in the ages 31–51 years (DD). The two measures of depression were closely associated (chi-square P
0.0001). Among the respondents 38% had a recall of being bullied at some time during school. Table 1 shows the frequency of depression by each of the included covariates (imputed data). Bully victims had a significantly higher frequency of depression, both MDI and DD, (chi-square trend test range over the five imputed dataset 0.00001 < P < 0.04). Likewise, individuals from social classes V and VI had a higher frequency of depression (chi-square trend test P < 0.0001) as had those who had at least one parent with mental illness or who did not know about their parent's mental health.
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In addition, bullying was found to be unequally distributed between social classes with an increasing risk of bullying from social classes II–VI, whereas social class I had experienced bullying to the same degree as social class III.
Likewise, those who reported to have at least one parent with a mental illness significantly more often recalled to have been bullied at school (data not shown).
In the univariate logistic regression analyses there was a significant association between bullying and the measure of doctor diagnosed depression and a nearly but non-significant association with MDI. For instance, subjects who experienced bullying in school had a significantly increased risk for having a first diagnose of depression at the ages 31–51 years [OR = 1.33(1.11–1.60)]. The odds for having or having had a depression was significantly increased for respondents reporting high intensity and high duration of bullying (table 2).
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After inclusion of the possible confounders the association between occurrence of bullying and MDI attenuated. However, the association between bullying occurrence and DD remained significant with an OR of 1.24 (1.03–1.50) among the bullied (table 2).
Both bullying duration and bullying intensity were significantly associated with both measures of depression after adjustment for possible confounders. Individuals exposed to bullying for a long period or at a high intensity were at the highest risk of an outcome of depression (table 2). Furthermore, there was no evidence of interaction between adult social class and recall of bullying or between parental mental health and recall of bullying (data not shown).
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The present study suggests that adult men who recall being bullied at school have significantly worse odds of being diagnosed with depression during the ages 31–51 years or of having severe depressive symptoms at the age 51 years, even after adjustment for social class and parental mental illness. The association between bullying and depression is in concordance with most literature in this field showing an increased risk of depression and anxiety in adolescence among those who were exposed to bullying during school life.32–37 Our study suggests that bullying may be one of several psychosocial factors contributing to an increased risk of development of depression in midlife. Our study supports the findings of Jordanova et al.38 who pointed at an increased risk of common mental illnesses among older individuals who reported to recall bullying at earlier phases in life including school time. However, we cannot clearly establish the causational direction due to the limitations of the study discussed below.
The study population was drawn from a complete birth cohort consisting of all males born in 1953 in the Metropolitan area of Copenhagen; and, as such, it was not a random sample. This lowered the risk of selection bias, but the study population was not representative of the country as a whole. Twenty percent of the subjects who returned a questionnaire had missing data on the included variables and we decided to impute the missing values. The results of the initial analyses of subjects with full records and the results from analyses of the imputed dataset were nearly identical except that the univariate associatation between the dichotomous bullying variable and MDI became insignificant in the imputed analyses. Obviously we have no information on the experience of bullying in school among individuals missing due to death prior to survey 2004 (1015) and non-responders to the survey (n = 3215). However, register-based information on hospitalization for depression and use of antidepressants was available. Non-responders to the 2004 survey had a higher incidence of first admission to hospital for depression than responders and had more often claimed an anti-depressive drug prescription.39 This presumably leads to a risk of underestimation in the present analyses.
The present study includes retrospective answers on bullying, which involves a risk of recall bias. Optimally, information on the experience of bullying should have been collected at several points in time during school ages. Unfortunately, the articulation of the concept of bullying was not present in 1960s Denmark, which is the major explanation for the question not to be available from the 1965 school questionnaires. Consequently, individuals with a present depression or individuals who had experienced a depression at the ages 31–51 years may have stronger recollection of adverse life events like being bullied at school. The risk of recall bias was reduced by the fact that the respondents were not aware of what exposure and outcome we were studying. Moreover, Rivers40 has found that memory of bullying is relatively stable over time, so it is likely that respondents who report incidents of bullying actually have experienced bullying during their school life.
Perhaps the most important risk factor for depression is earlier depression. We did have access to register based information on admission to psychiatric hospital from 1968 to 2002, and it was possible to include this variable into our analyses. However, we chose not to do so since we consider depression occurring between the experience of bullying at school and e.g. major depression in 2004 as a mediating variable which should not be controlled for.
The measure of bullying is based on one self-reported measure with five possible answers. This breakdown does not encompass all aspects of bullying, which is a complicated phenomenon, but it does take into account the occurrence, intensity and duration of bullying. Olweus41 found a consistency between student reports and teacher reports of bullying, which suggest that the validity of self-reported bullying is high. Our measure is not formally validated. However, to our knowledge internationally no validated measure of bullying is available, and we therefore decided to develop a new. The proportion of individuals with a recall of bullying at school was 38%. It is hard to judge whether this high occurrence is due to some amount of over reporting. But as there was no focus on the psychosocial environment among children at school in the 1960s, it is not unlikely that bullying occurrence was this high. In addition, cross-sectional studies of school aged children in the late 1990s showed a point prevalence of being a bully victim of 25%.42 It is most likely that a cumulated measure of bullying as the one used in the present study will indicate a higher proportion of bullied.
The variable DD indicates the number of respondents who at the ages 31–51 years have had a first diagnose of depression. However, we are aware that the first depression diagnosed by a doctor is not necessarily the first depression, undiagnosed depressions may have occurred earlier. This also introduces the possibility of undiagnosed depressions or depressive symptoms to have been prevalent already in childhood. By excluding the diagnoses reported in the ages up to 30 years we believe we have prevented at least some risk of reverse causation.
The MDI scale is a suitable instrument in measuring present depression in questionnaires and has been validated in comparison with the Schedule for Clinical Assessment in Neuropsychiatry (SCAN). In addition, the sensitivity and specificity of the scale have been found to be acceptable.43
In the literature on the possible long-term health effects of bullying the question of causality has often evolved. The question is relevant since most studies have been based on either cross sectional studies of children or like the present study on adolescent or adult cohorts where the respondents are asked to recall if bullying had occurred at school. Several researchers have debated whether bullying is merely a result of a certain personality trait which is also a predictor of depressive or anxiety related symptoms.44 Unfortunately, we did not have the possibility to include measures of personality in the present study, although this variable may explain some of the association between bullying and later depression. However, today there are large variations in the prevalence of bullying between school classes, schools and countries.45 It is unlikely that such large structural and geographical prevalence differences are caused by personality differences. Also, several intervention studies have shown that pedagogical intervention programs help reduce bullying at school.46,47 If the association between bullying at school and depression later in life is causal the incidence of depression consequently may also be reduced.
Depression is a multifactorial disease in which several pathways interact. Physical and social exposures in childhood, as well as in adulthood, have long-term effects on the risk of disease.48 Thus, there are various possible explanations for the association between bullying and depression. One such is articulated in Stress Theory, which states that if the fight-and-flight response is overloaded (e.g. as a consequence of bullying) it could lead to depression through increased levels of cortisol.49 Another explanation is that risk factors accumulate over time due to chains of risk (e.g. that one adverse exposure leads to another, etc).50 Furthermore, differences in health behaviours e.g. alcohol intake and drug abuse between bullied and not bullied individuals may play a role as possible mediators. Likewise, the availability of social support and the respondent's marital history or cohabitation status could be candidates for the causal pathway. Information on these variables from a period between childhood and 2004 was however not available. We included measures of alcohol intake, emotional support and cohabitation status from the follow-up study in 2004 the multivariate models although not optimal. However, they did not change the results and were omitted from the final models. In our multivariate models we included a measure of adult social class since a strong social gradient in both measures of depression was present and bullying at school was associated with social class. But the gradient in depression was to a large extent due to the high risk in social class VI and there is a risk that some individuals in this group are in this category because of illness (e.g. depression) i.e. reverse causation. This has however, been contradicted as an unemployment period of six months is enough to bring out impairment in mental wellbeing including depression.51 There is also a risk that bullying leads to a higher risk of ending up in social class VI which in turn increases the risk of depression and then social class acts as a mediator which should not be controlled for. However, we decided to adjust for social class since there was a gradient in depression also for social groups I–V and since the association between bullying and social class not necessarily is causal. The net result is that our results presumably are slightly over-adjusted.
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Conclusion |
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The present comparisons of the long-term incidence of depression among middle-aged men who experienced high and low levels of bullying at school might indicate that being bullied at school is a contributing factor in the development of adult depression.
For the pathways between bullying and depression to be found, more longitudinal studies, with displaced measurements over a long period of time, are required. As bullying has shown to be preventable, our findings underline the importance of further prevention of bullying in school.
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Danish Heart Association (grant no. 01-2-9-16-22916).
Key points
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We also thank those who initiated the study and/or continued the Metropolit study: K Svalastoga, E Høgh, P Wolf, T Rishøj, G Strande-Sørensen, E Manniche, B Holten, IA Weibull and A Ortman. We thank professor Per Kragh Andersen and Martin Theil Jensen BSc, Department of Biostatistics, Institute of Public Health, University of Copenhagen for help with the multiple imputation procedure.
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