The association between school bullying and depression: evidence from TwinssCanChina study

Abstract

BACKGROUND Recently, the prevalence of depression in adolescents has been found to be rising in China. Evidence from Western population has suggested that both genetic and environmental factors are involved in developing of adolescent depression symptoms (1). Furthermore, the association between school bullying and depression has been reported by line of studies in Western population (2), which indicates school bullying victimization will increase level of depression and suicidal thoughts. As in China, school bullying phenomenon is also quite worrying and has shown to be exacerbated in recent years, it is important to increase the public awareness of school bullying and its negative impact on mental health in adolescents. However, little research has addressed this issue in Chinese population. Therefore, the purpose of present study is to investigate whether school bullying experience will predict the depression in Chinese adolescents, and furthermore, how genetic and/or environmental factors influence the level of depression based on subclinical Chinese population. METHODS In this pilot study, 72 healthy Chinese twins, including 29 monozygotic (MZ) twins and 43 dizygotic (DZ) twins, were recruited. High school bullying history was measured by Retrospective Bullying Questionnaire (RBQ) which covered six types of victimization, including two physical (beaten/hit/kicked, stolen from), two verbal (nicknames, threatened), and two social (lies/gossip, excluded). The symptom of depression was measured by the Symptom Checklist 90-R (SCL-90-R), which was a self-reported clinical rating scale. Data analysis was carried out by multilevel linear model utilizing maximum likelihood estimation rationale. RESULTS The mean age of the twins is 17 years old, with 66.3% female and 33.7% male. There is no significant difference in the mean level of depression between MZ twins (M=7.92, SD=8.46) and DZ twins (M=6.43, SD=6.08). In both types of twins, totally 19 participants (16.4%) have suffered one type of bullying (beaten/hit/kicked, or stolen from, or nicknames, or threatened, or lies/gossip, or excluded), 18 participants (15.5%) have suffered two types of bullying, 10 participants (8.6%) have suffered three types of bullying, and 9 participants (7.8%) have suffered four types of bullying. No participant suffering five or six types of bullying is detected in our dataset. Multilevel linear modelling has revealed school bullying experience significantly predicting depression level (b=1.3, P=0.001) in Chinese adolescents. Furthermore, genetic factor explains 47% of the variation in depression symptoms in Chinese adolescents, while common environmental and non-shared environmental factors explain 25% and 28%, separately. DISCUSSIONS In this pilot study based on Chinese twin adolescents, consistently with Western population, high school bullying experience has demonstrated a significant association with depression, with more bullying experience predicting higher level of depression symptoms. Moreover, the depression has shown moderate heritability in Chinese adolescents, which is in line with findings in Western population (47% in Chinese population vs 44% in Western population) (1). Additionally, both common and non-shared environmental factors are identified to be significant contributors to individual differences in depression symptoms, which highlight the importance of exploring environmental risk factors to understand the mechanism of mental health problems. In conclusion, our findings suggest that school bullying acts as an environmental risk factor contributing to developing depression in Chinese adolescents. As our research is an ongoing one, further study employing larger sample size would be used to confirm these findings.