The WHO has reported that depression is the leading cause of morbidity and disability in children and adolescents worldwide and the third most common cause of disability-adjusted life years in adolescents. When severe, depression can lead to suicide[48]. Adolescent depression has symptoms that can persist into adulthood, resulting in high suicide risk and a high recurrence rate[49]. However, the risk factors for depression in children and adolescents are affected by many aspects, different researchers have different views, and the interactions among these factors are complex[50, 51].
4.1 Relationship between being bullied and depression in children and adolescents
This meta-analysis showed that bullying is related to depression in children and adolescents, which is consistent with the findings of Fan H and Gao L[52, 53]. This association may be because adolescents who experience bullying perceive themselves more negatively, are more closed off, and are reluctant to seek outside support. A study by Duan S et al. showed that when children and adolescents are bullied, their emotions are greatly affected; this emotional harm is difficult to treat and reduces the individual's mental health, resulting in depression[54]. Additionally, victims of bullying are regarded as weak, even though they may have excellent grades in school. After being bullied, the victims are typically threatened with harm if they tell an adult[55].
4.2 Relationship between bullying and depression in children and adolescents
This meta-analysis found that bullying is a risk factor for depression in children and adolescents. This finding is consistent with that of Choi JK[55]. One explanation may be that bullying individuals have depression and choose to use aggression as a coping mechanism. In addition, bullying individuals are unable to normally communicate with their peers; thus, they are rejected and experience depression(Lee 2021). Relevant studies have shown that aggressive bullying individuals are prone to depressive symptoms and even self-loathing thoughts and behaviors, which may be related to symptoms that have a high co-occurrence with aggression, such as impulsivity and anger[56].
4.3 Relationship between bullying-bullied behavior and depression in children and adolescents
The meta-analysis also found that of the three bullying categories, bullying-bullied behavior had the strongest association with depression in children and adolescents. A study in Macau, China showed that bullying-bullied individuals experienced the most negative emotions, such as depression and anxiety, and the lowest life satisfaction[57]. The probable cause is that bullying-bullied individuals experience the negative effects of both bullying and being bullied. These individuals exhibit poor psychosocial functioning, poor self-control, vulnerability to rejection from peer groups, and the highest levels of depression[58]. The meta-analysis results suggest that reducing bullying among children and adolescents will help to prevent and control the occurrence of depression. Additionally, psychological intervention may be needed for children and adolescents after experiences of bullying and being bullied.
4.4 Subgroup analysis
The number of victims of cyberbullying has increased over the past decade, accompanied by increasing concern about the harmful effects of cyberbullying on victims. Multiple studies have linked traditional bullying among teenagers with depression, suicidal ideation, and nonfatal suicidal behavior[59, 60]. However, the psychological outcomes of cyberbullying are inconsistent and unclear, possibly because of its recent development. Some authors have argued that the consequences of cyberbullying are similar to those of traditional bullying[34, 61]; others believe that cyberbullying is more distressing than traditional bullying[62].
The subgroup analysis in this study showed that the risk of depression after being bullied in children and adolescents was significantly higher after 2015 than that before 2015. One explanation is that the recent technological advances and the internet age have facilitated the appearance of cyberbullying in the lives of children and adolescents. Thus, some children and adolescents may not only experience traditional bullying but also cyberbullying. As mentioned earlier, adolescence is a critical time for psychological development; thus, adolescents are at higher risk of depression. The results of this meta-analysis also indicate that the risk of depression in children and adolescents after being bullied is higher in Europe than in Asia; this may be because European countries carry out universal screening for depression in adolescents. In addition, in terms of screening tools, the Epidemic Investigation Center Depression Scale, the Children's Depression Scale, and the Patient Health Questionnaire are widely used to screen for depression in Chinese children and adolescents[63, 64]. However, the screening ability of these scale need to be further verified and revised, and their psychometric properties (such as sensitivity, specificity, and diagnostic accuracy) should be determined according to different regions and survey samples.
This study also found that the risk of depression was similar in children and adolescents who experienced traditional bullying or cyberbullying, suggesting that, while cyberbullying merits attention, school bullying should still be addressed. A variety of support should be included (e.g., from schools, relevant departments, and families) to bolster children’s mental health and provide timely support to promote healthy development.
Lutrick K[65]reported a positive relationship between being bullied and depression in children and adolescents through a meta-analysis, results consistent with those of this study; however, their meta-analysis only evaluated the relationship between being bullied and depression in children and adolescents and focused on Latino populations, which are understudied. In contrast, the present meta-analysis covered a wider range of populations (including those from Europe and Asia) and a larger sample size. We also evaluated the effects of bullying and bullying-bullied behavior on depression. The results may inform the prevention and control of depression in children and adolescents.
In this meta-analysis, the heterogeneity of the included studies was high; after subgroup analysis (according to sex, sampling method, publication year, and region), I2 was still greater than 50%, suggesting that these factors may not have been the source of heterogeneity. Since the 31 included studies were from 13 countries, the definition or assessment of bullying and depression, location, participant ethnicity, and culture may have all contributed to the heterogeneity.
4.6 Limitations
Only cross-sectional data were included in this meta-analysis, which precludes determination of causal associations. Additionally, few studies have been conducted on bullying, bullying-bullied behavior and depression in children and adolescents, which may reduce the reliability of the results. Subgroup analysis was carried out to analyze the heterogeneity among studies. The results of the funnel plot and Egger's test showed that publication bias was present, suggesting that it may be caused by publication type (i.e., gray literature).