1. IntroductionSchool bullying is a worldwide phenomenon and one of the main challenges for the education community. At present, bullying in schools is common in many countries and regions. A European multicenter study reported a bullying victimization rate of 18.2% among children [1]. A Global School-based Health Survey showed that 20.6% of Indonesian students and 28.3 to 51.0% of Southeast Asian countries aged 13–17 years were bullied [2]. Previous studies also showed that 32% of students were bullied in some form by their peers at school on one or more days over the period of a month. Bullying and cyberbullying behaviors represent nowadays a major social problem, affecting 37% of adolescents [3]. Like in any other countries, school bullying was prevalent among students in China: cross-sectional studies revealed that the self-reported prevalence of traditional bullying victimization can be as high as 22.7%. In addition, the estimated prevalence was 20.8% for face-to-face bullying victimization, while 9.6% for cyberbullying victimization [4].School bullying refers to any unwanted, repeated, and harmful aggressive behavior among children and adolescents. It can encompass four possible dimensions, including physical bullying (e.g., kicking, hitting, pushing, grabbing and destroying objects), verbal bullying (e.g., name-calling, teasing, abusing, mocking), relationship bullying (e.g., excluding from social situations, spreading rumors) and cyberbullying (e.g., threating through phones, text messages, WeChat, online emails, personal websites or online forums to attack others) [5,6,7]. Previous studies have shown that bullying victimization not only leads to increased behavioral and emotional problems [8], psychiatric symptoms [9], and reduced academic performance [10], but was also associated with increased risk of suicidal ideation and behaviors [11]. In addition, there is evidence that bullying is strongly associated with depression symptoms from adolescence to young adulthood [12]. Therefore, it is imperative to reduce the risk of depression associated with school bullying victimization in students. However, direct intervention on school bullying victimization is likely to be ineffective. Victims of bullying may choose to hide their experiences for fear of rejection by peers [13]. Under these circumstances, exploring intervenable factors in the association between school bullying victimization and depression among students will provide useful information. Previous studies suggested that the personality was a risk profile for bullying. It reported that bullies showed low agreeableness and conscientiousness, high extraversion and neuroticism, and, unexpectedly, also lower openness [14]. Nasti’s models showed a significant negative association between bullying and openness, conscientiousness and agreeableness [15]. Tani et al. indicated that children with low conscientiousness tend to act in antisocial ways [16]. Personality traits describe individual characteristics such as cognitive, emotional, and behavioral aspects that may play a role in diatheses or an increased propensity to psychopathologic states [17,18], including depressive symptoms. The Big Five personality traits define the five dimensions of personality such as neuroticism, extraversion, openness to experience, agreeableness, and conscientiousness. Different types of personality may also be associated with variable reactivities, such as emotional regulation or coping styles [17,19]. It was previously reported that neuroticism was a strong mediator in the association between gender and depression in addition to various social and psychological factors [20]. However, the role of Big Five personality traits in the occurrence of bullying and depression has not severely been explored among the Chinese population. Under this scenario, exploring intervenable factors in the association between school bullying victimization and depression among Chinese college students will provide useful information.

This study hypothesizes that the Big Five personality traits would have a mediating effect on the relationship between school bullying and depression. It aimed to analyze the influence of the Big Five personality traits on depression in students who had been bullied, and to provide reference data to help develop targeted psychological education to aid in the prevention of bullying. We tested three specific aims in the present study. First, we examined associations among each of the five personality factors, bullying, and depressive symptoms. Second, we explored the direct contribution rate of depressive symptoms after being bullied in school. Third, we examined the mediating roles of each personality factor in the relationship between bullying and depressive symptoms, meaning the indirect contribution rate of the Big Five personality traits generated after being bullied to the production of depressive symptoms.

4. DiscussionIn recent years, school bullying has become increasingly recognized as a major social issue, with research focusing more on this topic. Our study found that males were more likely to be bullied at school than females, but females were more prone to depression than males, same as some previous studies [26,27]. Males in adolescence have rapid physical development and strong physical strength, but their psychological development is still immature, competitive and impulsive with emotional ups and downs. Compared with females, it is easier for males to use violent means to solve problems in life, and thus to bully by insulting, beating or threatening persecution [28]. Females are more mentally sensitive than males, are more susceptible to external environmental influences, and are prone to emotional disorders, such as depression [29]. Adolescents living in a family environment where their parents often quarrel or single-parent families are emotionally indifferent and psychologically inferior due to the lack of care and education of one parent. Those bring not only material and affection deficits to young people, but more serious psychological and spiritual damage. They are unwilling to have more contact with other classmates in school, experience a lack of friends, and are easily affected emotionally and in terms of personality [30]. Therefore, it is easy for this group to experience bullying on campus and negative emotions. Our survey found that there were significant differences in campus bullying and depression experience for research subjects depending on caregivers; the same goes for living expenses per month. Due to the rapid economic development, a large number of rural laborers go out to work to change their living conditions in China instead of taking their children with them, resulting in many left-behind children. Some evidence has indicated that left-behind children have many psychological problems due to the parent–child separation, poor living environment, and lack of effective education and supervision [31]. In order to attract parents’ attention and concern, left-behind children deliberately engage in antisocial behavior, which leads to bullying on campus, or lack of parents’ company for a long time and low economic conditions may lead to children’s inferiority complex and depression. Therefore, perhaps increasing parents’ company and family economic status can reduce the occurrence of campus bullying and depression, which can be used to prevent the occurrence of campus bullying and depression.The occurrence of school bullying seriously affects students’ study and life, resulting in negative emotional states, leading to depression, fearfulness, and even suicide [5]. Our study showed that students who had been bullied were more likely to suffer from moderate to severe depression, with a significant correlation between previous school bullying and depression scores, which indicated that school bullying was a risk factor for depression. These findings are consistent with those of Kaltiala-Heino et al. [32] and Lutrick et al. [33], where it was found that the higher the frequency of bullying, the more often there were cases of depression. Therefore, along with seeking to prevent the occurrence of school bullying, it is also necessary to pay close attention to psychological changes likely to be occurring among the victims of bullying and strengthen the relevant psychological aspects of education among students. Further, attention must be paid to improving student social skills, interpersonal skills, psychological robustness, and the use of appropriate skills when confronted with bullying. In addition, some studies have shown that there was a great relationship between personality and bullying on campus, which could be used as an important research field in the prevention of bullying [15]. Our findings are similar to those of the above researches. Consequently, the prevention of bullying on campus may be considered from the perspective of development of a stable personality in childhood and adolescence. This requires parents and teachers to pay attention to the daily changes of bad emotions and psychology in children and guide them.The depression and bullying displayed a positive correlation with neuroticism and a negative correlation with conscientiousness, openness, agreeableness, extraversion. These findings fit the historic notion that neurotic introverts are especially vulnerable to depression [34]. Personality factors are known to be involved in the regulation of emotion and cognitive vulnerability that contribute to a predisposition to depression. Neuroticism is associated with negative emotions such as anxiety, fear, and anger [35]. Extraversion is the tendency to be active and sociable [36]. Openness to experience is the tendency toward preferring unconventional ideas and experiencing diverse emotions [37]. Agreeableness refers to interpersonal characteristics such as altruistic and cooperative tendencies. Conscientiousness is characterized by persistence, organization, and goal-directed behavior [38]. Bullying was a specific type of interpersonal violence. Victims of bullying usually reported stronger negative emotions and higher levels of emotion dysregulation compared with those who were not bullied [39]. It has been documented that lower levels of personality functioning in depressed patients were associated with a broader spectrum of negative emotions [40]. This was probably because individuals with a higher level of personality have more promoting and compensatory factors, and therefore may experience fewer symptoms of depression even if being bullied. Meanwhile, we determined that personality had an importantly intermediate association between school bullying and depression. Among all dimensions of personality, neuroticism played the strongest mediating role. Neuroticism had a significant positive predictive effect on developing depression, while conscientiousness, agreeableness, openness, and extraversion had a significant negative predictive effect, similar to the findings of previous studies [41]. This major finding highlighted the significance of personality in interventions designed to prevent depression associated with school bullying in Chinese students. Therefore, the incidence of depression caused by school bullying could be reduced through seeking to adapt students’ personality traits. Specifically, tendencies towards neuroticism, which leads to poor emotional stability and tends to orient behavior towards impulsivity, irritability, and psychological stress, need to be confronted and students should try their best to develop the ability to control their own emotions and form a stable personality. After the appropriate development of these character traits, students would be more likely to exhibit greater compassion and a sense of responsibility, engage in positive communication and establish affirming relationships with others, have good resilience and be optimistic concerning the future. They would also not be inclined to arbitrarily abuse, fight with, mock others, or spread malicious rumors, and, when dealing with instances of bullying, they would have appropriate psychological and emotional stability to enable them to confront bullying effectively, addressing negative emotions. Studies have also shown that individuals with extroverted, cheerful, easy-going, and self-disciplined personalities have better mental health, while individuals with greater levels of emotional instability or neurotic tendencies are more likely to have more psychological problems [42].

There are several strengths in the present study, including study sample size and methodological approaches. Although the relationship between previous school bullying and the occurrence of depressive symptoms were earlier reported across different countries, our study is the first to be performed in the China population examining the mediating roles of five factor personality traits in the link between bullying and depressive symptoms. The large sample size derived from a population cohort was useful for studying the risk factors and etiology that may contribute to the development of depressive disorders in Chinese college students.

Despite these strengths, there are also several limitations. First, this was a cross-sectional study, therefore causal inference should be avoided. Second, the retrospective self-report method used in collecting data may be prone to information bias. Finally, study participants were chosen from a specific province in northeast China, and representativeness should be considered when interpreting the results.