Bullying is a universal social-health problem, having an impact on a large number of adolescents. In our study, 46% of the school sample reported involvement in bullying as current or former targets. Earlier studies have found similar prevalence rates ranging from 40 to 43% [
27,
28]. An additional 24% of the students had prior involvement in school conflicts or victimization. Although the definition criteria for bullying were not fulfilled by the conflict group, the study showed a high prevalence of school victimization in a representative sample of school children in Germany (70%). In accordance with the discussion of earlier research [
5,
33], and the recommendation of Fischer and Riedesser [
38], that the term bullying in the context of psychological traumatology should be reserved to describe a “severe, potentially traumatic situation”, we differentiated moderate from severe bullying. Our results showed that 40% of the overall bullying group comprised the severe bullying group, which was comparable to the findings of Solberg and Olweus [
5], who reported that among targets of bullying 38.3% were bullied at least weekly in the last couple of months. Altogether, every sixth student (17.3%) was subject to severe bullying according to our definition (longer than 6 months and more than once per week). This finding supports Rigby [
40] who reported that 15% of the school sample had been bullied once a week or more. Although the association between the frequency or duration of bullying and symptoms of PTSD were examined in earlier research, as far as we know, the combination of duration and frequency has rarely, if ever, been investigated before. In line with Mynard et al. [
27], boys and girls were equally likely to have been bullied. However, these results are in conflict with other studies that report more targets among boys [
5,
28].
Bullying and posttraumatic stress
Results show a high symptom level of PTSD among bullied students. Around 50% (range 46.2–61.5%) of the severe bullied adolescents had scores within the clinical range. These findings are consistent with the meta-analysis by Nielsen et al. [
11] in which, on average, 57% of bullied persons reached the clinical threshold in PTSD questionnaires. In our clinical sample for comparison, 65.2%–72.5% reached the critical range with no significant differences between the severe bullying group and the clinical sample. This suggests that severe bullying targets show clinically relevant symptoms of PTSD. Matthiesen and Einarson [
10] compared adult targets of bullying to a traumatized group using the PTSS-10, and reported even higher symptom rates among the bullying targets. This result might be explained by the type of recruitment because their bullying group was recruited from a help seeking population. In our study, the traumatized sample was drawn from a help-seeking population, whereas the severely bullied students were recruited from a randomly selected school sample.
Maltreated children are more likely to be bullied than children who have not been maltreated [
8]. Therefore, high scores on PTSD symptom questionnaires could potentially be caused by experiences of serious and adverse life events in the past. To alleviate this potential bias in our analysis, we excluded this group from a second sensitivity analyses. Although the statistical effects were slightly reduced, the severe bullying and clinical groups reached parity on the PTSD symptom scales even after the exclusion of those with additional experiences (CRIES). Additionally, the PTSS-10 scores were still high among those in the severe bullying group, especially girls. Furthermore, the severe bullying group still showed the greatest risk of reaching critical scores (40.9–59.1%, controls = 0%). As the exclusion of students with additional serious life events did not change our main results, it is likely that the high scores are specifically associated with bullying and not largely influenced by multiple traumatic events. This finding confirmed our hypothesis that symptoms of PTSD mainly resulted from bullying, supporting Nielsen et al. [
11], who found that PTSD symptoms were overrepresented in bullying targets. Thus, prevention of bullying at school may reduce traumatic experiences and consequent PTSD development.
In the PTSS-10 girls scored higher than boys. This is consistent with studies reporting higher rates of PTSD among females within the general trauma field [
12,
15]. Questions remain on whether gender is a risk factor for PTSD per se or if this effect is influenced by characteristics such as levels of symptom reporting, e.g. women have been shown to be more willing to disclose traumatic experiences than men [
15]. However no gender differences could be found in the CRIES where boys and girls were equally likely to score within the clinical range. The inconsistent gender effect within our study may point to the methodological problem of heterogeneity in definitions and operationalization of PTSD symptom measures [
41]. Interestingly, our CRIES results are similar to Mynard et al. [
27] who found no gender differences in the long version of the CRIES (Impact of Events Scale; [
37]) but contrary to Idsoe et al. [
28] who found higher rates for girls in the CRIES and more girls who reached the clinical range. Overall, gender differences in PTSD symptoms might arise due to questions that are more applicable or even just easier reportable for girls (like nightmares and anxiety) while boys tend to deny these symptoms because of their social role. As another hypothesis, girls tend to cope with stressors by asking for social support [
42]. If this support is affected by bullying and exclusion it may be more difficult for girls than for boys to solve their problems on their own, resulting in higher levels of PTSD symptoms [
41]. Overall, the results on gender differences of PTSD symptoms remain inconsistent (in particularly with regards to bullying and PTSD symptoms); therefore further studies should examine gender specific reactions and coping strategies following bullying among adolescents.
As expected, there was a linear trend in the degree of PTSD symptoms and experiences of verbal or physical aggression (control group < conflict group < moderate bullying group < severe bullying group). The conflict group showed slightly more symptoms than the control group, but fewer symptoms than the moderate bullying group. Given the definition of bullying stating that targets of bullying are unable to defend themselves [
32], one might assume that the conflict group represents harassed students who can defend themselves rather than become helpless [
43]. Contrary to the discussion that the use of the term bullying is inflated [
44], we found a group of students who experienced peer aggression but did not assign the term carelessly; they were able to discern between bullying and other kinds of victimization. Further research should reveal whether this group is more likely to become bullying targets in the future, or if they might be even more resilient.
In the CRIES, the severe bullying group reached clinical ranges of scores indicating higher levels of PTSD symptoms, i.e. three times more often (61.5%) than the moderate bullying group (23.8%). The interrelationship between the symptoms in the CRIES and duration and frequency of bullying is also reflected in the significant correlation scores. Hence, duration and frequency of bullying had a considerable influence on the level of symptoms in the CRIES. In the PTSS-10, twice as many students of the severe as the moderate bullying group reached the clinical range (46.2% vs. 19.5%). The differences in the averages between the severe and the moderate bullying group, however, was not significant, which is also reflected in the non-significant correlations of duration and frequency with the PTSS-10 scores. Hence, longer or more frequent bullying did not lead to more symptoms in the PTSS-10. Although further research is necessary, these results might suggest that there is a critical threshold where longer duration and higher frequency is no longer associated with an increased severity of PTSD symptoms.
The elapsed time since the events did not automatically lead to a decrease in the symptoms, neither in the traumatized, nor in the bullying groups. This underscores the relative time stability found in other research, which characterizes PTSD [
10,
12] contrary to adjustment disorder where the symptoms last no longer than 6 months [
22]. This implies that bullying in children and adolescents may negatively affect their wellbeing, even months or years after an incident. Other studies also note the long-term effects of bullying [
6]. Furthermore, this gives weight to the assumption that the students’ symptoms are more than simple stress reactions or short bursts of mood swings in response to negative experiences, indicating that this group of students is a clientele that needs help. In the present study, the presence of symptoms, even after the bullying had ceased, can also be explained in part, by external factors. As schooling is mandatory, students are reminded regularly of their negative experiences by the setting and ongoing contact with their abusers. Our study and the literature show that bullying is associated with the three symptom clusters of PTSD [
11]. A discussion on whether or not bullying constitutes a causal factor of PTSD development is indicated. If so, the current validity of the Criterion A needs reviewing. Other authors have already questioned the functionality of PTSD diagnostic criteria [
18,
20]. Van Hoof et al. claim that the clarification of events as either traumatic or non-traumatic is determined by rater’s subjective interpretation of the diagnostic criteria, and hence a matter of opinion [
18]. At the moment, bullying targets receive little or no help to deal with their short and long-term consequences. A proper diagnosis could increase support and treatment availability to those affected. This is even more important as post-event factors may play a major role in determining whether or not a child develops PTSD following a traumatic event [
45]. Further research should investigate whether access to PTSD treatments could support bullying targets to cope with long-term effects.