Introduction

Longitudinal and case-control studies have established that self-harm (defined as intentional self-poisoning or self-injury, irrespective of the motive or the extent of suicidal intent [1]) during adolescence is a risk factor for numerous adverse clinical and social outcomes [2–4], including suicide [5]. As such, the occurrence of self-harm in pre-teen children is a particularly concerning event [5, 6]. Yet to date, knowledge about self-harm in young people derives almost exclusively from adolescent populations [7–12], and few studies have examined self-harm in non-treatment-seeking young people aged 12 years or under [13–18]. This is a key period in the life course when the first symptoms of common mental disorders, such as depression and anxiety, begin to emerge [19, 20]. As such, no prevalence estimates of self-harm in primary school-aged children in the community have been published [1]. Much of what we know comes from treatment-seeking samples of adolescents [5, 10, 21–23], and the degree to which these findings are applicable to the general population remains unknown. Additionally, almost nothing is known about the characteristics of primary school-aged children who engage in self-harm, particularly in relation to pubertal stage [11]. This is important because pubertal stage has been linked to differing levels of risk for the onset of mental disorders and early substance abuse [19].

We addressed these notable gaps in the literature by analysing data from a prospective cohort study in Victoria, Australia: the Childhood to Adolescence Transition Study (CATS) [24]. We aimed to describe the cross-sectional correlates and antecedents of self-harm in primary school-aged children. In the absence of comparable data in the published literature on which to formulate specific a priori causal hypotheses, we conducted an exploratory analysis of CATS data to generate the first health, social and demographic profile of self-harm in this population.

Methods

Results

The recruited sample (n = 1 239) contained a slightly lower proportion of boys than girls (46% boys; 54% girls) compared with census data for 8–9 year-old schoolchildren across the state of Victoria (51% boys; 49% girls) [36]. Participants scored slightly higher than the Australian population average for SES (mean SEIFA = 1 012, standard deviation [SD] = 67 vs. M = 1 000, SD = 100) [36]. A higher percentage of participants identified as Indigenous compared with all schoolchildren of the same age in Victoria (5% vs. 1%) [36]. At wave 4, 1 067 of the 1 239 recruited students (86.1%) completed the student questionnaire and this is the sample we used for the study (see Table 1).

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Table 1. Participant baseline demographic characteristics and self-harm reported at wave 4 (age 11–12 years) (multiple imputation analysis; N = 1067).

https://doi.org/10.1371/journal.pone.0242802.t001

Self-harm data were available for 1 059 participants (96.4% of all 1 067) participants at wave 4. Twenty-eight participants (2.6%) reported past-year self-harm, of whom 18 (64.3%) were females and 10 (35.7%) were males. Ten (35.7%) reported self-battering, 8 (28.6%) reported cutting, 2 (7.1%) reported choking/hanging, 2 (7.1%) reported scratching, and one (3.6%) reported jumping from a height. A further five participants (17.9%) did not specify the nature of their self-harm. Table 1 shows descriptive estimates (obtained via multiple imputation) of the characteristics of the whole sample (n = 1 067): 47.5% were males and 52.5% were females, with a mean age of 11.9 years (SD = 0.39, range 10.7 to 13.4 years) at wave 4.

Table 2 displays unadjusted and adjusted estimates of the associations between participants’ demographic, health, social, educational, and family-related factors at wave 4 and self-harm at wave 4 (obtained via multiple imputation). These cross-sectional results indicated that the presence of depressive symptoms, being in mid-puberty or late/post-puberty, and experiencing difficulties with peer relationships were most strongly associated with self-harm at wave 4. Estimates obtained from available case analysis (conducted as a sensitivity analysis) were similar and are displayed in S1 Table.

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Table 2. Cross-sectional associations between participant characteristics at wave 4 (age 11–12 years) self-harm reported at wave 4 (multiple imputation analysis; N = 1067).

https://doi.org/10.1371/journal.pone.0242802.t002

Table 3 displays the estimated associations between health, social, academic and family characteristics in waves 1–3 and self-harm at wave 4 (obtained via multiple imputation). A similar pattern of variables that were concurrent correlates of self-harm at wave 4 were also antecedents of self-harm. Importantly, the presence of these characteristics at two or more waves was typically associated with larger adjusted odds ratios than the presence at one wave only. Estimates obtained from available cases analysis (conducted as a sensitivity analysis) were similar and are displayed in S2 Table.

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Table 3. Associations between wave 1–3 (age 8–9 years to 10–11 years) health, social, academic and family characteristics and self-harm reported at wave 4 (age 11–12 years) (multiple imputation analysis; N = 1067).

https://doi.org/10.1371/journal.pone.0242802.t003

Discussion

In this prospective cohort study examining the prevalence, correlates and antecedents of self-harm in a population-based sample of children in Melbourne, Australia, we found that 3% of 11-12-year-olds had self-harmed during the previous 12 months. To our knowledge, these are the first prevalence estimates of self-harm among a community-dwelling sample of primary school-aged children internationally. Our findings suggest that mental health, puberty and peer relationships are most strongly associated with self-harm among primary school-aged children. Participants who had few friends, and those who had experienced bullying victimisation, were seven and 24 times more likely to have self-harmed at age 11–12 years, respectively. Additionally, participants were experienced frequent social exclusion by their peers, and those who had been teased frequently, were four and 15 times more likely to have self-harmed, respectively. These findings add weight to the predictive impact of bullying victimisation on self-harm in young people [37, 38] and demonstrate that this impact is observable at a considerably younger age than has previously been reported. In a recent prospective multicentre study from Europe examining life events as risk factors for self-harm in adolescents aged 14–15 years, Kaess et al. [38] demonstrated that prior bullying victimisation was associated with the first onset of self-harm at 12 month follow-up. Our finding also supports the conclusion of a recent meta-analysis which demonstrated an association between bullying and cyberbullying victimisation and self-harm in young people [39]. In terms of mental health, participants who self-harmed were more than seven times more likely to experience depressive symptoms and five times more likely to experience anxiety than their peers who had not self-harmed.

Life-course theorists have long posited that the period between childhood and adolescence is a critical developmental period during which social learning and interaction with peers become central developmental functions [40]. Young people begin to enact various social and behavioural strategies during this period and receive prompt feedback about the success (or lack thereof) of the chosen strategies [41]. As such, experiences during this period can have broad-ranging implications for children’s later social development and can be expected to affect many areas of behaviour, from attachment to aggression and sexuality. For participants in our study who experienced persistent difficulties with their peer groups and later self-harmed, it is possible that this self-harm may contribute to further social isolation due to the stigma associated with such behaviour [42]. Other potential mechanisms may also contribute to self-harm in children and adolescence. These include the influence of sex hormones, which act on hippocampal and hypothalamic systems and shift control of affect and cognitive process, may contribute to the onset and rate of self-harm [11]. Potential mechanisms also include factors related to cognitive development, as evidence suggests that many young people who self-harm demonstrate specific deficits in problem-solving abilities [43]. Similarly, difficulties with emotion regulation may precipitate self-harm in young people; participants who reported poor emotional control in our sample were more than four times as likely to have self-harmed. This finding supported those of a recent study by Palmer et al. examining the association between emotion dysregulation and the onset of self-harm in adolescents aged 14–15 years in the UK [44]. Palmer et al. reported that, prior to the onset of first self-harm, participants experienced difficulties regulating their emotions and, specifically, a lack of emotional clarity and poor impulse control. These difficulties in regulating emotions, the authors argued, might help to identify adolescents at increased risk of engaging in self-harm in the future.

In our study we did not assess the presence or degree of suicidal intent when measuring self-harm and were therefore unable to make inferences about the prevalence, antecedents or characteristics associated with non-suicidal self-injury (NSSI). We adopted this approach, in part, due to the many limitations of NSSI as a concept [45]; first, the definition of NSSI is restricted to methods such as cutting, burning, stabbing, and hitting, thus ensuring that any act of non-suicidal self-poisoning cannot be classified as NSSI [45]. Second, longitudinal research has identified NSSI as one of the most important risk factors for suicide attempts [46], indicating that intentionality can change over time. Third, as many people use different methods of self-harm on different occasions [47], it is possible that people may engage in both NSSI and self-poisoning at different times, which would result in an under-ascertainment of self-harming behaviours if the NSSI categorisation alone was used.

Strengths and limitations

Strengths of our study include its prospective design, multiple assessment points across a narrow age range, large population-based sample size, comprehensive measures of emotional and behavioural problems, and inclusion of both males and females. Additionally, we scrutinised all participants’ free-text descriptions of self-reported self-harm events in order to improve case ascertainment. Unlike many previous studies [13, 16, 18], our sample was a non-treatment-seeking sample and participants did not need to have sought medical help for self-harm to have been included. Our study also contained some potential limitations. First, the number of participants who had self-harmed by age 11–12 years was relatively small; however, these participants formed part of a much larger, population-based community cohort for whom we have comprehensive demographic, mental health, social, family composition, and educational data. Second, an active parental consent process was employed at recruitment, with only 54% of parents providing written consent. Third, the sample was skewed towards higher SES and had a higher proportion of Indigenous children than the general Australian population. Fourth, we did not ask about self-harm prior to wave 4 and this may represent a missed opportunity to collect informative data. Fifth, self-harm was measured via self-report and this may have contributed to an under-ascertainment of cases [48]. Finally, we did not assess the presence or degree of suicidal intent when measuring self-harm and were therefore unable to make inferences about the prevalence, antecedents or characteristics associated with NSSI. As discussed above, however, this approach resulted in a larger proportion of all self-harming behaviours being detected.

Conclusion

Primary school-aged children who experience persistent difficulties within their peer group, including bullying, social exclusion and teasing, are at increased risk of self-harm as they progress through the pubertal transition. These peer problems, along with the early onset of puberty, dominate the risk profile for self-harm among children at this age and represent tangible, modifiable risk factors which may benefit from targeted prevention initiatives. The focus of existing intervention efforts aimed at preventing and reducing adolescent self-harm should extend to primary school-aged children, with a particular focus on mental health and peer relationships during the pubertal transition.

Supporting information

Acknowledgments

The authors would like to thank all the families and schools who have participated in this study. We would like to thank all staff and volunteers involved in data collection and processing at MCRI. Permission was granted from the Victorian Department of Education and Training and the Catholic Education Office Melbourne to recruit through their schools.