Sex Differences in the Global Prevalence of Nonsuicidal Self-Injury in Adolescents

This meta-analysis evaluates sex differences in the prevalence of nonsuicidal self-injury among adolescents within and between geographical regions.


Introduction
Nonsuicidal self-injury (NSSI) is defined as "the deliberate, self-inflicted destruction of body tissue resulting in immediate damage, without suicidal intent and for purposes not culturally sanctioned." 1 The mean age of onset for NSSI is approximately 13 years, 2 and the behavior is common among adolescents, [3][4][5] with global prevalence estimates between 18% and 23%. 1,2,6Historically, there has been debate on the validity, and therefore merit, of distinguishing suicide-related behavior as with or without suicidal intent. 7][10][11][12] Moreover, NSSI as a phenotype has been studied among youths across the world, 2 and knowledge of this behavior in a global context is especially important because of the associated risk of suicide.
0][41][42][43][44][45] In North America, the rates of NSSI are rising among all adolescents, 46 but disproportionately among younger female adolescents. 47,48There is also preliminary evidence to suggest that the prevalence of NSSI among adolescents differs by sex across geographic regions.38 To date, studies reporting prevalence of NSSI among adolescents tend to present the findings disaggregated by sex; however, a systematic approach is needed to evaluate and quantify the geographic variability in the prevalence of NSSI among female vs male adolescents worldwide. This nformation is essential to develop sex-, regional-, and potentially culture-specific interventions for NSSI among adolescents across the globe that can ultimately mitigate the risk of suicide in these populations.Therefore, the objective of this study is to examine sex differences in the global and regional prevalence of NSSI among adolescents (henceforth defined as those aged 10 to 19 years).

Methods
A systematic review of the literature on sex differences in the prevalence of adolescents with NSSI was performed according to the Preferred Reporting Items for Systematic Reviews and Metaanalyses (PRISMA) reporting guideline. 49The protocol for this review was registered with the international prospective register of systematic reviews before data extraction and analysis (CRD42022347504).This protocol describes a systematic review that provided data for 2 studies: the current meta-analysis and a separate systematic review of sex differences in the clinical correlates of NSSI among adolescents.

Search Strategy
We searched MEDLINE and APA PsycINFO with the terms adolescents, self-injury, sex factors, and synonyms in May 2022.A complete search strategy can be found in the eMethods in Supplement 1.

Inclusion Criteria
Studies were included if they were published in English and reported on NSSI in female and male adolescents aged 10 to 19 years.Studies were only included if they reported original data and explicitly defined self-harm as not having suicidal intent.Given the paucity of research on sex differences in NSSI, studies were not excluded on the basis of study design.Similarly, studies were included regardless of whether they defined a specific period during which the NSSI occurred (eg, recent or lifetime occurrence).The reference lists of included articles were also reviewed for relevant citations.

Exclusion Criteria
Studies were preliminarily excluded if the data for adolescents were not reported separately from other age groups, if they did not clearly differentiate self-injury based on suicidal intent, or if they did not report on prevalence in female and male adolescents separately.Articles were also excluded if

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Sex Differences in the Global Prevalence of Nonsuicidal Self-Injury in Adolescents they had limited relevance to the topic of this meta-analysis, including only reporting on NSSI in individuals with a specific diagnosis or characteristic.Grey literature and conference abstracts were also excluded.Articles published before January 2000 were excluded, as the early 2000s marked a turning point in research in the area, in part due to the creation of the first validated measure for what was then called deliberate self-harm (including NSSI). 50Lastly, all eligible articles were quality assessed by 2 independent reviewers (F.M. and J.A.) using the Joanna Briggs Institute Critical Appraisal Checklist for Cross-Sectional Studies 51 or the Joanna Briggs Institute Critical Appraisal Checklist for Cohort Studies. 52Articles were excluded if they received less than 4 on their respective checklists.The article selection process is shown in Figure 1.

Article Selection Process
Articles were screened by title and abstract by 2 independent unblinded reviewers (F.M. and R.H.B.M.).Full-text reports were reviewed for study eligibility by the same reviewers.Discrepancies were resolved by consensus.

Data Collection
Data were recorded for the following categories: (1) author(s); (2) year of publication; (3) country of origin (where the study was published or conducted); (4) aims or purpose; (5) population and sample size; (6) methods.For characteristics of studies included, please refer to eTable 1 in Supplement 1.
Geographical regions were defined by continent.

Statistical Analysis
Prevalence data were extracted and quantitatively pooled.Random-effects meta-analysis was conducted using Meta-Essentials version 1.5 for Microsoft Excel (Erasmus Research Institute of Management). 53A random-effects model was chosen given the high expected heterogeneity of included studies.Odds ratios (OR) were used as the indicator of effect size.The weighting method used was inverse variance.ORs were calculated for community samples only, as clinical samples are less generalizable to the overall population.Post hoc analyses were conducted using 2-way analyses of variance (ANOVAs) to assess regional variation of NSSI prevalence within sex.
Heterogeneity was assessed using the I 2 statistic.For analyses with an I 2 greater than 75%, which generally indicates that a large proportion of variability can be accounted for by heterogeneity between studies, moderator analyses were conducted.Moderator analyses were run for year of publication (mean-centered on 2015), survey time period (lifetime NSSI vs past month to 5 years), and geographical region (Europe vs Asia and North America and Asia vs Europe and North America) separately using regression analyses to attempt to determine sources of heterogeneity.These moderators were chosen given that trends in suicide-related behavior change over time, that studies looking only at recent NSSI may show different characteristics than those looking at lifetime occurrences, and that certain regions may show greater differences in reporting and analyzing NSSI than others, respectively.Presence of publication bias was assessed with visual inspection of funnel plots and Egger regression.The threshold for statistical significance was P = .05,using 2-sided tests for statistical analyses.Data were analyzed from July 2022 to April 2023.

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Sex Differences in the Global Prevalence of Nonsuicidal Self-Injury in Adolescents
Publication bias (see eFigure 5 in Supplement 1) did not reveal any obvious asymmetry on visual inspection, with the exception of 1 outlier.Egger regression was also not statistically significant (t = −1.08;P = .29),indicating that publication bias is unlikely to have significantly affected the findings of this analysis.

Prevalence by Region
The community prevalence of NSSI among adolescents was significantly higher in female adolescents than male adolescents in North America (  3).It was also higher in the single Australian sample (18.8% and 16.8%, respectively; OR, 1.15).In contrast, prevalence rates were not significantly different between sexes in Asia (24.1% for female adolescents vs 24.8% for male adolescents; OR, 1.00; 95% CI, 0.71-1.41)(Figure 4).[42][43][44][45] Post hoc analyses using ANOVA did not reveal a regional difference in the prevalence of NSSI among female adolescents (F 2,30 = 5.31; P = .01).In contrast, there was some regional variation in the prevalence of NSSI among male adolescents (P = .01),with higher prevalence reported among male adolescents in Asia compared with North America (t 17 = 3.15; P = .003)and Europe (t 20 = 2.29; P = .02).There was no regional variation in the prevalence of NSSI among male adolescents in Europe as compared with North America (t 10 = 0.45; P = .33).ANOVA and t test statistics can be found in eTable 2 in Supplement 1.

Discussion
This meta-analysis summarizes sex differences in the prevalence of NSSI among adolescents around the world.We report a higher prevalence of NSSI among female adolescents compared with male adolescents overall, although considerable regional variation exists.In North America, Europe, and Australia, the prevalence of NSSI was higher among female adolescents; whereas in Asia, the prevalence of NSSI did not significantly differ by sex, and in some samples was higher in male adolescents.There were no regional differences in the prevalence of NSSI in female adolescents; however, male adolescents in Asia had a higher prevalence of NSSI than male adolescents in other regions.

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Sex Differences in the Global Prevalence of Nonsuicidal Self-Injury in Adolescents To date, the literature on sex differences in the prevalence of NSSI among adolescents has reported inconsistent results, with some studies recording higher prevalence in female adolescents and others recording no sex difference. 1,2This meta-analysis reports a higher overall prevalence of NSSI among all pooled female adolescents compared with male adolescents, with an OR of 1.60, denoting a moderate effect size. 60This higher prevalence of NSSI among female adolescents builds on findings from a recent meta-analysis 2 of 158 samples showing female adolescents report significantly more self-harm than male adolescents, with an OR of 1.72 (ie, moderate effect size), although direct comparisons are not possible as Gillies et al 2 use a broader definition of self-harm that does not specify suicidal intent.While the large proportion of Western samples in both metaanalyses likely influences the effect sizes reported, it is nevertheless worth considering if higher rates of NSSI in female adolescents across the globe reflect known sex differences in depressive disorders, internalizing symptoms, and identity disturbance that are more common in female adolescents compared with male adolescents, and if these factors moderate the relationship between NSSI and female sex, regardless of the region of the world. 31,61,62e portrayal of NSSI in the media and the ensuing effects of contagion are also elements likely contributing to the increased prevalence of NSSI among female adolescents across the globe.NSSI is often portrayed through a young female character in psychological distress who uses self-injury as a coping mechanism. 63Given the influential role of media and popular culture in the day-to-day life of adolescents and how media cross-permeates different cultures, it may be that female adolescents who identify with characters who engage in self-injurious behavior are more likely to engage in NSSI regardless of where they live. 63,64though we found NSSI to be generally more common among female compared with male adolescents, there were some notable geographical exceptions.In Asia, the prevalence of NSSI was either equivalent in female and male adolescents, or the female-to-male ratio was reversed compared with other regions.A parallel trend in Asian populations is suicide rates by sex that are incongruent with the commonly reported gender paradox of suicide-more suicide attempts in female adolescents and more deaths by suicide in male adolescents 65 -seen in Western countries. 66,67These contrasting findings demonstrate the need to explore regional factors influencing sex differences in the prevalence of all suicide-related behaviors and not solely NSSI.
Notably, post-hoc analyses did not reveal a significant difference in the regional prevalence of NSSI among female adolescents but did reveal a higher prevalence of NSSI among male adolescents in Asia as compared with male adolescents in other regions.Our findings appear to suggest that female adolescents engage in equivalent rates of NSSI regardless of the region, whereas male adolescents in Asia engage in NSSI more often than in other regions.The reasons for this are unclear, and research is lacking, but it may be attributable to different gender roles and/or sociocultural factors in Asia vs Western cultures such as North America, Europe, and Australia. 68For example, Western media presents self-harm as a stereotypically female-gendered behavior 69 ; hence, male adolescents living in these regions of the world may be less socialized than female adolescents to engage in NSSI.Previous research also demonstrates that NSSI may be used more commonly in non-Western countries as an attempt to regulate negative emotion specifically arising from interpersonal conflict, rather than to cope with emotion dysregulation more generally. 19,70,71Why this would be more relevant for male adolescents living in Asia warrants further examination.Lastly, it is possible that regionally rooted sociocultural factors may have affected participants' willingness to self-report NSSI, depending on the stigma and/or perceived consequences of disclosing NSSI. 72ese factors may have influenced underreporting of NSSI in certain groups, thus contributing to the geographical variations of NSSI prevalence by sex reported in this study.Given the regional variation of NSSI in male adolescents, there is a strong argument for further research to elucidate possible reasons for this and to allow for the development of regionally and perhaps culturally specific interventions for the treatment and prevention of NSSI among adolescents.

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Sex Differences in the Global Prevalence of Nonsuicidal Self-Injury in Adolescents

Limitations
Limitations of this meta-analysis include the significant heterogeneity of study designs and sample characteristics (eg, age ranges) and that not all possible moderators or combinations of moderators could be accounted for.Furthermore, while all studies defined NSSI as self-injurious behavior without suicidal intent, participants were classified into an NSSI group based on varying criteria (eg, past month engagement in NSSI vs lifetime engagement in NSSI vs a certain frequency of NSSI in the past year).At minimum, participants classified as having engaged in NSSI would have done so at least once in their lifetime, but some studies employed more stringent criteria, making it difficult to directly compare findings between studies.It should also be noted that the majority of studies included in this analysis were conducted in upper-middle income and high-income countries across the globe, with limited numbers of samples from certain regions such as Australia and an absence of representation from South America or Africa, and thus these findings may not be able to be extrapolated to countries not represented here.The restriction of our search to English language articles is also a limitation given our focus on geographic variability.Lastly, most studies used participants' self-reported sex and may not have accounted for cisgender vs transgender identity, limiting our ability to make any conclusions regarding the effect of gender identity vs sex assigned at birth.Given that gender dysphoria is a known risk factor for NSSI and suicide, 73 future work in this area, with a focus on sex differences and regional variation, will be of paramount importance.

Conclusions
In summary, this meta-analysis provides the important global and regional estimates of the femaleto-male prevalence of NSSI among adolescents.Globally, NSSI was more prevalent among female adolescents than male adolescents, with equivalent prevalence estimates among female adolescents across regions.By contrast, the prevalence of NSSI among male adolescents was higher than that in female adolescents in Asia and male adolescents in other regions.Future research with sex-, region-, and culture-specific lenses will be essential in clarifying how and why NSSI phenotypes differ by sex in different regions.Research in this area will be foundational in developing and evaluating effective interventions for all adolescents engaged in NSSI regardless of location and sex.

Figure 1 .
Figure 1.Flow Diagram of the Study Selection Process 1074 Records identified from databases a

Figure 2 .
Figure 2. Geographical Distribution of Included Community Samples, Separated by Continent

eReferences. SUPPLEMENT 2. Data Sharing Statement
Moderator Analysis for Year of Publication Mean-Centred Around 2015 eFigure 2. Moderator Analysis for Asia (14 Samples) as Compared With Other Geographical Regions eFigure 3. Moderator Analysis for Europe (Samples) as Compared With Other Geographical Regions eFigure 4. Moderator Analysis for Lifetime NSSI vs Recent NSSI (Past Month to Past 5 Years) as Survey Measure eFigure 5. Funnel Plot of Effect Size (Log or Female:Male) vs SE to Assess for Publication Bias eTable 1. Characteristics of Included Studies eTable 2. ANOVA and t-Test Values for Post-Hoc Comparisons SUPPLEMENT 1. eMethods.Full OVID Search Strategy eFigure 1.