Nonsuicidal self-injury (NSSI) is defined as a direct [1], repeated and socially unacceptable injury to a person’s body tissue [2], a deliberate destruction of one's own body tissue [3], and without suicidal thoughts [4]. It includes cutting, scratching, or burning the surface of one’s body, as well as hitting with objects, and inflicting direct damage to one’s skin or bones by a person [5]. This is a common character in borderline personality disorder [6], and was one of the initial criteria of borderline personality disorder [7]. The criteria for NSSI were presented in Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), but it needs further empirical study to be mentioned in the next version of DSM. The criteria for NSSI include the following: the person having intentionally injured surface parts of their own body such as cutting, burning, biting, and scratching skin for at least 5 days over the past year; this act cannot have been culturally influenced [8]; the intentional injury should have been associated with negative emotions and feelings such as pervasive anxiety, stress, anger, and confusion; and this act must have been committed to achieve a specific goal [9].

It usually begins in adolescence, the age of onset has been reported to be between 14 and 16 years [10], and its rate decreases in late adolescence [11]. As a common psychiatric symptom, it puts adolescents at greater risk than other age groups [12]. The prevalence of this disorder in teenagers has been reported 16% [13]. Sex differences are also important in this disorder, which occurs more commonly in girls [14]. The prevalence of NSSI in teenage girls is reported to be 19% [15]. There are also cultural differences in the prevalence of these behaviors. The overall prevalence of NSSI (32.6%) is higher in Asian countries compared with Western countries (19.4%) [16].

According to the prevalence of this disorder in different ages and cultures, there are various predisposing factors based on the results of qualitative research. One of the etiological factors, for instance, is attachment styles [17,18]. Generally, the predisposing factors for the development of NSSI can be divided into individual and environmental factors [19]. Adolescence is a period of physical and psychological growth, and it is important because of changes related to puberty, identity, and independence. This stage is accompanied by important and sudden changes such as physical, sexual, cognitive, emotional, and moral changes, as well as changes in desires, values, and patterns. All these factors together create difficulties for teenagers, and training, support, time, and patience are required to answer their questions or have their problems solved. If teenagers cannot manage and solve their problems, they may turn to inappropriate practices such as NSSI [20].

On the other hand, attachment styles are known to be one of the environmental factors, which play an important role in the onset, formation, and maintenance of NSSI in adolescents [21]. Based on the research results, insecure attachment plays an important role in the tendency of people to NSSI [22]. Unfavorable family conditions in the early years of life have been identified as risk factors for presenting NSSI. There are correlations between NSSI and parental absence [23], poor quality of relationship with parents [24,25], and a lack of social support [26].

Attachment is a deep and enduring emotional bond that connects one person to another across time and space. Attachment is an emotional bond between child and the primary caregiver. The caregiver’s response pattern can help regulate the child’s emotional states so that the child can develop a secure attachment style. On the other hand, an inconsistent response pattern or less responsive caregiving can lead to an insecure attachment pattern that causes emotional dysregulation [21]. Some empirical studies have discussed the relationship between insecure attachment and NSSI. In a sample of adult psychiatric patients, they found that preoccupied attachment was associated with lifelong NSSI, but emotional pain partially moderated the relationship between a dysfunctional attachment pattern and NSSI [27].

A longitudinal study showed that the dysfunctional attachment between adolescents and their mothers is associated with a higher frequency of NSSI, which is mediated by increased rates of confused identity style [28]. The quality of attachment to parents through self-compassion, as self-reported by patients, provides us with insights into how such a disorder can emerge in adolescents. All of these results indicate that an insecure attachment pattern increases the likelihood of engaging in NSSI or can maintain it [29]. Despite the importance of these findings, studies in this field, especially among the adolescent population, are scarce [21]. Therefore, the authors will conduct a systematic review and meta-analysis to investigate the relationship between NSSI and attachment.

The primary goal of this systematic review is to investigate the relationship between NSSI and attachment. The secondary objectives are as follows: (1) examine the relationship between NSSI and attachment according to the study type (interventional and observational studies); (2) examine the relationship between NSSI and attachment according to the study approach (cross-sectional, retrospective, and longitudinal); (3) examine the relationship between NSSI and attachment based on different types of attachment (attachment to the primary caregiver, romantic attachment, attachment to peers, etc); (4) examine the relationship between NSSI and attachment based on gender; (5) examine the relationship between NSSI and attachment based on different age groups; and (6) examine the relationship between NSSI and attachment based on studying the difference in findings (heterogeneity) in this relationship and investigating its potential reasons.

This study is a systematic review that will be reported based on the recommendations of the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) 2009 checklist. The study protocol was registered with the International Prospective Register of Systematic Reviews (CRD42021226455).

The quality assessment of the included studies will be performed using the Joanna Briggs Institute checklist. Two reviewers will conduct this process separately. Then, the agreement between the 2 reviewers will be assessed, and in case of inconsistency, consensus will be used.

To assess the heterogeneity between the primary studies, we will use Cochran Q test and the inconsistency index (I²). In addition, heterogeneity classification will be conducted using the classification method proposed by Higgins as follows: 0% to 24.9% (mild heterogeneity), 25% to 49.9% (moderate heterogeneity), 50% to 74.9% (severe heterogeneity), and 75% to 100% (highly severe heterogeneity).

Subgroup analysis will be performed based on variables such as age, gender, type of the study, design of the study, quality of the study, different types of attachments, and time periods.

In order to assess publication bias, funnel plots (if 10 or more primary studies are included) will be depicted and examined. In addition, the Begg and Egger statistical tests will be used to assess the publication bias. P values of <.1 will be considered as considerable publication bias.

Sensitivity analysis will be conducted using the one-out remove method and the “metaninf” command in Stata software. Furthermore, subgroup analysis or meta-regression will be used in order to explore the effect of methodologic quality on the meta-analysis results. In case of considerable difference between high- and low-quality studies, the conclusion will be made using high-quality studies.

Since this systematic review is limited to the published articles, it does not require ethical approval.