Childhood adversity, social support, problematic internet use, psychological vulnerability, and pathways to non-suicidal self-injury and suicidality in adolescents and young adults: a prospective cohort study protocol.

Background: Non-suicidal self-injury (NSSI) and self-harm among adolescents and young adults are increasingly recognised as major public health concerns. Although childhood adversity, family and social support, problematic internet use, psychological vulnerability, and psychiatric symptoms have each been associated with NSSI and suicidality, these factors have often been examined separately. Less is known about how environmental adversity, interpersonal support, and psychological vulnerability interact within an integrated developmental pathway leading to NSSI and suicidality. Methods and analysis: Adolescents and young adults aged 13-29 years with a recent history of self-harm or suicidality will be recruited from National Taiwan University Hospital. Community participants without an established psychiatric diagnosis will be recruited from collaborating schools as controls. Participants will complete self-report and caregiver-report measures assessing NSSI, suicidality, childhood adversity, family and social support, problematic internet use, reflective functioning, self-efficacy, impulsivity, and internalising and externalising symptoms. Descriptive and group-comparison analyses will compare psychosocial and clinical characteristics between the clinical and community groups. The primary analysis will use structural equation modelling within the NSSI group to examine whether childhood adverse environment, inadequate family and social support, and problematic internet use are associated with NSSI and suicidality through psychological vulnerability and downstream internalising and externalising problems. Sample size estimation was based on Monte Carlo simulation-based power analysis in R. Exploratory network analysis will identify observed variables most directly connected to NSSI and suicidality and detect bridge nodes across psychosocial and symptom domains. Discussion: Previous studies have identified multiple psychosocial and psychological correlates of NSSI and suicidality, but these factors have rarely been examined within an integrated developmental framework. By jointly examining early adversity, interpersonal support, problematic internet use, mentalization-related vulnerability, impulsivity, self-efficacy, and psychiatric symptoms, this study may clarify how distal environmental risks and proximal psychological vulnerabilities become clinically connected in young people with self-harm. Findings may inform earlier, mechanism-informed, and context-sensitive prevention strategies targeting upstream vulnerabilities before progression to repeated NSSI or suicidal behaviour.