Examining the Association Between Internet Addiction and Nonsuicidal Self-Injury Among Chinese Middle School Students: Prospective Cohort Study.

Background: While cross-sectional studies have consistently reported an association between nonsuicidal self-injury (NSSI) and internet addiction (IA), longitudinal evidence regarding the directionality and dose-response relationship remains limited. Furthermore, the roles of sex and varying degrees of problematic internet use in predicting new-onset NSSI are not fully understood. Objective: This prospective cohort study aimed to investigate whether baseline IA and its intermediate states predict the subsequent new onset of NSSI among Chinese adolescents over a 6-month period and to explore potential sex differences in this longitudinal association. Methods: A prospective cohort design was used. A total of 1315 junior high school students without a history of NSSI were recruited at baseline, and 704 (53.5%) students completed the 6-month follow-up. IA and NSSI were assessed using the Chinese Internet Addiction Scale-Revised and a self-report questionnaire from the Adolescent Health-Related Risky Behavior Inventory, respectively. Logistic regression analysis was conducted to examine the predictive value of IA exposure for incident NSSI, adjusting for key covariates, including sex, age, ethnicity, only child status, anxiety, and depression. Restricted cubic spline regression was used to model the dose-response relationship between distinct states of IA and NSSI risk. Results: The baseline prevalence of IA was 9.09% (64/704). At the 6-month follow-up, the incidence rate of NSSI was 9.8% (69/704). Restricted cubic spline regression revealed a linear dose-response relationship, where the risk of incident NSSI escalated with increasing IA severity. In the fully adjusted model for the total sample, baseline IA was a significant predictor of subsequent NSSI (odds ratio [OR] 2.185, 95% CI 1.031-4.627; P=.04). Crucially, stratified analyses revealed significant sex disparities: the longitudinal association between IA and subsequent NSSI was statistically significant among female adolescents (OR 3.271, 95% CI 1.101-9.717; P=.03) and the intermediate internet-dependent state (OR 2.593, 95% CI 1.002-6.710; P=.049) but not among male adolescents (IA: P=.44; internet-dependent state: P=.87). Conclusions: NSSI incidence is notably prevalent among Chinese junior high school students. While IA serves as a robust, independent risk factor for predicting the new onset of NSSI in the overall adolescent population, sex-stratified analyses revealed that this longitudinal association was statistically significant (P=.03) only among female students. These findings underscore the critical need to integrate IA assessments into school-based mental health screenings and highlight the necessity of developing sex-specific, emotion-focused prevention strategies to mitigate NSSI risk.