The development of Non-Suicidal Self-Injury (NSSI) during adolescence: A systematic review and Bayesian meta-analysis

Despite a surge in research on self-injury in the last decade, a summary of research findings about the development of Non-Suicidal Self-Injury (NSSI) over time in community youth samples is not yet present in the scientific literature. This study aims to summarize the empirical literature on this topic, examining both the occurrence (Study 1) and frequency (Study 2) of NSSI over time, and for this reason, a Systematic Review and Bayesian Meta-Analysis were conducted. Following the PRISMA guidelines, the longitudinal studies included in the systematic review consisted of 41 papers (Study 1 = 16; Study 2 = 25). Only studies with available data were included in the meta-analysis (Study 1 = 12; Study 2 = 11). First, the findings highlight limits related to methodological aspects, the design of the studies, and the availability of data. Meta-analytic results shows that across development, the frequency (i.e., not the occurrence) of NSSI increases for the group of younger adolescents, remains stable in the group of middle adolescents, and it decreases for older adolescents. This study highlights some limitations that can be summarized in three different macro categories: the first refers to methodological aspects (e.g., the lifetime prevalence of NSSI), the second to the design of the studies (e.g., not homogeneous cohort; short-term covered), and the third to the availability of data. The current meta-analysis tries to shed light on the longitudinal research on NSSI behavior and how this behavior develops in terms of both occurrence and frequency, providing practical and methodological indications for future research. •The development of NSSI - occurrence and frequency, in adolescence•A systematic review and a meta-analytic Bayesian approach to synthesize data•Few longitudinal studies with multiple waves and covering long span-time•For younger adolescents, the frequency of NSSI increases over time•For older adolescents, the frequency of NSSI decreases over time