Developmental trajectories and sex differences in first-recorded self-harm episodes among children and young people: A population-based modelling study.

OBJECTIVE: To investigate the developmental onset and sex-specific nonlinear trajectories of first-recorded episodes of self-harm among children and young people, examining variations across methods and settings. METHODS: We conducted a population-based surveillance study using statewide data from Santa Catarina, Southern Brazil, including individuals aged 5-25 years. Generalised additive models and spline-based mixed models were used to estimate continuous age-specific incidence rates. Spatial and contextual risks were assessed using incidence rate ratios (IRRs). RESULTS: Out of 63,773 notifications, 12,322 first-recorded cases were identified (68.5% female; 81.1% aged 15-25 years). Overall incidence peaked at 16.0 years. Distinct non-linear developmental trajectories emerged by sex: among females, incidence rose abruptly from age 11 and peaked at 14.0 years, whereas males showed a more gradual increase, peaking five years later at 19.0 years. Sex differences temporarily converged between ages 15 and 19. Method-specific analyses revealed a marked female predominance for self-poisoning (peak at 14.7 years) and sharp objects (peak at 14.0 years), whereas hanging was heavily male-predominant, peaking in young adulthood (20.5 years). School-based self-harm showed the earliest and most concentrated developmental peak (13.7 years), disproportionately affecting females (IRR = 1.73; 95% CI: 1.22-2.46). Conversely, sex differences were attenuated and no longer statistically significant in public spaces (IRR = 0.81; 95% CI: 0.40-1.64). CONCLUSIONS: First-recorded episodes of self-harm follow highly distinct, age- and sex-specific trajectories. The early-adolescent peak among females contrasts with the delayed trajectory observed in males, highlighting critical, setting-specific windows of developmental vulnerability that argue against 'one-size-fits-all' prevention and support targeted clinical and school-based strategies.