Objectives: This study examined (non-)monotonic time trends in psychological and somatic complaints among adolescents, along with gender differences.
Methods: Repeated cross-sectional Health Behaviour in School-aged Children (HBSC) data from1994to2022covering15-year-oldadolescentsfrom41countries(N=470,797) were analysed. Three polynomial logistic regression models (linear, quadratic, cubic) were tested for best fit, including separate analyses by gender and health complaints dimension.
Results: Time trend patterns varied by gender and health complaints dimension. Increases were found in 82.3% of cases (linear 25%, quadratic U-shaped 28.7%, cubic 28.7%), while 14% showed no clear trend, and 3.7% decreased. Boys typically showed linear increases or no clear trend over time, whereas girls generally showed cubic or U-shaped trends. Psychological complaints often displayed U-shaped or cubic patterns, whereas somatic complaints mostly showed linear increases.
Conclusion: Psychological and somatic complaints demonstrated diverse time trend patterns across countries, with non-monotonic patterns (U-shaped and cubic) frequently observed alongside linear increases. These findings highlight the complexity of changes within countries over three decades, suggesting that linear modelling may not effectively capture this heterogeneity.