Cohort Profile: The Tokyo Adolescent Mental Health Study (TAMHS).

The Tokyo Adolescent Mental Health Study (TAMHS) is an ongoing, school-based longitudinal cohort designed to elucidate the developmental trajectories of mental health among adolescents in Japan and to inform evidence-based strategies for promoting adolescent well-being. Since its inception in 2009, TAMHS has annually tracked adolescents aged 12-18 from a Tokyo secondary school known for its tradition of enrolling twins and triplets. Participants are followed for up to 6 years. As of 2024, 2,435 adolescents (1,211 boys, 1,196 girls, 28 unspecified), including 551 twins and triplets (22%), have been enrolled. This unique sample composition allows for rigorous comparisons between singletons and multiples to examine the impact of birth plurality on neurodevelopmental and mental health trajectories. High annual retention (96.2-99.2%) ensures the integrity of longitudinal analyses. Data are collected through self-report questionnaires covering demographics, physical and mental health status, daily habits and lifestyle behaviors, social relationships, behavioral problems, healthcare access, and social and environmental determinants of health. The study has yielded important insights into associations between adolescent lifestyle behaviors and mental health outcomes.