Background Mental health problems are not distributed equally in society. Our understanding of when social inequalities in mental health emerge is limited. We sought to examine inequalities in trajectories of mental distress in diverse, representative cohorts of adolescents in inner-London. Methods We analysed longitudinal data from our cohort study of adolescent mental health, REACH (n=4663; 51% girls, 29% free school meals [FSM], 85% minoritised ethnic groups). We used latent growth curve models to estimate trajectories of mental distress (total, internalising, and externalising scores from the self-report Strengths and Difficulties Questionnaire) from age 11-16 years, overall and by gender, FSM, ethnic group, and their intersections. Results We found strong evidence of differences in trajectories of mental distress by gender and FSM. Higher mean internalising scores in girls (vs. boys) were evident at age 11-12 and this inequality widened year-on-year (difference in mean intercepts: 0.74 [95% CI 0.52, 0.96]; slopes: 0.50 [0.39, 0.61]). Higher mean levels of distress among those receiving FSM (vs. not) were evident at age 11-12 years (e.g., difference in intercepts, general distress: 0.79 [0.19, 1.39]), and this difference, though modest, persisted through adolescence. By ethnic group and intersecting identities, the picture was more complex and mixed. Broadly, Black African youth generally reported better mental health trajectories vs. their peers; Black Caribbean and Mixed Black-and-White youth shared similar trajectories, differing somewhat from Black African; and by age 16, internalising distress was highest among lower-income White British girls. Conclusions In diverse inner-cities, adolescence is an important period in the emergence and persistence of some of the inequalities in mental health reported in adults; others are more nuanced. Keywords: social inequalities, mental health, adolescence, trajectories. Abbreviations: FSM, free school meals; SDQ, strengths and difficulties questionnaire; LCM, latent growth curve models.

The authors have declared no competing interest.

This work was supported by the UK Economic and Social Research Council (ESRC) Centre for Society and Mental Health (ES/S012567/1); and the European Research Council (ERC) (REACH 648837). KB is in part supported by Oxford Health NIHR-BRC and NIHR-TVO-ARC. GK is supported by the Wellcome Trust (309118/Z/24/Z).

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The Psychiatry, Nursing and Midwifery Research Ethics Subcommittee (PNM-RESC) of Kings College London (15/16-2320) gave ethical approval for this work.

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We welcome and encourage requests from those wishing to access REACH data for specific research projects or collaborations. Our data access policy, which aims to make REACH data as accessible as possible while adhering to legal and ethical principles and protecting the privacy of schools and participants, can be found at www.thereachstudy.com/information-for-researchers.html. Further information about REACH is also available on the study website. Applications should be submitted to the Principal Investigator, Professor Craig Morgan, at craig.morgan@kcl.ac.uk.