Background Socioeconomic disadvantage increases risk for mental health difficulties, but inequalities may differ over childhood for externalising and internalising difficulties. Evidence on whether inequalities emerge early or widen during childhood can inform the developmental timing of interventions to effectively address inequalities. We therefore aim to characterise the developmental trajectory of mental health risk up to age 17 years in a nationally representative UK cohort.

Methods We use data from the UK Millennium Cohort Study. Parent-report of the Strengths and Difficulties Questionnaire at ages 5 (n=14707), 7 (n=13437), 11 (n=12793), 14 (n=11332) and 17 (n=9268) was used to assess for being-at-risk for internalising difficulties (borderline/abnormal peer or emotional score) or externalising difficulties (borderline/abnormal conduct or hyperactivity score). Relative and absolute inequalities by maternal education in the proportion at risk of internalising and externalising difficulties are described cross-sectionally by the relative and slope index of inequality (RII and SII), with 95% confidence intervals. Main analyses using Generalised Linear Mixed Effect models, to be conducted, will estimate the effect of education and equivalised household income on longitudinal, within-individual changes in mental health risk stratified by sex.

Results In the most disadvantaged group (no maternal education qualifications) and least disadvantaged group (maternal degree plus qualifications) respectively, 36.4% (95% CI 33.7%–39.0%) and 13.7% (12.1%–15.4%) of children were at risk for internalising difficulties at age 5, rising to 53.5% (47.1%– 59.9%) and 28.2% (25.1%–31.3%) by age 17. For externalising difficulties, 47.2% (44.6%-49.8%) compared to 16.6% (14.9%-18.2%) were at risk at age 5, decreasing slightly to 33.6% (26.6%–40.5%) and 12.2% (9.5%–14.9%) by age 17. Relative inequalities in internalising difficulties decreased from age 5 (RII: 3.35, 3.28–3.46) to age 17 (RII: 2.16, 2.09–2.22). Relative inequalities in externalising difficulties decreased from age 5 (RII: 3.35, 3.23–3.48) to age 11 (RII: 2.86, 2.75–2.97), and increased again to age 17 (RII: 3.61, 3.42–3.77). Absolute inequalities showed a fluctuating trend for internalising difficulties (SII range: 0.24–0.30) and decreased slightly for externalising difficulties (age 5 SII: 0.35, 0.34–0.36; age 17 SII: 0.26, 0.23–0.29).

Discussion Large and persistent inequalities in risk for internalising and externalising difficulties are evident from age 5, through to young adulthood in the UK. Relative inequalities appear sensitive to change in the risk prevalence over childhood, but there was little change in absolute inequalities. Early intervention is likely most effective to prevent socioeconomic inequalities in mental health risk. However, interventions that address childhood developmental mechanisms, such as within school settings, can reduce inequalities throughout childhood.