Socioeconomic inequalities in mental health difficulties over childhood: a longitudinal sex-stratified analysis using the UK Millennium Cohort Study

Stark socioeconomic inequalities in childhood mental health have been widely reported. Understanding whether they vary with age, by type of difficulty or sex can inform public health policies to tackle socioeconomic inequalities. We investigated the effects of early life childhood socioeconomic circumstances (SECs) (maternal education and household income) on developmental trajectories of externalising and internalising difficulties in childhood and adolescence, in males and females from the UK-representative Millennium Cohort Study (N = 15383). We estimated the Slope Index of Inequality (SII) (absolute difference between the most versus least advantaged) using linear mixed-effects regression models, on parent-reported Strengths and Difficulties Questionnaire externalising and internalising difficulties score, at 5, 7, 11, 14, and 17 years(y). The mean externalising score was high at 5y (4.8 [95 %CI: 4.7, 4.9]) and decreased slightly, while mean internalising score increased over childhood, reaching 3.9 [3.8, 4.1] by 17y, with a steeper trend for females in adolescence. Lower maternal education was associated with greater externalising scores at 5y (SII, Male: 3.0 [2.7 to 3.3]; Female: 2.7 [2.4, 3.0]) with inequalities decreasing slightly up to 17y (SII Male: 2.4 [2.0 to 2.7], Female: 2.5 [2.1, 2.8]). Inequalities in internalising scores increased slightly over childhood (SII Female 5y: 1.3 [1.1, 1.6]; 17y: 1.9 [1.5, 2.3]; SII Male 5y = 1.6 [1.3, 1.8], 17y = 1.8 [1.5, 2.2]). Patterns were similar using household income. Disadvantaged SECs are associated with persistently higher levels of parent-reported mental health difficulties up to 17y, with larger effects on externalising than internalising difficulties, but little differences by sex or socioeconomic measure.