Timing of exposure to household poverty and adolescent mental health problems

AIMS: Mental health problems in adolescence are increasingly prevalent and have tremendous impacts on life-long health and mortality. Although household poverty is a known risk factor for adolescent mental health, evidence of the timing hypothesis is scarce. We aimed to examine the longitudinal associations of poverty across childhood with mental health in adolescence, focusing on the timing of exposure. METHODS: We used the data of 5,671 children from a Japanese population-based longitudinal cohort, which recruited the first graders (aged 6-7 years) and followed biannually until eighth grade (aged 13-14 years) in Adachi, Tokyo. Household poverty was defined as households having any of the following experiences: annual income less than Japanese yen 3 million, payment difficulties and material deprivations, measured in first, second, fourth, sixth and eighth grades. Adolescent mental health included parent-report internalizing and externalizing problems (the Strengths and Difficulties Questionnaire), self-report depression (the Patient Health Questionnaire-9) and self-esteem (the Japanese version Children's Perceived Competence Scale) in eighth grade. We applied g-estimation of structural nested mean modelling to account for time-varying confounders. RESULTS: If adolescents were exposed to household poverty at any grade across childhood, on average, they would report more severe depressive symptoms (ψ = 0.32 95% CI 0.13; 0.51) and lower self-esteem (ψ = -0.41 -0.62; -0.21) in eighth grade. There were also average associations of household poverty at any grade with more internalizing (ψ = 0.19 0.10; 0.29) and externalizing problems (ψ = 0.10 0.002; 0.19). Although the associations between household poverty and mental health were stronger in younger ages (e.g., poverty in the second grade → depression: ψ = 0.54 -0.12; 1.19 vs. poverty in the eighth grade → depression: ψ = -0.01 -0.66; 0.64), overlapping 95% CIs indicated no statistically significantly different associations by the timing of exposure. CONCLUSION: We found the average effect of exposure to household poverty at any grade on mental health outcomes in eighth grade, failing to support the timing hypothesis. The findings indicate that the effects of household poverty accumulate over time in childhood and impact adolescent mental health (cumulative hypothesis) rather than the effects differ by the timing of exposure. While cumulative effects suggest a persistent intervention in poor households across childhood, we highlight intervention at any timing in childhood may be effective in alleviating adolescent mental health problems.