Social gradients in ADHD by household income and maternal education exposure during early childhood : Findings from birth cohort studies across six countries

ObjectiveThis study aimed to examine social gradients in ADHD during late childhood (age 9-11 years) using absolute and relative relationships with socioeconomic status exposure (household income, maternal education) during early childhood (<5 years) in seven cohorts from six industrialised countries (UK, Australia, Canada, The Netherlands, USA, Sweden). MethodsSecondary analyses were conducted for each birth cohort. Risk ratios, pooled risk estimates, and absolute inequality, measured by the Slope Index of Inequality (SII), were estimated to quantify social gradients in ADHD during late childhood by household income and maternal education measured during early childhood. Estimates were adjusted for child sex, mother age at birth, mother ethnicity, and multiple births. FindingsAll cohorts demonstrated social gradients by household income and maternal education in early childhood, except for maternal education in Quebec. Pooled risk estimates, relating to 44,925 children, yielded expected gradients (income: low 1.83(CI 1.38,2.41), middle 1.42(1.13,1.79), high (reference); maternal education: low 2.13(1.39,3.25), middle 1.42(1.13,1.79)). Estimates of absolute inequality using SII showed that the largest differences in ADHD prevalence between the highest and lowest levels of maternal education were observed in Australia (4% lower) and Sweden (3% lower); for household income, the largest differences were observed in Quebec (6% lower) and Canada (all provinces: 5% lower). ConclusionFindings indicate that children in families with high household income or maternal education are less likely to have ADHD at age 9-11. Absolute inequality, in combination with relative inequality, provides a more complete account of the socioeconomic status and ADHD relationship in different high-income countries. While the study design precludes causal inference, the linear relation between early childhood social circumstances and later ADHD suggests a potential role for policies that promote high levels of education, especially among women, and adequate levels of household income over childrens early years in reducing risk of later ADHD.Funding Agencies|Canadian Institutes of Health Research [OCO-79897, MOP-89886, MSH-95353, ROG-110537, MOP-123079, HDF70335]; County Council of Ostergotland; Swedish Research Council [K2005-72X-11242-11A, K2008-69X-2082601-4]; Swedish Child Diabetes Foundation (Barndiabetesfonden); Juvenile Diabetes Research Foundation; Wallenberg Foundation [K 98-99D12813-01A]; Medical Research Council of Southeast Sweden(FORSS); Swedish Council for Working Life and Social Research [FAS20041775]; Ostgota Brandstodsbolag; Australian Government Department of Social Services; Australian Institute of Family Studies (AIFS); Australian Bureau of Statistics (ABS); Erasmus Medical Center, Rotterdam; Erasmus University Rotterdam; Netherlands Organisation for Health Research and Development (ZonMw) [ZonMw 907.00303, 916.10159]; Netherlands Organisation for Scientific Research (NWO); Ministry of Health, Welfare and Sport; Ministry of Youth and Families; lInstitut de la statistique du Quebec; Fondation Lucie et Andre Chagnon; Ministere de leducation et de lEnseignement superieur; Ministere de la Sante et des Services sociaux; Ministere de la Famille; GRIP Research Unit on Childrens Psychosocial Maladjustment; QUALITY Cohort Collaborative Group; Centre hospitalier universitaire SainteJustine; Institut de recherche Robert-Sauve en sante et en securite au travail; lInstitut de recherche en sante publique de lUniversite de Montreal; Centre de recherche du Centre hospitalier de lUniversite de Montreal (CRCHUM); Fonds de recherche du Quebec Sante (FRQS); Fonds de recherche du Quebec Socie te et culture (FRQSC); Social Sciences and Humanities Research Council (SSHRC); Human Resources and Skills Development Canada (HRSDC); Canadian Institute for Health Research (CIHR); Canadian Foundation for Innovation (CFI); Statistics Canada; Economic and Social Research Council; Office of National Statistics; U.S. Bureau of Labor Statistics; National Institute for Child Health and Human Development