Objective: This 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). Methods: Secondary 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. Findings: All 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). Conclusion: Findings 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 children's early years in reducing risk of later ADHD.
Bibliographical noteFunding Information:
This study is based on a comparison of seven international birth cohorts. EPOCH was partly supported by Canadian Institutes of Health Research (J. McGrath: OCO-79897, MOP-89886, MSH- 95353; L. Séguin: ROG-110537). ABIS and this research were supported in part by the County Council of Ostergotland, Swedish Research Council (K2005-72X-11242-11A and K2008-69X-20826- 01-4), the Swedish Child Diabetes Foundation (Barndiabetesfonden), Juvenile Diabetes Research Foundation, Wallenberg Foundation (K 98-99D- 12813-01A), Medical Research Council of Southeast Sweden(FORSS), the Swedish Council for Working Life and Social Research (FAS2004- 1775), and Ostgota Brandstodsbolag. Johnny Ludvisson founded the ABIS Cohort. Longitudinal Study of Australian Children (LSAC) was initiated and funded by Australian Government Department of Social Services, with additional funding from partner organizations Australian Institute of Family Studies (AIFS) and Australian Bureau of Statistics (ABS). The study was conducted in partnership with the Department of Social Services (DSS), the Australian Institute of Family Studies (AIFS) and the Australian Bureau of Statistics (ABS). The findings and views reported in this paper are those of the authors and should not be attributed to the DSS, the AIFS or the ABS. This paper uses unit record data from Growing Up in Australia, the Longitudinal Study of Australian Children. Generation R Study (GenR) was made possible by financial support from Erasmus Medical Center, Rotterdam; Erasmus University Rotterdam; Netherlands Organisation for Health Research and Development (ZonMw; additional grant received by V. Jaddoe, ZonMw 907.00303, 916.10159); Netherlands Organisation for Scientific Research (NWO); Ministry of Health, Welfare and Sport; and, Ministry of Youth and Families. Generation R Study (GenR) is conducted by Erasmus Medical Center in close collaboration with the School of Law and Faculty of Social Sciences of the Erasmus University Rotterdam, the Municipal Health Service Rotterdam area, Rotterdam, the Rotterdam Homecare Foundation, Rotterdam and the Stichting Trombosedienst & Artsenlaboratorium Rijnmond (STAR-MDC), Rotterdam; we gratefully acknowledge the contribution of children and parents, general practitioners, hospitals, midwives and pharmacies in Rotterdam. Québec Longitudinal Study of Child Development (QLSCD) 1996-2014 cohort was principally funded and supported by l'Institut de la statistique du Québec through partnership with Fondation Lucie et André Chagnon, Ministère de l'éducation et de l'Enseignement supérieur, Ministère de la Santé et des Services sociaux, Ministère de la Famille, GRIP Research Unit on Children's Psychosocial Maladjustment, QUALITY Cohort Collaborative Group, le Centre hospitalier universitaire Sainte- Justine, Institut de recherche Robert-Sauvé en santé et en securité au travail, l'Institut de recherche en santé publique de l'Université de Montréal, Centre de recherche du Centre hospitalier de l'Université de Montréal (CRCHUM), Fonds de recherche du Québec Santé (FRQS), Fonds de recherche du Québec Sociéte et culture (FRQSC), Social Sciences and Humanities Research Council (SSHRC), and Canadian Institutes of Health Research (MOP-123079, HDF- 70335). The paper used unit record data from the QLSCD (ELDEQ - Enquête longitudinale des enfants du Québec). Data for the QLSCD were collected by the Institut de la Statistique du Québec, Direction des enquêtes longitudinales et sociales. National Longitudinal Study of Children and Youth (NLSCY) was conducted by Statistics Canada and sponsored by Human Resources and Skills Development Canada (HRSDC); both agencies played a role in funding, development of survey content, research, and dissemination of findings. NLSCY and this research was supported by funds to the Canadian Research Data Centre Network (CRDCN) from the Social Sciences and Humanities Research Council (SSHRC), the Canadian Institute for Health Research (CIHR), the Canadian Foundation for Innovation (CFI), and Statistics Canada. Although the research and analysis are based on data from Statistics Canada, the opinions expressed do not represent the views of Statistics Canada. The UK Millennium Cohort Study (MCS) was supported by the Economic and Social Research Council, the Office of National Statistics, and various government departments. The study was led by the Centre for Longitudinal Studies at the Institute of Education of the University of London. We thank the Economic and Social Data Service and the United Kingdom Data Archive for permission to access the study data. The US National Longitudinal Survey of Youth (USNLSY79) is sponsored and directed by U.S. Bureau of Labor Statistics and conducted by Center for Human Resource Research at The Ohio State University. Interviews are conducted by the National Opinion Research Center (NORC) at the University of Chicago. The Children of the NLSY79 survey is sponsored and directed by the U.S. Bureau of Labor Statistics and the National Institute for Child Health and Human Development.
© 2022 Spencer et al.