The association between the relative age and ADHD diagnosis and pharmacotherapy

Background: Prior work indicates that the youngest children in a classroom are more likely to be diagnosed with and treated for Attention-Deficit/Hyperactivity Disorder (ADHD), a phenomenon known as the relative age effect (RAE). However, most existing literature predates recent developments, including updates to diagnostic criteria, growing overdiagnosis concerns, and increased RAE awareness, that have raised questions about whether this effect persists. Therefore, this thesis has two main goals. First, to review the published evidence with a focus on sex and age differences and second, to reexamine the influence of relative age on ADHD diagnosis and treatment in British Columbia, Canada. Methods: To address the objectives, two studies were completed. First, a comprehensive review searched five databases and identified thirty-four studies yielding forty-five independent samples that examined the RAE in ADHD in children and adolescents. Second, a population-based cohort study including 2,112,662 British Columbian children aged 3-17 years at any point between the years 2014 and 2023 was conducted. Stratified by age groups and sex, the risk difference and relative risk of diagnosis and treatment were calculated for children born in November/December (relatively younger) compared to those born in January/February (relatively older). Results: Most reviewed studies supported the RAE in ADHD diagnosis and treatment, with the effect appearing stronger in girls and attenuating with age. The BC cohort study confirmed the continued influence of relative age on ADHD under contemporary clinical conditions. Consistent with the review, the RAE on ADHD diagnosis was strongest in elementary school (aged 6-12) for both sexes, with stronger effects in girls (relative risk (RR) 1.35, 95% CI 1.27-1.47) than boys (RR 1.25, 95% CI 1.21-1.29). A novel finding of the cohort study was that while the effect attenuated among high school-aged girls (aged 13-17), it remained elevated among high school-aged boys (RR 1.24, 95% CI 1.20-1.29). Similar patterns were observed for treatment. The RAE was absent in preschool (aged 3-5). Conclusions: Consistent evidence supports the RAE internationally and in British Columbia. The effect peaks in elementary school for girls but persists into high school for boys. Increased awareness of the RAE is needed among providers.