Mental Health
Attention-Deficit/Hyperactivity Disorder and Relative Age in School Class: Evidence from the province of Quebec, Canada Geneviève Lefebvre* Geneviève Lefebvre Jesse Gervais Catherine Haeck Pierre Lefebvre Philip Merrigan
It is well known that the youngest children in a classroom have a greater chance of being diagnosed and medicated for attention-deficit/hyperactivity disorder (ADHD). This phenomenon has been observed over the years in many countries, including Canada. The aim of this study was to provide new Canadian evidence based on data from the province of Quebec, recognized as having among the highest rates of ADHD diagnoses and medication consumption in the world.
A cohort of children born between 1996 and 2005 was assembled from administrative data from the Régie de l’assurance maladie du Québec (n=748,075). Diagnostic codes (ICD-9 314.0) and ADHD-related medication purchases were identified for the elementary school period (kindergarten and grades 1 to 6). As the cut-off date for kindergarten entry is September 30 each year, the proportions of children born in September versus October with ADHD diagnoses and medications were compared using logistic regressions. Results stratified by sex and drug insurance type were also obtained.
We found that Quebec children born in September had an increased likelihood of being diagnosed with ADHD in elementary school compared with children born in October (marginal risk difference: 0.06, 95% CI: [0.05, 0.06]; odds ratio: 1.68, 95% CI [1.62, 1.74]). The relative age effect for ADHD medications was similar to the one found for diagnoses. We also observed a rising trend in the age relative effects among the birth year cohorts from 1996 to 2005. From an additive perspective, the relative age effect for boys was greater than the one for girls. Children covered by the type of drug insurance associated with the lowest socio-economic status were found more susceptible to be affected by this relative age effect phenomenon than children covered by other types of drug insurance.
The relative age effect for ADHD diagnostics and medications persists in Canada and is in fact more present than ever.