Birthday Behaviour: Relative Age Effects and Mental Health Disorder Misidentification in School

Attention Deficit Hyperactivity Disorder (ADHD) is diagnosed at higher rates in children who are young for their grade, without a biological or seasonal basis. Whether this higher incidence in younger students is due to over-diagnosis or if relatively older students are being overlooked is not fully understood. We address this question of directionality in ADHD misdiagnosis while also evaluating how a child's gender and ADHD sub-type, i.e., hyperactivity or inattentiveness, impact which children are more likely to be referred for diagnosis by teachers. Using a kindergarten sample of teacher- and parent behavioural assessments from Canada's National Longitudinal Survey of Child and Youth (NLSCY), we demonstrate that underassessment of older girls within a grade drive the initial age at school entry effect in ADHD. With this, we find evidence to support common thinking that, regardless of relative age, females are under-assessed for ADHD severity and males are over-assessed. These results are exacerbated for hyperactivity and diluted across inattentive-type ADHD. Within the classroom, we find peer misbehaviour increases over-assessment of the individual child, while teacher age and experience and special education training play mitigating roles. These results have important implications for children's long-term educational outcomes as well as policy implications for the appropriate allocation of mental health resources for children.