Although the alternative interpretations that underdiagnosis takes place in older children or that the social pressures on younger children amplify the symptoms of the disorder must also be considered, the evidence of a relative-age effect in our study raises the concern of overdiagnosis and overtreatment of ADHD in younger children within a grade. [...] Inappropriate diagnosis of ADHD in a child born late in the year might lead parents and teachers to treat the child differently or adversely change the child’s self-perceptions.
[...] Our data underscore the dimensional and developmental nature of the symptoms of ADHD and the impact of contextual expectations on the likelihood of the diagnosis being made. Age-corrected rating scales and developmentally appropriate evaluation are therefore essential, but such a strategy may not be enough to fully eliminate the relative-age effect. Confounding influences may still exist, such as the expectations of parents and teachers, or the child’s self-perception in the classroom. For example, inadvertent reinforcement may magnify the apparently inattentive, distracting or impulsive behaviours of the youngest children in a class, such as escaping from a difficult academic task (negative reinforcement) or receiving attention from teachers and peers for disruptive behaviour (positive reinforcement). A previous study found that teachers’ perceptions of child behaviour were more strongly related to a child’s age within a grade than were parental perceptions, suggesting that
[T]eachers’ opinions of children are the key mechanisms driving the relationship between school starting age and ADHD diagnoses.