The relative-age effect we report, based on population-wide public data in British Columbia, is consistent with analyses of relative age in two studies using survey and private health plan data in the US.14,15
In our study, a sudden change in the percentage of children who received a diagnosis of ADHD or treatment for the condition occurs near Dec. 31, the cut-off date for entry to school in the province of British Columbia. In the American studies, a discontinuity occurred around the dates for entry to school at the different times of year for the different states. In interpreting their findings, the authors of those studies considered that the relative immaturity of younger students within a grade may lead to the inappropriate diagnosis of ADHD.
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. The potential harms of overdiagnosis and unnecessary pharmacologic treatments are important to consider. Children who are given medications to treat ADHD are exposed to adverse effects on sleep, appetite and growth, in addition to an increased risk of cardiovascular events.23
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 analyses add weight to concerns about the medicalization of the normal range of childhood behaviours, particularly for boys.
Our data underscore the dimensional and developmental nature of the symptoms of ADHD24
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.14
It is possible that closer consideration of a child’s behaviour in multiple contexts, including those outside of school, may lessen the risk of unnecessary diagnosis when assessing children for ADHD.
We aimed to determine the influence of birth month on diagnosing and prescribing medications for ADHD, but our study faced certain limitations. To interpret the increased risk of diagnosis and treatment among younger children within a grade as overdiagnosis or overprescribing implies an assumption as to appropriate levels of diagnosis and treatment, such as that children born in January received appropriate levels of diagnoses and treatment. However, the true incidence of ADHD is unknown. The proportion of children born in January who received a diagnosis of ADHD might underestimate incidence of the disorder if more mature children within each grade are better able to cope with an underlying disorder. However, it is possible that the January proportion overestimates the incidence if a larger issue of overmedicalization of childhood behaviour exists.
Our analyses show a relative-age effect in the diagnosis and treatment of ADHD in children aged 6–12 years in the province of British Columbia. The strength of this effect remained relatively stable for the duration of our 11-year study. Although the prevalence of diagnosis and treatment for ADHD remains considerably higher among boys, it increased over time for children of both sexes in this age range.
Although the influence of relative age on diagnosis and treatment may lessen for older children, we found the effect was present at all ages from 6 to 12 years, for girls and boys alike. The potential harms of overdiagnosis and overprescribing and the lack of an objective test for ADHD strongly suggest caution be taken in assessing children for this disorder and providing treatment. Greater emphasis on a child’s behaviour outside of school may be warranted when assessing children for ADHD to lessen the risk of inappropriate diagnosis. Further research into the determinants of ADHD and approaches to its assessment and treatment should consider a child’s age within a grade.