The youngest girl in class is up to 70% more likely to be diagnosed with ADHD than the oldest girl in class. She’s also 77% more likely to be prescribed medications like Ritalin. That’s in Canada.
It’s similar for boys. The youngest boy in class is 63% more likely to be diagnosed with ADHD and 73% more likely to be medicated. That’s in Taiwan. But it’s similar around the world - Australia, Brazil, Germany, Israel, Norway, Turkey, and the USA have all found the same thing.
Why would that be? How could the relative ages of children influence an ADHD diagnosis?
In ADHD research this difference between the youngest and oldest children is called the Relative Age Effect (RAE). Despite using different methods to research the RAE, studies agree that
being“relatively younger within the school year is a risk factor for an ADHD diagnosis”.
The Relative Age Effect is very consistent in ADHD research. It’s been found in large population studies involving millions of children, in clinical samples assessing the relative immaturity of children with ADHD diagnoses, and in studies comparing teacher ratings of ADHD symptoms in their pupils. The RAE is most clear in younger age groups and has become stronger over the years that it has been studied. The RAE is much less marked in countries where it is common to let younger children in a class stay another year in that class.
The Relative Age Effect is not caused by seasonal factors or birth months. It is found in both the northern and southern hemispheres, where school calendars follow their own distinct patterns. Furthermore, when the cutoff for starting school is changed, the birth month of the children most likely to receive ADHD diagnoses changes too.
Finally, the Relative Age Effect is not found in other areas of children’s mental health. It is only evident for ADHD.
So how can we explain the Relative Age Effect, and why does it only appear in the diagnosis and medication of childhood ADHD?
Why is there a Relative Age Effect?
The simplest explanation is that we are diagnosing immaturity as ADHD.
Children born at the end of a school year are up to a year younger than their classmates born at the beginning of a school year. In a classroom, this age gap means the youngest child is relatively immature compared with the oldest child. For six-year olds, the oldest child in class has had almost a year longer - 15% of a lifetime - to learn and practise skills needed in a classroom.
The Relative Age Effect suggests that social factors influence which children are seen as ‘having’ ADHD. And that children who are young in their school year may be diagnosed inappropriately if teachers, parents and clinicians mistake their immaturity for ADHD.
Confusion about the cause of inattentive or active behaviours may cause harm, as ADHD diagnosis has been shown to lead to poorer outcomes. However, the Relative Age Effect is rarely acknowledged or discussed by ADHD authors.
Textbook silence
Despite extensive documentation that relative age is a significant risk factor for an ADHD diagnosis, there’s a resounding silence about it in textbooks, articles and guidelines. In a sample of 43 academic textbooks, none of the sections on ADHD referred to the Relative Age Effect. Most clinicians don’t seem to know about it, and parents certainly don’t.
Moreover, no guidelines or textbooks recommend that teachers and clinicians be aware of the potential for age-related immaturity to be misdiagnosed as ADHD, and to adjust their practices accordingly.
Where does this leave us?
The effect of relative age on ADHD diagnosis is consistent and significant. The structure of schools, and the way adults understand children’s behaviour, are influencing ADHD prevalence rates worldwide, especially for younger ages.
Parents and teachers need to know about the effect of relative age on ADHD diagnosis. And when evaluating children for possible ADHD, clinicians should consider the relative age of the child and the potential impact of their classroom environment on teacher and parent reports of ADHD symptoms.
Because of the risks of diagnosis itself, clinicians need to be especially cautious about ADHD diagnosis for mild difficulties. For these children, any harms associated with an ADHD diagnosis or psychiatric drug use, are more likely to outweigh any possible benefits.
Our schools need to provide children of all ages with environments, structures, timetables and teaching methods appropriate for their level of development and their way of being in the world — without requiring a diagnosis.