The findings indicate that younger students in the school year are more likely to be diagnosed with ADHD and receive medication compared to their older classmates.

The study, which focused on ADHD with two additional studies on ASD diagnoses, revealed that the youngest children in a class were 38 percent more likely to receive an ADHD diagnosis and 28 percent more likely to be prescribed ADHD medications. The effect of relative age varied across the studies, but overall, it indicated that teacher ratings for ADHD-related symptoms were more influenced by relative age, as opposed to parent ratings.

Researchers suggested several factors that contribute to the difference in parent and teacher reporting of ADHD symptoms. The more demanding and less flexible school environment, the presence of peers for comparison, and the shorter duration of the teacher-child relationship compared to the parent-child relationship were all factors that potentially influence teacher ratings. Additionally, parents may overlook certain behaviors due to social desirability bias towards their child.

The study authors highlighted that teachers' perceptions and susceptibility to relative age bias could impact a child's referral and diagnostic process. Teachers are more likely to identify ADHD symptoms in younger children and give higher scores on symptom scales, which are taken into account by clinicians during diagnostic assessments. Conversely, they may miss ADHD symptoms in older children when comparing them to their younger and less mature classmates. The impact of relative age on ADHD diagnosis and treatment was more pronounced in younger children attending primary school and gradually lessened as children reached adolescence.

In the case of ASD, similar findings were observed, with the youngest children in a class being more likely to be diagnosed with the condition compared to their older peers. Researchers speculated that traits such as immature speech or social skills in relatively younger children may be interpreted as signs of autism.

Although only a small number of studies on the 'relative age' effect for ASD diagnoses are available, researchers emphasized the need for more research in this area. Dr. Eleni Frisira, lead author of the study, stressed the importance of supporting teachers in considering the relative age of a child in the classroom when ADHD is being queried.

Dr. Josephine Holland, another author of the study, expressed concern that despite this phenomenon being known for over a decade, it has not yet translated into changes in practice. The study team emphasized the importance of considering a child's age in relation to their classmates when assessing and diagnosing conditions like ADHD and ASD, for healthcare professionals, teachers, and parents. Incorporating the 'relative age' effect into clinical guidelines and training for healthcare professionals specializing in neurodevelopmental disorders was suggested as a way to promote critical thinking about children's symptoms during assessments.

With an estimated 2.6 million people in the UK having ADHD, of which almost 700,000 are children, the findings of this study have significant implications. Early identification and understanding of ADHD symptoms are crucial, as they often become more noticeable when a child's circumstances change, such as starting school. It is worth noting that ADHD can be categorized into two main types - inattentiveness and hyperactivity, although not all individuals may exhibit symptoms of both.

The study, published in the journal European Child and Adolescent Psychiatry, highlights the need for a comprehensive approach to assessing and diagnosing neurodevelopmental disorders, considering not only the individual's behavior but also their relative age within their peer group.