Researchers Confirm That Relative Age Impacts ADHD Diagnosis

The youngest children in a class are more likely to receive an ADHD diagnosis than their peers


Researchers in Finland (led by British ADHD expert Kapil Sayal), have verified that the youngest children in a class are more likely to receive an ADHD diagnosis than their peers. Their study, published in The Lancet Psychiatry, found that children born between September and December (the youngest in the Finnish school system) were 64% more likely to be given a diagnosis of ADHD than their peers in the same class who were born between January and April (the oldest).

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Part of the controversy surrounding the diagnosis of ADHD is that studies have consistently shown that the youngest children in a class are up to twice as likely receive a diagnosis (for example, see here, here, and here)—and to be prescribed stimulant medications (see here and here). Researchers theorize that this is due to their developmental immaturity relative to their peers in the same class, who may be almost a year older. Previous researchers have referred to this as “evidence of medically inappropriate ADHD diagnosis and treatment in school-age children.”

According to Sayal:

“Teachers and parents might interpret the behaviour of younger and older children within the same class differently because they might not take relative age into account. This situation could result in a referral and diagnostic bias, meaning that relatively young children within the class are more likely to be clinically referred and subsequently diagnosed with ADHD—possibly incorrectly.”

Most of the previously mentioned studies were conducted in countries which have generally high rates of ADHD diagnosis. The authors of the current study wanted to examine whether these results would hold true in countries with lower diagnosis rates, such as their home country of Finland. Their results demonstrate that even in countries where ADHD diagnosis is less frequent, it is still given to the youngest children in a classroom.

Sayal notes that the ADHD diagnosis is based on comparing children to the others in their class. After all, “Findings from a recent study indicate that parent ratings of relative immaturity compared with peers are associated with ADHD symptoms.”

Additionally, the youngest children in a classroom consistently experience academic difficulties, most likely because they are a year less experienced, and their brains are a year less developed than their peers.

Concerningly, the researchers found that this effect is actually growing. That is, in recent years, the youngest children are making up an increased percentage of those diagnosed with ADHD in any given class. Children diagnosed at younger ages (prior to age 10) were far more likely to be diagnosed in this manner.

According to Daryl Efron, who wrote a commentary on the article also published in The Lancet Psychiatry, “If behavior congruent with relatively young age within the school year is being systematically misclassified as developmental immaturity, many incorrect diagnoses of ADHD could be made, and children might be treated unnecessarily with medication.”

Sayal’s study included all “singleton live births” between 1991 and 2004 in Finland (a total of 870,695). The data on ADHD diagnosis came from Finnish Hospital Discharge Register, which includes all ADHD diagnoses because Finland has a universal healthcare system in which children’s medical needs are attended to free-of-charge.

The researchers also examined whether the effect could reflect the presence of other diagnoses, such as conduct disorder, oppositional defiant disorder, and learning (developmental) disorder. The presence of these diagnoses did not affect the results. Thus, the researchers ruled out the possibility that the effect of relative age was due to these other diagnoses.

Sayal and the other researchers also wanted to investigate whether gender was predictive of this effect. However, so few girls were diagnosed with ADHD (only 932 over the 13-year period) that the analysis was underpowered. They noted a statistically significant finding of the same effect, however. 276 girls born between January and April were diagnosed with ADHD, while 313 girls born between May and August received a diagnosis. By comparison, 343 of the diagnosed girls were born between September and December.

For boys, the data was clearer: 1535 boys born between January and April were diagnosed with ADHD, while 1823 boys born between May and August received a diagnosis. By comparison, 1846 of the diagnosed boys were born between September and December.

Children born in December bucked this trend. Slightly fewer children born in December received diagnoses than those born in October or November. The researchers theorize that this may reflect some flexibility in when children born in December can begin school.

According to Sayal, “Teachers, parents, and clinicians should take relative age into account when considering the possibility of ADHD in a child or encountering a child with a pre-existing diagnosis.

The researchers add that education systems should reflect this information too: “From the perspective of education policy, there should be greater flexibility in school starting dates for children who are judged less mature compared with their peers within the same school year.”



Sayal, K., Chudal, R., Hinkka-Yli-Salomäki, S., Joelsson, P., & Sourander, A. (2017). Relative age within the school year and diagnosis of attention-deficit hyperactivity disorder: A nationwide population-based study. The Lancet Psychiatry. Published online October 9, 2017. (Full Text)