New study on the relative age effect and ADHD

I’m sure you already heard about the relative age effect, if not it describes “a bias, evident in the upper echelons of youth sport and academia, where participation is higher amongst those born early in the relevant selection period (and correspondingly lower amongst those born late in the selection period) than would be expected from the normalised distribution of live births.” (wikipedia) And a new study shows a possible link to ADHD-diagnosis.

From the press release:

Younger primary school children are more likely to be diagnosed with attention deficit hyperactivity disorder (ADHD) than their older peers within the same school year, new research has shown.

The study, led by a child psychiatrist at The University of Nottingham with researchers at the University of Turku in Finland, suggests that adults involved in raising concerns over a child’s behaviour – such as parents and teachers – may be misattributing signs of relative immaturity as symptoms of the disorder.

In their research, published in The Lancet Psychiatry, the experts suggest that greater flexibility in school starting dates should be offered for those children who may be less mature than their same school-year peers.

Kapil Sayal, Professor of Child & Adolescent Psychiatry at the University’s School of Medicine and the Centre for ADHD and Neurodevelopmental Disorders Across the Lifespan at the Institute of Mental Health in Nottingham, was the lead author on the study.

He said: “The findings of this research have a range of implications for teachers, parents and clinicians. With an age variation of up to 12 months in the same class, teachers and parents may misattribute a child’s immaturity. This might lead to younger children in the class being more likely to be referred for an assessment for ADHD.

“Parents and teachers as well as clinicians who are undertaking ADHD assessments should keep in mind the child’s relative age. From an education perspective, there should be flexibility with an individualised approach to best meets the child’s needs.”

Evidence suggests that worldwide, the incidence of ADHD among school age children is, at around five per cent, fairly uniform. However, there are large differences internationally in the rates of clinical diagnosis and treatment.

Although this may partially reflect the availability of and access to services, the perceptions of parents and teachers also play an important role in recognising children who may be affected by ADHD, as information they provide is used as part of the clinical assessment.

The study centred on whether the so-called ‘relative age effect’ – the perceived differences in abilities and development between the youngest and oldest children in the same year group – could affect the incidence of diagnosis of ADHD.

Adults may be benchmarking the development and abilities of younger children against their older peers in the same year group and inadvertently misinterpreting immaturity for more serious problems.

Previous studies have suggested that this effect plays an important role in diagnosis in countries where higher numbers of children are diagnosed and treated for ADHD, leading to concerns that clinicians may be over-diagnosing the disorder.

The latest study aimed to look at whether the effect also plays a significant role in the diagnosis of children in countries where the prescribing rates for ADHD are relatively low.

It used nationwide population data from all children in Finland born between 1991 and 2004 who were diagnosed with ADHD from the age of seven years – school starting age – onwards. In Finland, children start school during the calendar year they turn 7 years of age, with the school year starting in mid-August. Therefore, the eldest in a school year are born in January (aged 7 years and 7 months) and the youngest in December (6 years and 7 months).

The results showed that younger children were more likely to be diagnosed with ADHD than their older same-year peers – boys by 26 per cent and girls by 31 per cent.

For children under the age of 10 years, this association got stronger over time – in the more recent years 2004-2011, children born in May to August were 37 per cent more likely to be diagnosed and those born in September to December 64 per cent, compared to the oldest children born in January to April

The study found that this ‘relative age affect’ could not be explained by other behavioural or developmental disorders which may also have been affecting the children with an ADHD diagnosis.

However, the experts warn, the study did have some important limitations – the data did not reveal whether any of the young children were held back a year for educational reasons and potentially misclassified as the oldest in their year group when in fact they were the youngest of their original peers.

The flexibility in school starting date could explain why the rate of ADHD in December-born children (the relatively youngest) were slightly lower than those for children born in October and November.

And while the records of publicly-funded specialised services which are free at the point of access will capture most children who have received a diagnosis of ADHD, it will miss those who were diagnosed in private practice.

Abstract of the study published in The Lancet:

Background
Findings are mixed on the relationship between attention-deficit hyperactivity disorder (ADHD) and younger relative age in the school year. We aimed to investigate whether relative age is associated with ADHD diagnosis in a country where prescribing rates are low and whether any such association has changed over time or relates to comorbid disorders (eg, conduct disorder [CD], oppositional defiant disorder [ODD], or learning disorder [LD]).

Methods
We used nationwide population-based registers to identify all Finnish children born between Jan 1, 1991, and Dec 31, 2004, who were diagnosed with ADHD from age 7 years onwards (age of starting school). We calculated incidence ratios to assess the inter-relations between relative age within the school year, age at ADHD diagnosis, and year of diagnosis (1998–2003 vs 2004–11).

Findings
Between Jan 1, 1998, and Dec 31, 2011, 6136 children with ADHD were identified. Compared with the oldest children in the school year (ie, those born between January and April), the cumulative incidence of an ADHD diagnosis was greatest for the youngest children (ie, those born between September and December); for boys the incidence ratio was 1·26 (95% CI 1·18–1·35; p<0·0001) and for girls it was 1·31 (1·12–1·54; p=0·0007). The association between relative age and age at ADHD diagnosis reflected children diagnosed before age 10 years, and the strength of this association increased during recent years (2004–11). Thus, compared with children born between January and April, for those born between May and August, the ADHD incidence ratio was 1·37 (95% CI 1·24–1·53; p<0·0001) and for those born between September and December, the incidence ratio was 1·64 (1·48–1·81; p<0·0001). The relative age effect was not accounted for by comorbid disorders such as CD, ODD, or LD.

Interpretation
In a health service system with low prescribing rates for ADHD, a younger relative age is associated with an increased likelihood of receiving a clinical diagnosis of ADHD. This effect has increased in recent years. 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.

 

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Filed under At home, Education, Research

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