Relative age and ADHD symptoms, diagnosis and medication: a systematic review

Abstract

There is a growing international literature investigating the relationship between attention-deficit/hyperactivity disorder (ADHD) and younger relative age within the school year, but results have been mixed. There are no published systematic reviews on this topic. This study aimed to systematically review the published studies on the relative age effect in ADHD. Systematic database searches of: Medline, Embase, PsycINFO, Web of Science, ERIC, Psychology and Behavioral Sciences Collection and The Cochrane Library were conducted. Studies were selected which investigated the relative age effect in ADHD in children and adolescents. Twenty papers were included in the review. Sixteen (of 20) papers reported a significantly higher proportion of relatively younger children being diagnosed with ADHD and/or receiving medication for this. Meta-analyses involving 17 of these 20 papers revealed a modest relative age effect in countries with higher prescribing rates, risk ratio = 1.27 (95% CI 1.19-1.35) for receipt of medication. The relative age effect is well demonstrated in countries with known higher prescribing rates. Amongst other countries, there is also increasing evidence for the relative age effect, however, there is high heterogeneity amongst studies. Further research is needed to understand the possible reasons under-pinning the relative age effect and to inform attempts to reduce it.

Keywords: ADHD; Relative age.

Fig. 1

PRISMA flow diagram

Fig. 2

A Forest plot of studies comparing proportions receiving medication for ADHD between the oldest and youngest within the school year in higher prescribing countries. *Evans 2010 appears twice in this Figure due to presentation of data from Medical Expenditure Panel Survey (line 2) and Private Insurance claims (line 4) comparisons separately

Fig. 3

A risk-ratio plot of studies comparing proportions receiving a diagnosis of ADHD between the oldest and youngest within the school year in higher prescribing countries

Fig. 4

A risk-ratio plot of studies comparing proportions receiving a diagnosis of ADHD between the oldest and youngest within the school year in other countries. *Chen 2016 appears twice in this figure due to presenting comparison of the oldest ¼ of the year compared with the youngest ¼ (line 4) and the presentation of those born in the first month of the academic year and the last month (line 7). Rivas-Juesas 2015 appears twice in this figure due to data comparing the oldest 1/3 of the year compared with the youngest 1/3 (line 6) of the year and the oldest 6 months of the year compared to the youngest 6 months (line 3)

Fig. 5

A risk-ratio plot of studies comparing proportions receiving medication for ADHD between the oldest and youngest within the school year in other countries. *Chen 2016 appears twice in this figure due to presented comparison of the oldest ¼ of the year compared with the youngest ¼ (line 5) and those born in the first month of the academic year and the last month (line 6). Pottegard 2014 appears twice in this figure due to presented comparisons of those born in the oldest 1 month and the youngest 1 month of the year (line 3) and the oldest 2 months compared with the youngest 2 months (line 1)