Children who start school earlier due to being born just before the enrollment cutoff are 40-50% more likely to be diagnosed with attention-deficit/hyperactivity disorder during early primary school (ages 5-8) compared to their older peers, according to a new study.

Researchers examined the impact of deferring school entry for children born just before the enrollment cutoff on attention-deficit/hyperactivity disorder (ADHD) diagnosis and treatment in England. The study analyzed administrative data on ADHD prescription rates and found that children who start school earlier, due to being born just before the cutoff, have higher rates of ADHD diagnosis than their older classmates. This increase is primarily attributed to peer-comparison bias driven by differences in relative age.

The findings confirm that relative age is the main factor contributing to the increased ADHD diagnosis rate in early school starters, particularly between the ages of 5 and 8, when most first-time ADHD prescriptions occur. This gap in ADHD diagnosis decreases in later childhood, but early diagnosis leads to continued treatment over time.

Published by the IZA Institute of Labor Economics, the study found that early school starters have a 40-50% higher likelihood of receiving ADHD prescriptions compared to later-starting peers. Of those who began ADHD treatment between ages 5 and 8, 86.16% were still receiving treatment by age 9, with this percentage declining to 64.20% by age 15.

The researchers ruled out environmental and stress factors as significant drivers of this gap, reinforcing the relative age effect as the central mechanism. No conflicts of interest were disclosed by the researchers.