Many children diagnosed with attention deficit disorder may just simply be the youngest members of their class, Taiwanese researchers suggest.
Looking at data on 400,000 children, they found August-born children were almost twice as likely to have ADHD as those born the previous September.
The team said many cases may be down to teachers comparing the behaviour of the least and the most mature children.
ADHD is an inability to control activity levels and sustain attention.
It is a behavioural condition which affects up to 7% or 400,000 British children and impairs their ability to learn.
The study of Taiwanese children, in the Journal of Pediatrics, found 2.8% of the pre-school and primary school boys born in September are diagnosed with the condition compared with 4.5% of those born the following August. For girls the incidence increased from 0.7% to 1.2%.