New European research has found that children born later in the year are more likely to be diagnosed with ADHD or take ADHD medication than those born earlier on.

Carried out by the Norwegian Institute of Public Health, the study looked at the number of ADHD diagnoses and ADHD medication use among 509,827 Norwegian children aged six-14 years born between 1998 and 2006.

After analyzing the data the team found that the later in the year children were born, the more likely they were to be diagnosed with ADHD or prescribed medication for the condition.

Boys born later -- between Oct. and Dec. -- were 1.4 times more likely to be diagnosed with or use medication for ADHD than those born between January and March, with girls born in the last three months 1.8 per cent more likely.

The team also found that the incidence of medication use also increased with age.

At the end of the follow-up, when children were in ninth grade, 5.3 per cent of boys born between Oct. and Dec. had received ADHD medication, compared with 3.7 per cent of boys born between Jan. and March.

For girls 2.2 per cent of those born later received medication, compared to 1.3 per cent of those born earlier.

The research backs up the findings from studies in other countries, which have also shown a similar association between birth month and ADHD. However, this is the first time that a link has been found in Norway.

The team is unsure of the reason behind their findings, although they did comment that there is no reason to believe that children born later in the year are more exposed to environmental risk factors for ADHD than those born earlier.

They did suggest however one possible explanation. The criteria to diagnose ADHD from the International Classification of Diseases (ICD10) suggests that symptoms such as inattention, hyperactivity and impulsivity should be more pronounced than expected in relation to the child's age, and as Norwegian children start school the year they turn six, making children born in December youngest in their grade, professionals may be comparing the behavior of younger children to older ones -- that is those born earlier in the year yet are in the same class.

The results can be found published online in the Scandinavian Journal of Public Health.