Being among the youngest children in a school class increases the chance of being medicated for Attention Deficit Hyperactivity Disorder (ADHD). Although the study reporting this is the fifth to produce similar results, it is the first in the southern hemisphere. This rules out seasonal effects, and supports theories of over-medication.
Three previous studies in the United States and one in Taiwan have found that children born just in time to make it into a school year are more likely to be diagnosed with ADHD.
When all the studies were undertaken in the northern hemisphere, the possibility remained that some common factor was skewing the results, although the fact that the US and Taiwan have different cut-off dates for school entry probably disappointed astrology fans. In Australia, however, the school year starts in January and the annual cutoff for entry is a birthday before June 30.
Dr Martin Whitely of Curtin University obtained birth dates for children prescribed ADHD medication. Whitely discovered differences between the Australian states. In some, children who are legally old enough to go to school are frequently held back a year by their parents. However, in Western Australia (where Whitely was once a member of parliament), 98 percent of children start school in the first year they are allowed to, making the data easier to analyze.
In the Medical Journal of Australia, Whitely reports that Western Australian children aged 6-10 who were born in June were twice as likely to be medicated for ADHD as those born in July, and therefore among the older members of their class. The effect is weaker for older children.
Whitely told IFLScience: “The paper looked at numbers, we can only hypothesize about the causes.” However, he noted that “teachers provide crucial information” that informs diagnoses and may mistake age-related immaturity for a neurological condition.
Although it is theoretically possible children born in July are being under-diagnosed, Whitely thinks it far more likely that over-diagnosis is widespread, and worst among the June births.
Interestingly, while the overall rate of medication in Western Australia was much lower at 1.9 percent than in American studies (2.6-5.8 percent), the ratio between the oldest and youngest students was very similar in all cases.
Two further lines of research interest Whitely. He's keen to study whether other states' culture of delaying the start to school for many younger children changes things. “Increased flexibility should provide protection for the children held back,” he told IFLScience, “but it creates a wider age range in a class and may actually make the birthday effect stronger for those who do start early.” He also wants more investigation into the long-term effects of ADHD medication, something he has fought against for many years.