The Centers for Disease Control and Prevention (CDC) report that as of 2011, ADHD has been diagnosed in around 6.4 million U.S. children between 4-17 years of age.
While scientists do not currently know what causes ADHD, research has indicated a variety of risk factors. The disease seems to be more common among males than females, with boys being more than twice as likely to be diagnosed with ADHD.
Other risk factors include genetics, brain injuries, low weight at birth, tobacco or alcohol intake during pregnancy, as well as gestational exposure to some environmental toxins.
New research, published in the Medical Journal of Australia, suggests that there might also be a connection between children's birth date and the risk of receiving an ADHD diagnosis.
Assessing the birth date effect in Australian children
A team of researchers, led by Dr. Martin Whitely from Curtin University in Perth, Western Australia, set out to analyze data by year and month of birth from 311,384 schoolchildren divided into two 5-year bands.
One group of schoolchildren was aged between 6-10 years and born between July 2003 and June 2008, while children in the other group was aged between 11-15 years and were born from July 1998 through June 2003.
Of these, 5,937 children (or 1.9 percent) received ADHD medication, with the boys receiving medication at a higher rate than girls - 2.9 percent compared with 0.8 percent, respectively.
The analysis revealed that the youngest children in a school class are more likely to receive ADHD medication than their older classmates.
More specifically, from the 6-10-year-old group, children born in June - which is the last month of the recommended school entry age - were approximately twice as likely to receive medication than those born in the first school entry month, which is the previous July.
In the second band, comprising children aged between 11-15 years, the effect was less pronounced, but still statistically significant.
Results may indicate overdiagnosing
The authors say that these results confirm previous international studies.
For instance, the 1.9 percent medication prescribing rate is comparable with the rate determined by a Taiwanese study quoted by the authors - 1.6 percent. Additionally, the authors note, the Taiwanese study, along with three other North American studies, found the ADHD medication prescribing rate to be "at least" twice as high.
The findings could mean that ADHD is overdiagnosed, suggest Dr. Whitely and team. Referring to a previous article that seems to support this conclusion, they write:
"Frances, who led the DSM-IV development taskforce of the American Psychiatric Association, argued that similar findings in North America indicated that developmental immaturity is mislabelled as a mental disorder and unnecessarily treated with stimulant medication. While he considers ADHD a legitimate diagnosis, he asserted that it is overdiagnosed and overmedicated."
The authors conclude that even if the prescribing rates are relatively low, "there are significant concerns about the validity of ADHD as a diagnosis."
Influence of birth month on the probability of Western Australian children being treated for ADHD, Marin Whiteley et al., Medical Journal of Australia, doi:10.5694/mja16.00398, published online 23 January 2017.
The Medical Journal of Australia news release, accessed 20 January 2017.
CDC, ADHD Data and statistics, accessed 20 January 2017.
Don’t throw out the baby with the bath water, Allen Frances, Australian & New Zealand Journal of Psychiatry, doi:10.1177/0004867415579467, published online 28 April 2015.
NIH, Attention deficit/hyperactivity disorder, accessed 20 January 2017.