Attention deficit hyperactivity disorder affects millions of children in the United States every year. While it is not yet known what causes the condition, scientists have pointed to a variety of risk factors. New research suggests that a child’s birth date may affect the chances of being diagnosed with the condition.

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A new study suggests that ADHD may be overdiagnosed and overmedicated in younger schoolchildren.

Attention deficit hyperactivity disorder (ADHD) is a mental disorder characterized by inattention, hyperactivity, and impulsivity, which all interfere with daily functioning in children.

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.

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.

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.”

Read how allergies during pregnancy might be linked with autism and ADHD.