But how are the poor pharmaceutical companies supposed to make money if not enough kids are diagnosed with things they don’t have?

A child that is easily distracted, fidgety and interruptive in school might not have a clinical case of attention deficit hyperactivity disorder (ADHD), but might rather just be acting his or her age, posit researchers behind two new studies of diagnosis trends.

Kids whose birthdays fall around school enrollment cutoff dates can find themselves as either the youngest or oldest among their peers, depending upon which side of the date they fall on. And that coincidence of the calendar can make a significant difference in a child’s chances of getting diagnosed with ADHD, according to the new research.

Two separate studies, both set to publish in a future issue of the Journal of Health Economics, found that students whose birthdays fell just before their school’s age enrollment cutoff date—and thus were among the youngest in their class—had a substantially higher rate of ADHD diagnoses than students who were born just a day or two later and were the oldest in the grade below.
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After reviewing U.S. diagnosis and treatment records across a decade, she and her colleagues found that “being young for [a] grade more than doubles the change that a student is diagnosed with or treated for ADHD.” And as they pointed out in their paper, “ADHD is an underlying neurological problem and incidence rates should not change dramatically from one birth date to the next.”