Two new studies found that children who start school at a younger age are much more commonly diagnosed with attention deficit hyperactivity disorder (ADHD) than their older classmates. The researchers, from Michigan State and North Carolina State universities, suggest this means that doctors are often misdiagnosing immaturity as ADHD, and improperly treating up to a million kids. (Watch an ad explaining ADHD.) Here's a brief look at the findings:

In a nutshell, what did the researchers discover?
Michigan State economist Todd Elder focused on kindergarteners, and found that the youngest kids in the class were 60 percent more likely to be diagnosed with ADHD than their older classmates. By middle school, the younger kids were twice as likely to be prescribed Ritalin or other psychostimulants. The NCSU study found similar results among thousands of 7- to 17-year-olds.

How many kids are diagnosed with ADHD?
More than 4.5 million in the U.S. alone, according to the federal Centers for Disease Control. It is the most common behavioral disorder diagnosed in the U.S. About 900,000 of those kids are probably just acting their age around older classmates, Elder estimates, and improperly medicating them wastes between $320 million and $500 million a year — $90 million of which is billed directly to Medicaid.

How could you mistake immaturity for ADHD?
The symptoms of ADHD — scattered attention or inattentiveness, impulsiveness, and hyperactivity — are also common signs of immaturity. "If a child is behaving poorly, if he's inattentive, if he can't sit still, it may simply be because he's 5 and the other kids are 6," Elder says. "Many ADHD diagnoses may be driven by teachers' perceptions of poor behavior among the youngest children in a kindergarten classroom." Some commentators find this unsurprising: "Is it wrong to simply say, 'Duh?,'" asks Carolyn Castiglia at Babble.

Could the researchers be misinterpreting the data?
It's possible. They note that while their findings show convincingly that younger children in a class are diagnosed with ADHD more frequently, they don't show why. An alternative explanation is that older kids are under-diagnosed with ADHD.

How can I be sure my child isn't being misdiagnosed?
Diagnose ADHD by age, not grade — and get a second opinion, advise the Michigan State researchers. Pediatrician William Sears suggests parents keep a diary of their child's symptoms. "With immaturity, you will notice steady improvement as your child learns to settle into a school routine. On the other hand, true ADHD, if left undiagnosed and untreated, can become worse with time," he says in Parenting.

Sources: Guardian/British Medical Journal, Time, AFP, LA Times, NPR, USA Today, MSU, NCSU, Parenting, HelpGuide