That's hard to say exactly, because researchers have used different techniques to answer that question.
Many reports estimate that anywhere from 5% to 8% of school-age kids have the disorder. But the CDC now puts it at 11%.
Some studies have relied on parents saying that a health care provider told them their child has the disorder. But that doesn't necessarily mean that the child meets all the criteria for the condition, says Craig Surman, MD, co-author of Fast Minds: How to Thrive if You Have ADHD (Or Think You Might).
Some doctors might give children an ADHD label even when they may have another educational, behavioral, or mental issue.
The flip side is that some kids who truly have the disorder aren't getting diagnosed with it.
"ADHD often overlaps with a lot of other problems, and many clinicians don't know how to categorize what they're seeing," Surman says.
What experts do know for sure: ADHD is one of the most common behavioral disorders that happens in childhood, according to both the CDC and the National Institutes of Health.
Another ADHD-related mystery is that rates seem to vary a lot based on geography. More than 13% of children in certain states -- including Alabama, Arkansas, Kentucky, Ohio, Rhode Island, and South Carolina -- have been diagnosed with the disorder. Yet other states -- including California, Colorado, and Nevada -- report diagnosis rates of 7% or lower.
Why the discrepancy? Experts say it's possible that there aren't enough well-trained mental health professionals in certain areas, so children aren't being properly diagnosed in some regions. Or, parents in some areas may be reluctant to have their children checked for the condition.
There are also big differences in the number of people who are diagnosed with ADHD around the world. For example, some surveys have found that 11% of children in Australia have been diagnosed, compared to less than 3% in the United Kingdom.
Again, these numbers may be misleading: Health care providers around the world may use different criteria to spot the disorder. And cultural factors, including stigma and what is considered "normal" behavior, and economic issues like the cost of evaluation and treatment, likely play roles, too.
A recent review of studies concluded that the true rate of ADHD among children is pretty similar throughout Asia, Africa, Australia, Europe, and the Americas.
Many people mistakenly think ADHD is a male problem. Right now, boys are twice as likely as girls to be diagnosed with it, Silverman says. But those numbers might start to shift as researchers learn more about how the condition shows up in females.
There are three types of ADHD: inattentive, hyperactive-impulsive, and combined inattentive and hyperactive-impulsive. Girls with the disorder are more likely to have inattentive symptoms as opposed to hyperactive ones, Silverman says. So it can be harder for them to get properly diagnosed. The combined type is most common in boys.
About 4% of adults also have the disorder.
To officially meet the criteria for ADHD as an adult, your symptoms must have started by age 12, even if you were never properly diagnosed in childhood.
Those symptoms can change over time, too, so ADHD in adults rarely looks the way it does in kids. While some young children with the disorder, especially boys, might have problems sitting still and end up running all over a classroom, adults are much more likely to have trouble focusing and staying organized.
About 60% of people who had the condition in childhood still have symptoms as adults. Do the other 40% "grow out" of it? Not exactly, he says.
"ADHD is a neurobiological disorder, and we know that brains develop," he says. "So either their brains changed or life has compensated for it." For example, someone with ADHD who is thriving as a traveling salesman might suddenly have problems again if he were to take on a desk job.
The gender gap in ADHD also narrows a little with age: 1.6 men are diagnosed for every 1 woman who gets the diagnosis, Silverman says.
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