Before the early 1990s, fewer than 5 percent of school-age children were thought to have attention deficit hyperactivity disorder (a.k.a. ADHD). Today, that number has more than doubled, with doctors diagnosing almost 6.4 million American kids with the disorder before the age of 17, The New York Times reports.

According to several health experts, this rapid increase in ADHD diagnoses may have less to do with medical precision and more to do with sociological factors, such as current schooling methods and changing perceptions of the mental disorder.

ADHD is defined as “a persistent pattern of inattention or hyperactivity-impulsivity.” Though scientists say there is strong evidence that this pattern does have a genetic basis, a large percentage of kids who are given diagnoses are unlikely to have any kind of physical differences that would make them more easily distracted than normal children.

So what’s the cause for the increase? Stephen Hinshaw, PhD, a professor of psychology at the University of California, Berkeley says the No Child Left Behind Act might have helped incentivize ADHD diagnoses in schools. This act, signed under former president George W. Bush, was the first federal effort linking school financing to standardized test scores.

Hinshaw said that because of the policy, many parents and teachers regarded treatment of the disorder as a success if it boosted a child’s test performance. The problem was no one first delved into biological factors associated with the disorder. Also, in some districts, a child’s ADHD diagnosis resulted in their scores being removed from the school’s official average.

During the first four years after the act was implemented, ADHD diagnosis rates across the country increased by 22 percent. The most drastic rise in ADHD rates occurred in state systems that adopted the policy earlier or more stringently than others.

In addition, in 1991, doctors placed ADHD under the Individuals With Disabilities Act. (This designation allowed educators to hire more tutors and increased test times for diagnosed students.) What’s more, a 1997 overhaul of the Food and Drug Administration allowed drug companies to advertise ADHD medications more directly to the public. The policy changes brought ADHD into public view as “just another part of the experience of childhood,” says Maggie Koerth Baker, a science editor at and author of the Times article.

ADHD diagnoses could also be part of a broader trend in America to medicalize traits that previous generations might have dealt with in other ways, said Joel Nigg, PhD, a professor of psychiatry at Oregon Health and Science University. Thirty years ago, schools gave children far more time for unstructured play during the day and teachers simply punished kids who couldn’t sit still. Now, these academic institutions are more apt to give kids therapy and medicine before loading them with massive amounts of homework and higher educational standards than ever before.

Children with a history of trauma, violence and abuse are often misdiagnosed as having ADHD. Click here for more information.