We’ve known for some time that the prevalence of attention deficit hyperactivity disorder (ADHD) has risen among American children over the past several years. Whether there are more kids with ADHD now than ever before — or we simply didn’t know how to identify and treat them in the past — is left to debate, but new research shows that the diagnoses keep rising.

Published in The Journal of Clinical Psychiatry, a new study carried out by researchers at Milken Institute of Public Health at the George Washington University finds that the number of U.S. school children who were diagnosed with ADHD has risen by 43 percent, particularly among girls. Now, there are some 5.8 million U.S. children between the ages of 5 and 17 who are diagnosed with the disorder, which is characterized by inattention, inability to focus, and often hyperactive, reckless behavior.

“Traditionally, boys have been more likely to get a diagnosis of ADHD,” Sean Cleary, associate professor of epidemiology and biostatistics at Milken Institute and lead author of the study, said in the press release. “We found the parent-reported prevalence for girls diagnosed with ADHD rose from 4.3 percent in 2003 to 7.3 percent in 2011. That’s an increase of 55 percent over an eight-year period.”

The researchers examined children under the age of 17, using data from the Maternal and Child Health Bureau and the National Center for Health Statistics of the CDC from 2003-2011. They analyzed surveys that gathered information about the health of the kids, whether their parents had reported an ADHD diagnosis, as well as ethnic background and sociodemographic factors. They found that there was an 83 percent increase in ADHD diagnoses among Hispanic children from 2003 to 2011.

The researchers note, however, that they still aren’t sure what has caused the striking rise in ADHD diagnoses. The increase could indeed be a true rise in prevalence, but it could also mean a rise in over-diagnosis as well. With diagnosis means more prescriptions filled for Adderall, Ritalin, and other stimulant drugs. More research will be needed to understand the cause behind the increase, but it's important to note that ADHD is a mental health issue, something that is often nuanced, complicated, and difficult to pin down with specific symptoms.

“The hard part is that ADHD is just like depression, just like autism, just like schizophrenia in that it's a symptom-based mental disorder,” Stephen Hinshaw, a psychologist at University of California, Berkeley, told the American Psychological Association. “We don't have a blood test or a brain scan yet that's definitive. I believe that ADHD is a real condition, but it's on a spectrum, just the way that high blood pressure and autism are. It's always a bit arbitrary as to who is actually above the cut and who is below because we don't know exactly where the cut is. ”

Source: Cleary S, Collins K, et al. The Journal of Clinical Psychiatry. 2015.