During the last two decades, autism rates skyrocketed in this country. But findings published in the journal Autism suggest that as many as 9 percent of kids given this diagnosis may not actually have the condition, NBC News reports. Autism spectrum disorder (ASD) is a broad range of neuro-psychological conditions that affect a child’s social and communication skills. Symptoms can include mildly debilitating repetitive behaviors, difficulty communicating and severe intellectual disabilities. (Today, there’s still no one specific set of rules for diagnosing the condition.)

In this latest study, researchers at the National Center for Health and Statistics evaluated a national survey of more than 1,500 parents of kids with autism. Scientists found that up to 13 percent of children ever diagnosed with the condition had had their diagnosis changed. Why? Well, 74 percent of these children’s parents said it was due to new information about their child’s mental health. Researchers explained this meant up to 9 percent of children originally diagnosed with autism in the study didn’t actually have the disorder. Instead, mental health specialists later re-diagnosed many of these kids with attention deficit hyperactivity disorder, or ADHD. Study authors said it’s possible the re-diagnosis resulted because of “the high overlap between the symptoms of these disorders.” In addition, findings from a previous report noted the rise in U.S. autism cases might be owed to doctors re-classifying many kids with developmental delays as having autism, not because of an actual uptick in the condition.