Some kids with autism lose the label as they age. Now a new study is helping to explain why the diagnosis sticks around for some and not others.

Researchers looked at parent-reported data on more than 1,300 children ages 3 to 17 with a current or previous autism diagnosis. About one-third of the children had been diagnosed with autism, but were no longer considered to have the disorder, according to the study published Monday online in the journal Pediatrics.

What set the two groups of children apart was whether or not a child had other co-occurring psychiatric or developmental disorders in addition to autism, the researchers found. In fact, those who retained their autism diagnosis as they got older were more likely to have two or more co-occurring conditions, they said.

The other conditions affecting kids who retained an autism diagnosis often included learning disabilities, developmental delay, speech problems, anxiety and seizures.

It’s unclear why some children lose their autism diagnosis as they get older, the researchers said. It could be because they were misdiagnosed in the first place or because early intervention was successful.

“Core features of ASDs are often similar in presentation to commonly diagnosed co-occurring conditions,” the researchers wrote. “As the child grows older, it is likely that symptoms become clearer to medical professionals and that a more accurate diagnosis for the presentation of symptoms is more consistent with ASD… This can also be true in the opposite direction, in which a child might have been diagnosed with an ASD because of the presence of common ASD co-occurring conditions or diagnoses and then was later reclassified as not having an ASD.”