New research suggests that autism prevalence rose as much as 500% in recent decades, but it still may be underdiagnosed.

In an analysis of data on 8-year-olds in the New York-New Jersey metropolitan area between 2000 and 2016, researchers found that the number of children with autism and intellectual disability doubled over the time period. Among those without intellectual disability, however, the increase was fivefold.

Still, the study published online Thursday in the journal Pediatrics finds that disparities in diagnosis persist and many kids may remain overlooked. Children from affluent areas were 80% more likely than those from underserved areas to be flagged as having autism with no intellectual disability while Black children in this category were 30% less likely to be identified as compared to white kids.

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“Better awareness of and testing for ASD does play a role,” said Walter Zahorodny, an associate professor at the Rutgers New Jersey Medical School and a senior author on the study. “But the fact that we saw a 500% increase in autism among kids without any intellectual disabilities — children we know are falling through the cracks — suggests that something else is also driving the surge.”

The study is based on data from the New Jersey Autism Study, which looks at 8-year-olds in four New Jersey counties biannually as part of the Centers for Disease Control and Prevention’s efforts to track autism prevalence. Across the 16-year study period, researchers found 4,661 kids with autism in the region, just over 32% of whom had co-occurring intellectual disability while the remainder did not.

“One of the assumptions about ASD is that it occurs alongside intellectual disabilities,” said Josephine Shenouda, an adjunct professor at the Rutgers School of Public Health and the lead author of the study, who pointed to older studies showing that as many as 75% of kids with autism have intellectual disability. “What our paper shows is that this assumption is not true.”

Shenouda said the findings highlight how important it is to have early screening, identification and intervention and she said universal screening for autism is imperative, particularly in underserved communities.

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