Autism rates tripled among children in the New York and New Jersey metropolitan area from 2000 to 2016, according to a study published Thursday in the journal Pediatrics.
The authors, a team from Rutgers University, calculated the trend by analyzing Centers for Disease Control and Prevention estimates of the number of children who've been identified as having autism spectrum disorder by age 8.
Although there is no medical test for autism, the CDC has established a network of 17 sites across the country that estimate autism rates based on a combination of formal medical diagnoses and records from schools and health care providers.
Nationally, the rise in autism rates has been similar to the trend in New York and New Jersey, according to a 2021 CDC report. One in 54 children had been diagnosed with autism by age 8 in 2016, compared to 1 in 150 in 2000.
Advances in diagnostic capabilities and greater understanding and awareness of autism spectrum disorder seem to be largely driving the increase, the Rutgers researchers said. But there’s probably more to the story: Genetic factors, and perhaps some environmental ones, too, might also be contributing to the trend.
Precisely what those other factors are is still unknown, but researchers are at least clear on one fact: Autism has nothing to do with vaccines.
"We know for sure, for so many years now, that vaccines don’t cause autism," said Santhosh Girirajan, an associate professor at Pennsylvania State University who studies the genetic underpinnings of neurodevelopmental disorders and wasn't involved in the new study.
"On the other hand, what we really don't know is: What are the real, clear environmental factors that you should be avoiding?"
The Rutgers study found that in New York and New Jersey, the share of 8-year-olds diagnosed with autism who don't have intellectual disabilities rose more sharply than the share of those who do — a fivefold increase from 2000 to 2016, compared to a twofold increase.
That's most likely because doctors have gotten better at identifying cases of autism without intellectual disability — in other words, children with average or above-average IQs who display characteristics of autism, like impaired social skills, repetitive behaviors and difficulty communicating.
Such cases may be less obvious to parents, teachers or doctors than cases among kids with intellectual disabilities, who tend to have a harder time performing daily tasks on their own and are more likely to struggle in the classroom.
The new study also found that although racial disparities in autism diagnoses have narrowed, they persist. Historically, Black and Hispanic children have been diagnosed with autism at lower rates than white children. The new analysis showed that among children without intellectual disabilities, Black children were 30% less likely than white children to be diagnosed with autism. However, the gap has narrowed among children with intellectual disabilities.
"A reduction in disparities has led to increases in autism [rates], but that doesn't mean that the disparities have disappeared," said an author of the new study, Josephine Shenouda, an epidemiologist at Rutgers.
Shenouda and her team recommended that all toddlers be screened for autism during routine checkups at their pediatricians, echoing guidance from the American Academy of Pediatrics.
"A lot of major pediatricians in most major areas will have universal screening as part of their developmental surveillance for children during their child-well visits. However, it's not happening consistently, and even when it happens, the follow-through — where the parents are referred to appropriate services — that's also lacking," she said.
She estimated that half of kids in the U.S. are being screened for autism.
But more kids' getting screened and broader criteria for diagnosing autism also create more opportunities for misdiagnoses, Girirajan said. That might further contribute to the rising rates.
"You might see a child and you won’t be able to tell if it's ADHD or autism or just mild intellectual disability. And what happens is you need to use more standardized testing to have a uniform way of identifying children who have similar features," he said.
Beyond the improvements in diagnostics, genetic factors could be driving up autism cases on their own, experts said.
Much of the risk of developing autism — around 83%, according to one analysis — comes from inherited genetic factors. Girirajan estimated that 2,000 to 3,000 genes contribute to autism, although just 100 are consistently associated with the disorder.
A child can inherit risk factors from one or both parents, and older parents carry higher risks of having children with autism.
"We do see trends over time in parents waiting to have children, so certainly some of the increase could be explained by parental age," said Jennifer Durocher, a clinical associate professor of psychology at the University of Miami.
Girirajan said environmental factors, like exposure to air pollution, probably aren't significant contributors, but they could aggravate underlying genetic risks.
A 2021 study suggested that viral infections in childhood could also be a risk factor for autism in boys, while other studies have hinted at associations between autism and vitamin D deficiencies, gut inflammation and exposure to toxic chemicals.
"These are novel, emerging concepts," Girirajan said. "We don't have solid evidence that these are indeed causing autism."