WASHINGTON — For many experts, Health Secretary Robert F. Kennedy Jr. ‘s promise for “pulling back the curtain” to find autism’s causes in a few months is jarring — and unrealistic.

That’s because it appears to ignore decades of science linking about 200 genes that play a role — and the quest to understand differences inside the brain that can be present at birth.

“Virtually all the evidence in the field suggests whatever the causes of autism — and there’s going to be multiple causes, it’s not going to be a single cause — they all affect how the fetal brain develops,” said longtime autism researcher David Amaral of the UC Davis MIND Institute.

“Even though we may not see the behaviors associated with autism until a child is 2 or 3 years old, the biological changes have already taken place,” he said.

Kennedy on Wednesday announced the National Institutes of Health would create a new database “to uncover the root causes of autism and other chronic diseases” by merging Medicaid and Medicare insurance claims with electronic medical records and other data. He has cited rising autism rates as evidence of an epidemic of a “preventable disease” caused by some sort of environmental exposure and has promised “some of the answers by September.”

What Is Autism?

Autism isn’t considered a disease. It’s a complex brain disorder better known as autism spectrum disorder, to reflect that it affects different people in different ways.

Symptoms vary widely. For some people, profound autism means being nonverbal and having significant intellectual disabilities. Others have far milder effects, such as difficulty with social and emotional skills.

Autism rates are rising — not among profound cases but milder ones, said autism expert Helen Tager-Flusberg of Boston University.

That’s because doctors gradually learned that milder symptoms were part of autism’s spectrum, leading to changes in the late 1990s and early 2000s in diagnosis guidelines and qualifications for educational services, she said.

What’s the State of Autism Research?

The link between genes and autism dates back to studies of twins decades ago. Some are rare genetic variants passed from parent to child, even if the parent shows no signs of autism.

But that’s not the only kind. As the brain develops, rapidly dividing cells make mistakes that can lead to mutations in only one type of cell or one part of the brain, Amaral explained.

Noninvasive testing can spot differences in brain activity patterns in babies who won’t be diagnosed with autism until far later, when symptoms become apparent, he said.

Those kinds of changes stem from alterations in brain structure or its neural circuitry — and understanding them requires studying brain tissue that’s available only after death, said Amaral, who’s the scientific director of a brain banking collaborative called Autism BrainNet. The bank, funded by the nonprofit Simons Foundation, has collected more than 400 donated brains, about half from people with autism and the rest for comparison.

What About Environmental Effects?

Researchers have identified other factors that can interact with genetic vulnerability to increase the risk of autism. They include the age of a child’s father, whether the mother had certain health problems during pregnancy including diabetes, use of certain medications during pregnancy, and preterm birth.

Any concern that measles vaccinations could be linked to autism has been long debunked, stressed Tager-Flusberg, who leads a new Coalition of Autism Scientists pushing back on administration misstatements about the condition.

What About Kennedy’s Database Plan?

The U.S., with its fragmented health care system, will never have the kind of detailed medical tracking available in countries like Denmark and Norway — places with national health systems where research shows similar rises in autism diagnoses and no environmental smoking gun.

Experts say Kennedy’s planned database isn’t appropriate to uncover autism’s causes in part because there’s no information about genetics.

But researchers have long used insurance claims and similar data to study other important questions, such as access to autism services. And the NIH described the upcoming database as useful for studies focusing on access to care, treatment effectiveness and other trends.

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