Complex questions generally have complex answers — such as the question of what causes autism. Experts in the field have debated the topic for decades. Now, US President Donald Trump claims to have solved the mystery. 

The US Department of Health and Human Services launched a "massive testing and research effort" to determine "what has caused the autism epidemic" by September, HHS Secretary Robert F. Kennedy Jr. said in April. 

The number of people diagnosed with autism in the United States has been rising for decades. About one in 36 children were identified with autism spectrum disorder in 2020. That's up from one in 150 children in 2000, according to data from the U.S. Centers for Disease Control and Prevention (CDC). 

At Sunday's memorial service for right-wing activist Charlie Kirk, who was shot and killed on September 10, Trump teased an "amazing" announcement forthcoming regarding autism spectrum disorder.

"I think we found an answer to autism," Trump said.

At an announcement from the White House on Monday, the president blamed the painkiller Tylenol for high autism rates.

"Taking Tylenol is not good — I'll say it: It's not good," Trump said. He and Health Secretary Kennedy said they advised pregnant women not to take Tylenol, and would also give a recommendation to this effect to doctors. 

According to the government, people who take Tylenol during pregnancy have a significantly increased risk of giving birth to a child with autism spectrum disorder. Tylenol is available over the counter in the US; its active ingredient, acetaminophen, is chemically identical to popular European painkiller paracetamol. 

From a scientific point of view however, focusing on one active ingredient as the cause of autism isn't tenable. Extensive studies have shown no causal effect, only a minimal increase in risk, according to Christine M. Freitag, director of the Department of Child and Adolescent Psychiatry, Psychosomatics and Psychotherapy at Frankfurt University Hospital. 

Researchers: One ingredient in medication doesn't cause autism

Taking painkillers during pregnancy is generally considered a risk. But all neural developmental disorders, including autism, are linked to polygenetic risks, according to Freitag. In other words, neither a single active ingredient in medication, nor individual genes are responsible.

Hundreds to thousands of gene variants are likely to play a role. People who carry a single gene variant from that group have a minimal risk of developing autism. Only when these variants accumulate does the risk increase.

Researchers have also repeatedly pointed out that, in addition to hereditary gene mutations, environmental effects such as pollutants, fine dust, microplastics or environmental toxins might also increase the risk of autism.

With scientists having spent decades researching the genetic and neuroscientific causes of autism, "the idea that we can suddenly find the causes by September is unrealistic," Geoff Bird, a cognitive neuroscientist and autism expert at University of Oxford and University College London, told DW in April.

What causes autism?

There are many different signs of autism, and people don't necessarily experience it in the same way . For some, social communication can be challenging or even overwhelming. Others might have difficulties with learning or hypersensitivity to sensory stimuli like touch or light.

As described in medical texts, autism spectrum disorder arises from alterations in the brain's development during early life. Research has shown that in people with autism, there can be a wide range of changes in how the brain functions. 

Scientists are "very sure there is a genetic basis," Bird said. About 80% of autism cases can be linked to inherited gene mutations. 

Changes in genes such as MECP2 have been found to alter brain development, but the evidence that specific changes are directly linked to autism isn't clear.

"Diagnosis of autism has always been the biggest challenge in autism research, because we don't have a biological marker of autism," Bird said. He added that this has created challenges for researchers in trying to understand the biological causes of autism.

Autism awareness and broader diagnostics

Experts say the biggest reason why diagnoses of autism have increased is that clinical and social definitions have changed frequently since autism was first described 80 years ago. 

"Now it's common to diagnose people with much more subtle difficulties, so that explains some of the increased prevalence," said Bird.

Changes in screening methods have also helped experts catch signs of autism in girls more often.

"Autism was mostly defined by how it presents in boys, and girls' diagnoses were fit to that. Now we are expanding what counts to meet diagnostic criteria for autism to account for female representations," said Bird. "The natural consequence is that autism prevalence increases."

The neurodiversity movement has also contributed to broader diagnostic criteria. Autism awareness movements have helped people understand how their own experiences might not be neurotypical. 

"Awareness has likely increased folk seeking an assessment and diagnosis, and so [they] can feel relief when they find answers and potential next steps," said Suzy Yardley, CEO of Child Autism UK, a nonprofit organization. 

Scientists are also investigating whether factors like pollutants, changes in the gut-brain axis or the immune system might have a direct effect on neurodevelopment and autism.

However, Bird said the evidence around these theories is "not convincing".

"No doubt pollutants do bad things, but I would be surprised if they were increasing rates of autism," he said.

Vaccines do not cause autism

The claim that vaccines are behind rising rates of autism has been repeatedly and vigorously disproved. 

Over the past two decades, scientists have carried out many large-scale and rigorous studies to determine whether any aspect of vaccination could cause autism. None showed any links whatsoever between autism and vaccines given either during pregnancy or after birth.

"No link has been found between autism and vaccines, including those containing thimerosal, a mercury-based compound," according to the US-based National Institutes of Health (NIH).

The false claim that vaccines cause autism originally came from a paper published in 1998 suggesting a link between the measles, mumps and rubella (MMR) vaccine and problems with brain development.

Later, the study was found to have serious errors and was retracted. Yet the damage done by the paper lives on.

Kennedy's history of anti-vaccine advocacy is well-documented. And in early 2025, he told the CDC to study the links between vaccines and autism, despite the agency's own previous research showing no link.

The HHS secretary also downplayed a measles outbreak in Texas that infected 500 people and killed two unvaccinated children.

Autism community advocates skeptical of Kennedy's goals 

Advocates for the autism community met the HHS secretary's announcement with skepticism. The UK's National Autistic Society called Kennedy's claim a "fake news publicity stunt."

"We're stunned by the callous and anti-science way autistic people are talked about by Trump and RFK Jr." said Tim Nicholls, assistant director of policy, research and strategy at the UK's National Autistic Society, in April. "Wouldn't it be better if they could deploy their huge financial resources to making life better for autistic people and their families, and improving society's understanding of autism?"

Bird said "tensions" in the way people think about and research autism are common, particularly when it comes to the idea of reducing it or wiping it out. Some advocacy groups argue that autism is not an illness, and "therefore there is not anything to 'cure'," Yardley told DW.

But others argue that those who argue autism isn't a disorder "outweigh the voices of a large number of people with autism who feel their lives have been negatively affected by having autism," Bird said. 

Edited by: Derrick Williams

Editor's note: This article was originally published on April 11, 2025. It was updated on September 23, 2025, to include current events.