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The feature article that makes up this month's Long Read podcast was written before US President Donald Trump claimed that use of acetaminophen or paracetamol during pregnancy is linked to autism.
This is an audio Long Read from Nature.
In this episode, autism is on the rise.
What's really behind the increase?
Written by Helen Pearson and read by me, Benjamin Thompson.
On the 16th of April, Robert F Kennedy Jr held a press conference about rising diagnoses of autism.
The U.S.
Health and Human Services, or HHS, Secretary pointed to new data showing that autism prevalence in the United States had risen steeply from 1 in 158-year-olds in the year 2000 to 1 in 31 in 2022.
He called it an quote epidemic end quote caused by quote, an environmental toxin end quote and said he would soon be announcing a study to find the responsible agent.
The next month, the US National Institutes of Health, the NIH, part of the department that Kennedy leads, announced the Autism Data Science Initiative, the ADSI.
The initiative offered up to US$50 million to fund studies on the causes of autism.
The winning applications are expected to be announced in September.
Usually, big investments in research are welcomed by scientists, but not this time.
Many were dismayed that these developments seemed to ignore decades of work on the well-documented rise in autism diagnoses and on causes of the developmental condition.
Although Kennedy said that environmental factors are the main cause of autism, research has shown that genetics plays a bigger part.
Population studies have linked a handful of environmental factors, mostly encountered during pregnancy, to increased chances of autism, but their precise role has been hard to pin down.
More than anything, research has shown that the drivers of autism are fiendishly complicated.
There will never be a soundbite answer to what causes autism, says Helen Tager Flussberg, a psychologist who studies neurodevelopmental conditions at Boston University, Massachusetts.
The rise in prevalence many researchers say is predominantly caused by an increase in diagnoses, rather than a true rise in the underlying symptoms and traits.
We don't see an epidemic of autism, but we see an epidemic of diagnoses, says Sven Bulte, a specialist in child and adolescent psychiatric science at the Karolinska Institute in Stockholm.
Researchers are concerned that Kennedy, an anti-vaccine advocate, will use the ADSI to promote the disproven idea that vaccines are linked to autism.
Kennedy's comments prompted equally strong reactions from autistic people and autism groups, says Alicia Halliday, chief science officer at the Autism Science Foundation, a non-profit research and advocacy group in New York City.
Autistic people have a wide range of traits and symptoms.
That means that many live and work independently, whereas others need intensive support.
Some people want more research into the causes, she says, and others don't.
But many people in the autism community found it demeaning and offensive when Kennedy said at the press conference that quote these are kids who will never pay taxes.
They'll never hold a job.
End quote.
These factors do not determine a person's value, Halliday says.
And many autistic people are employed and pay taxes.
Scientists and autistic people alike are also concerned that the administration of US President Donald Trump is actually cutting the overall amount of money going towards autism research and support for autistic people, including through cancelled or delayed grants.
Everyone is really, really upset that money is being taken away from these important research projects, Halliday says.
Many studies suggest that the prevalence of autism is rising, and not just in the United States.
The trend has been observed in other high-income countries, including the United Kingdom Denmark, South Korea and Japan.
Everywhere people are doing research and are looking at the numbers, they are going up, Bolter says.
In some less wealthy countries however, a lack of health services and data mean the prevalence is difficult to assess.
There are several reasons to think that a rise in diagnoses explains a large part of this trend.
One is that the diagnostic criteria have changed over time in two periodically updated tools used by health professionals.
The Diagnostic and Statistical Manual of Mental Disorders, the DSM, used mainly in the United States, and the International Classification of Diseases, the ICD.
These describe autistic people generally as showing differences in social interaction and communication, as well as having restricted and repetitive behaviours and interests.
But until the early 1990s the manuals had a narrow definition for autism, says Diana Schendel, an epidemiologist who studied autism at Drexel University in Philadelphia, Pennsylvania.
For instance, the third edition of the DSM recognised autism only in young children and required that they meet a minimum number of criteria.
Updated versions of both handbooks, ICD-10 in 1990 and DSM-4 in 1994, broadened the diagnostic criteria so that they could be applied to people with fewer symptoms and of different ages.
That dramatically changed the opportunity for individuals to be classified under the autism umbrella, says Schendel.
In 2013, DSM-5 dropped some separate diagnoses, such as Asperger's syndrome, and subsumed them under the term autism spectrum disorder.
Autism is commonly diagnosed with co-occurring conditions such as intellectual disability or attention deficit, hyperactivity disorder, ADHD.
A 2015 study that Schendel co-authored tried to quantify the causes of increased autism prevalence among Danish people born between 1980 and 1991 and diagnosed up to 2011.
The authors estimated that 60 of the increase could be explained by changes in the early 1990s to diagnostic criteria and to the way in which diagnoses were reported to national health registries.
Diagnoses have also risen as health professionals have adjusted how they interpret and apply the criteria and as they have adopted improved standardized diagnostic tools such as structured interviews and observations.
Ava Lute, a cognitive neuroscientist at King's College London, says that she now diagnoses people in research projects whom she would have excluded ten years ago.
Our understanding of what autism is has changed, says Lute.
The other change that has turbocharged diagnoses, researchers say, is increased awareness of autism and declining stigma among teachers, health professionals and the broader public, alongside increased availability of diagnostic and support services.
Parents are more likely to observe signs of autism in children and are often incentivised to seek a diagnosis if it can help their child to access support such as at school.
Where before a child might be classified as difficult problematic in a classroom, now they might go seek a diagnosis, says Nick Putz, who studies neurodevelopment at King's College London and is autistic.
Children are also receiving diagnoses at a younger age than in the past.
More adults are seeking diagnoses, and they have become more common in girls and women, in whom autism has historically been overlooked.
Boulter argues that the high demands of modern schools, such as extensive group work and digital information, could also play a part.
The learning environment is extremely complex today, he says, and there's more and more children with maybe lesser traits or symptoms who are just kind of overwhelmed.
Similarly, more adults navigating a highly complex world might be seeking help and advice, leading to more autism diagnoses.
How much of the rise in autism is accounted for by increased diagnoses and awareness?
It's certainly the vast majority, Schendel says, although the exact proportion is difficult to calculate.
The figure that Kennedy cited a prevalence of 1 in 31 among 8-year-olds stems from an April report by the Autism and Developmental Disabilities Monitoring Network, a US surveillance program that tallies autism in children using health and education records.
These show who has received the diagnosis and who is eligible for autism-related education.
The results from this type of study are sometimes called registry or administrative prevalence, because they are based on administrative databases.
But some researchers view such studies with caution, says Damien Santomauro, a mental health epidemiologist at the University of Queensland in Brisbane, Australia.
Identification of autism can come from various sources, including psychiatrists, psychologists and educational settings.
Some children who have extra behavioral or educational needs might be included in an administrative database but would not meet stringent criteria for diagnosis in an independent clinical evaluation.
Santamaro says.
Conversely, some people with undiagnosed autism might be excluded from registries.
A more accurate way to estimate prevalence is through a population survey, in which researchers screen everyone in a representative group of children or adults for autism.
Santamaro led the most recent report on autism by the Global Burden of Disease Study, the largest scientific project on impacts of health conditions which is based only on survey data.
This estimated that autism spectrum disorder had a prevalence of 1 in 127 people globally less than 1 in 2021.
That's 62 million people overall.
A long-term study of Swedish children published this year, compared registry and survey prevalence in children who were born between 1993 and 2001.
It found that the prevalence of autism symptoms that parents reported when their children reached 18 had remained stable, even as the prevalence of registered diagnoses had gone up.
All of this adds weight to the argument that spiking autism statistics are driven mostly by improved detection and diagnoses, but Santomaro says that other high-quality surveys have made him think twice.
A study published in 2020 that used rigorous survey methods reported a prevalence of more than 3 in five-year-old children in Japan, higher than previous estimates for autism in some other Asian countries.
So although Santamaro agrees that most of the rise in autism is explained by increased detection and diagnosis, he says it remains possible that something could be pushing up.
Quote true prevalence, end quote as well.
But what?
Researchers have been hunting for the causes of autism for decades and have run up against difficulties at every step.
We just don't really understand a lot about how any human brain functions, says James Cusack, chief executive of Autistica, a UK autism research and campaigning charity in London, who has worked in autism research and is autistic.
So trying to find causal mechanisms for autism is, I think, much harder than people thought it was going to be 20 years ago.
One broad conclusion to emerge from the available data is that genetics plays a huge part.
Family history risk is probably the strongest risk factor there is, says Schindel.
Researchers captured the genetic contribution using measures of heritability, an estimate of how much the differences in autism traits between individuals can be explained by inherited genes rather than by environmental factors.
A large five-country study published in 2019 estimated the heritability of autism as about 80, equivalent to the heritability of height.
For comparison, the heritability estimates for depression are roughly 30-50%.
Identifying the individual genes involved, however, has proved tough.
The genetics themselves are wildly complex, says Craig Neuschafer, an autism researcher at Pennsylvania State University in University Park.
Researchers have sought genes by comparing the genomes of autistic people and people who are not autistic, in families and in bigger population studies.
These have helped to pinpoint a selection of rare genetic variants variations in DNA sequence that cause a relatively large increase in the chances of autism.
Many of these are de novo mutations that probably arose in a sperm or egg cell and were passed to a child.
These large-impact variants are thought to be a major driver of autism in an estimated 10-20 of autistic people.
But quote the genetic architecture is very different from one individual to another, says geneticist Thomas Bougeron, who studies autism at the Pasteur Institute in Paris.
Researchers have also found that hundreds, perhaps thousands of common variants have a tiny effect individually, making them hard to pinpoint, but a big effect when combined.
If someone has enough variants, they could end up with symptoms and a diagnosis.
After genetics, environmental factors are.
Quote a smaller slice of the pie end.
Quote in driving autism, Schendel says, although there is debate about the size of the slice.
Some researchers argue that the influence of environmental factors is vanishingly small, and others disagree.
Most research suggests that any environmental factors exert their influence before birth however, and studies have convincingly shown no link to vaccines.
Many studies suggest that older parental age for mothers or fathers is linked to an increased likelihood of autism in children.
The average childbearing age has been increasing in many high-income countries, so this could be playing a small part in increasing autism prevalence, researchers say.
One possible explanation is that children of older parents are more likely to have de novo mutations than are children of younger parents.
Infections during pregnancy have also been linked in many studies to a greater chance of autism, as has exposure to air pollution before birth.
For instance, a US study published in June involving more than 8000 children and their mothers found that exposure to higher levels of ozone was associated with autism.
But Neuschafer says that other studies have produced conflicting results about the pollutants behind the link.
And although researchers have linked a string of other environmental factors to increased likelihood of autism, including gestational diabetes and obesity in mothers, antidepressants and certain other medications taken during pregnancy and folic acid deficiency, again the results have been mixed.
The problem with studies of environmental factors is, a quote lack of consistency and replicability.
Neuschafer says.
There's almost certainly no single environmental cause of quote, huge magnitude end quote to be found.
But hunting for definitive environmental influences is still worthwhile.
The enormous variability among autistic people is perhaps the biggest challenge for scientists, looking for causes.
It is probably the result of combinations of genetic and environmental factors interacting in convoluted ways.
And when we smush them together, and smush them together in different proportions in different studies, we're going to see different things.
Neuschafer says.
Complexity is our enemy here.
That's why some researchers are taking a different tack.
They are studying traits common in autism, such as features of mental health or ways in which people sense touch and sound, rather than attempting to find general causes.
That's one focus of AIMS2 Trials, a major autism research consortium running from 2018 to 2026, funded largely by the European Union and pharmaceutical industry, involving nearly 50 research institutions, companies and other groups worldwide.
Putz, for example, is investigating whether altered levels of the brain's chemical messengers, such as glutamate, might underlie sensory processing traits in autistic people.
The idea is to move to more quote personalized approaches.
He says that focus on each person's experiences and on how to support that individual.
New Schaefer says that the NIH's Autism Data Science Initiative is quote not out of sync with mainstream science.
End quote.
It aims to investigate the causes of autism and the effectiveness of current interventions, mainly using existing health databases and other data sources.
A key goal, according to the funding announcement, is to integrate and analyse those data, such as on genetics and environmental exposures.
That fits with one research goal to explore how interacting genetic and environmental factors contribute to autism.
This requires large numbers of people to detect the small effects of these interactions.
One large US project to study genetic and environmental factors together, called the Gears Autism Centre of Excellence Network, is combining information on about 175000 people from 18 sites in the United States, Canada and Denmark.
This includes data on air pollution, prenatal infections and many other factors.
E.J.
Gung, a biologist at the University of Washington in Seattle, says that he welcomes any extra support for studies of the environmental causes of autism.
These quote have been long under-supported.
End quote by public and private research funders.
Compared with genetic causes, he says, and the ADSI could work quote towards bridging this historical funding gap.
End quote.
Other scientists are concerned about the ADSI because of Kennedy's anti-vaccine history and his comments at the April press conference.
That quote threatens the credibility of the platform on which this initiative is built, says Neuschafer.
In April, more than 250 US researchers, including Tager Flussberg Schendel Amaral, Halliday and Neuschafer, formed the Coalition of Autism Scientists to express concerns about Kennedy's statements on autism and to advocate for rigorous research.
Researchers and advocacy groups have also expressed fears about whether the privacy of individuals in databases used for research will be protected.
At the end of it, I don't think we will be any closer to the causes of autism, Neuschafer says.
And researchers and autism groups are concerned that funding cuts and policies introduced by the Trump administration will ultimately set back autism research and support much more than the ADSI could further it.
A nature analysis of NIH-funded research projects using the Reporter tool shows that autism research received 62 million less funding in the first half of 2025 than in the same period in 2024.
$212 million compared with $274 million.
Investment in the ADSI quote cannot offset the broader erosion of support end quote caused by cuts to research grants and to Medicaid, the US public health insurance system for people with low incomes, education funding and other services.
These cuts threaten the infrastructure that supports both scientific progress and quality of life for autistic individuals and their families.
A spokesperson for the HHS, did not respond directly to the claim that the Trump administration is cutting autism research and support, but rejected claims that Kennedy has spread misinformation about autism and its causes.
HHS is taking action on autism as the public health emergency.
It is, they said, including by launching the ADSI, which could further our understanding of the condition and improve the well-being and health outcomes for people on the autism spectrum.
All NIH-managed databases follow the highest standards of security and privacy, they said.
And quote the NIH is fully committed to leaving no stone unturned in confronting this catastrophic epidemic, employing only gold standard, evidence-based science.
End quote.
David Amaral, an autism researcher at the MIND Institute at the University of California Davis, says that if he had 50 million to find the causes of autism, he would first convene a group of researchers clinicians, advocacy groups and autistic people.
He'd ask them to reach a consensus on the outstanding questions regarding causes and what research should be pursued.
Many researchers and funding agencies already work with members of the autism community in planning and conducting research.
In the past, the HHS received advice on autism, including research directions, from the Interagency Autism Coordinating Committee, the IACC, a group of federal employees, researchers and members of the public.
However, researchers say the committee has not met this year and that there was little external consultation with autism researchers or advocates over development of the ADSI.
The HHS spokesperson said the NIH is committed to incorporating input from the autism community and key stakeholders into autism-related initiatives and that the ADSI looks forward to working with the IACC and quote seeking its guidance once convened.
End quote.
Other consultations have suggested that finding causes is not a priority for some autistic people.
In 2016, Autistica and a group of other UK autism charities consulted more than 1000 autistic people, their families and professionals and asked them to devise a top 10 list of questions they wanted research to answer.
It included finding ways to improve and support mental health communication, education and other needs.
None of the listed priorities was about finding causes of autism.
Cusack says he is encouraged by advances over the past 30 years in understanding autism, how it's portrayed and how to provide support.
His biggest concern, he says, is that Kennedy and the HHS are quote going to falsely declare the cause for autism and that that gets in the way of a great deal of progress.
End quote.
To read more of nature's long form journalism, head over to nature.com slash news.
This moment, Alex's mile swim, it means everything to him and to all of us.
At Evernorth Specialty Services, we help providers make sure patient care doesn't get interrupted, especially for rare, chronic and complex conditions.
Now Alex and his care team have his hemophilia under control with life-saving medication and care management from Evernorth's specialty pharmacy Acredo.
So Alex can be the last one out of the pool.
Evernorth Specialty Services, because every moment counts.
Visit evernorth.com slash specialty to learn more.
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