US President Donald Trump and Health Secretary Robert F. Kennedy Jr. have long suggested vaccines may be playing a role in the increase in autism diagnoses, a theory that has been discredited by dozens of scientific studies. Photo / Getty Images
For decades, researchers have looked for links. Most believe the disorder springs from a complex interplay of genetics and environmental factors.
President Donald Trump and Health Secretary Robert F. Kennedy jnr have today given direct and unproven medical advice about the use of a common painkiller, acetaminophen, in pregnancy and infancy despite not having any medical background. He declared that there was a link between rising autism rates and the use of the painkiller, the active ingredient in Tylenol (paracetamol). That potential link has been studied over the years, but the evidence has been inconclusive.
Autism diagnoses in the United States have greatly increased over the last 25 years. Autism experts say the increase is at least partly caused by an increase in awareness and the gradual expansion of the disorder’s definition. But Trump and Kennedy have long suggested that vaccines also may be playing a role, a theory that has been discredited by dozens of scientific studies.
Here is what scientists know about autism and its potential causes and risks.
Autism spectrum disorder, as it is officially called, is a far-ranging mix of social and communication issues, repetitive behaviours and thinking patterns. The severity of the symptoms varies widely, from mild to very serious impairment. A child with an autism diagnosis may simply struggle with social cues; other children with severe cases may not be able to speak or use the toilet without help. There is no blood test or brain scan to determine who has autism, just the observations of clinicians and what parents tell them.
Hundreds of genes have been associated with autism, a neurodevelopmental disorder, but scientists say it appears to result from a complex combination of genetic and environmental factors.
“I do remember 30 years ago the geneticists being quite optimistic that we were talking about maybe six to 10 genes that might be contributing to autism risk, and now we know that number is literally in the hundreds,” said Helen Tager-Flusberg, a psychologist at Boston University. “Finding even the genetic basis of autism is turning into a far more complex a picture than we might have imagined.”
The Centres for Disease Control and Prevention has done a large-scale study of the risk factors that can contribute to autism, and researchers have examined dozens of potential contributors, including air pollution, exposure to toxic chemicals and viral infections during pregnancy.
Some research suggests that babies born to older parents may be at increased risk. Other studies have found hints that premature birth or low birth weight could be associated with the condition.
Scientists have been studying acetaminophen, the active ingredient in Tylenol and other painkillers, for over a decade. Some studies examining the use of acetaminophen by pregnant women have found an increased risk that children will develop neurodevelopmental disorders later in life. Other studies that have attempted to control for other factors that could underlie such disorders – including genetics – have found no link.
In August, researchers at the Harvard T.H. Chan School of Public Health and the Icahn School of Medicine at Mount Sinai published a review of 46 previous studies on acetaminophen during pregnancy, eight of them looking specifically at autism.
The researchers evaluated only existing studies and did not provide new data on the effects of acetaminophen. They concluded that there was a link between women who had used acetaminophen during pregnancy and attention-deficit/hyperactivity disorder and autism, but said this did not mean that the drug caused autism. The women who used Tylenol could differ in important ways from those who didn’t, including in the health issues that arose during their pregnancies or their underlying genetics. One major study in 2024 that looked at 2.5 million children in Sweden found that the association between acetaminophen use and neurodevelopmental disorders went away when they compared siblings born to the same mother.
After the paper was published, Dr Nathaniel DeNicola, who advises the American College of Obstetricians and Gynecologists, also said the findings did not change what doctors advised their pregnant patients.
“The conclusion of the paper is that Tylenol should be used judiciously in the lowest dose, least frequent interval,” he said, “which is exactly the current standard of care for Tylenol and for so many medications, and really so many things we may encounter in pregnancy.”
The notion that vaccines could cause autism gained traction in the late 1990s, when a British researcher named Andrew Wakefield published a study of 12 children purporting to reveal a link between the vaccine for measles, mumps and rubella and autism.
That thesis has been thoroughly discredited in the ensuing years by many larger studies, including one that enrolled the entire child population of Denmark. Regardless of the types of vaccines, the ingredients in them or the timing of the childhood vaccine schedule, researchers have not seen a link to autism.
Wakefield’s 1990 article was retracted in 2010, and he lost his medical licence.
An estimated 1 in 31 US children have now received an autism diagnosis, up from 1 in 150 in the year 2000 – a rise that has been fuelled by an expansion of the way autism is defined and diagnosed over the last several decades.
Autism first appeared in the third edition of the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorders in 1980.
In a 1987 revision, the definition of the disorder was adjusted to include children whose symptoms appeared at later ages, after 30 months. The number of criteria for a diagnosis of autism was also increased, to 16 from six, and children needed to exhibit only half of the 16, rather than all six, as with the previous listed criteria.
A fourth edition of the DSM, published in 1994, included Asperger syndrome, a social disorder marked by preoccupation with a single interest and other characteristics, on the autism spectrum. That was a significant shift, since it meant that people with milder impairments and average or even above average intellectual abilities could receive the diagnosis.
The fifth edition of the diagnostic manual, released in 2013, folded autism, Asperger syndrome and a condition called PDD-NOS – short for “pervasive developmental disorder-not otherwise specified” – into a single diagnosis of autism spectrum disorder. It also allowed clinicians to give a combined diagnosis of autism spectrum disorder and ADHD.
Data on autism diagnoses from the CDC found that the prevalence of profound autism, defined as having an intellectual disability and severe language impairments, rose slightly from 2000 to 2016, while other autism diagnoses rose more sharply.
Increased awareness of autism has also played a significant role in the rising diagnoses. Broad availability of special services in schools beginning in the 1990s gave parents an incentive to seek out a diagnosis for their children, while recommendations to paediatricians for universal screening of infants at their 18- and 24-month-old well-child visits ensured early detection.
Researchers also point to more discussions about autism on social media as a recent trend behind the rise in diagnoses. Videos about autism on TikTok and YouTube have been viewed billions of times, while discussions about autism on forums like Reddit have helped people find community and identity.
“Is there room also for there being an actual increase beyond those variables? I would say probably yes, because some of the things that are associated with autism, the nongenetic factors, those are also changing,” Tager-Flusberg said.
“Over time we have come to the understanding that the biology of this disorder is far more complex than we might have ever imagined.”
This article originally appeared in The New York Times.
Written by: Emily Baumgaertner Nunn and Azeen Ghorayshi
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