Autism is better known and diagnosed than ever before, leading to misconceptions that cases are skyrocketing.

When it came to her son's autism diagnosis, in some ways Shannon Des Roches Rosa feels she was lucky. What autism spectrum disorder (ASD) looks like can vary greatly between individuals, as well as between groups of people – girls' symptoms often differ from boys', for example. But as a young, white boy, her son, Leo, was part of a group with especially well-established diagnostic criteria.

And some of these criteria had been noticed by other people. As a toddler, Leo rarely made eye contact. In nursery school, he didn't respond to speech like the other kids. He frequently showed compulsive behaviours, like flapping his hands and chewing his clothes. He was easily stressed or overwhelmed. As a result, his diagnosis in 2003, at age two, was relatively straightforward.

But Rosa wasn't given much guidance about next steps. "I felt very lost," says Rosa, an editor and mother of three who lives in California. "I was really angry." Like so many other parents, what she wanted to know, above all, was why her child had autism. Online, she found other parents seeking the same answers.

Some believed their children had been injured by vaccines. Numerous studies on hundreds of thousands of children have consistently found no link between vaccinations and autism, and vaccinated children don't have higher autism rates than non-vaccinated children. But without a clearer understanding of why Leo was autistic, Rosa chose not to vaccinate her other children.

Over the years, Rosa realised she had been mistaken. Not only did Rosa eventually renounce her opposition to vaccination, she started a website, Thinking Person's Guide to Autism, to help others. "I have sympathy for people who believe this stuff because I've been there," she says. "But now I know that that was misinformation."

Rosa's story is an example of how, following an autism diagnosis, some families just want clear answers. Earlier this year, the US Health Secretary Robert F Kennedy Jr promised a "massive" research effort involving hundreds of scientists, to get to the bottom of what he has called an autism "epidemic" before September 2025.

But decades of research into autism have shown that its causes are complex – and mostly genetic. (Read more from the BBC about the genetic mystery of why some people develop autism.) Experts say the biggest reason for the rise in autism diagnoses, though, likely isn't these causes at all – it's a shift in how we screen for and diagnose the condition.

Autism is a neurodevelopmental disorder which often involves differences in thinking patterns, sensory processing, communication and social interactions. Autism prevalence is estimated at roughly 1% to 3% across the countries where it has been studied, although data is lacking in many low- and middle-income nations.

In places that have been tracking this data over decades, there has been a steady rise in autism diagnoses since the mid-20th Century. Based on health and education records, from 2000 to 2022, autism prevalence in the US increased from 1-in-150 to 1-in-31. Prevalence has also grown in Australia, Taiwan, and other countries.

"That can look alarming to people who don't know [the context of the statistics]", says Zoe Gross, the director of advocacy at the Autistic Self Advocacy Network (ASAN), a US nonprofit run by autistic people. It's not necessarily that autism cases are increasing, Gross emphasises: rather, diagnoses are on the rise.

As a recognised separate condition, autism is relatively new. It wasn't included in the Diagnostic and Statistical Manual of Mental Disorders (DSM) – a book with details on the symptoms and recommended treatment of hundreds of mental conditions........

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