Autism spectrum disorder (ASD) is rooted in atypical brain development that begins very early in life, even in the womb. It affects the structure and sizes of some brain regions as well as how brain cells form, organise and communicate, resulting in too much or too little signalling between them.
According to the Centres for Disease Control and Prevention, ASD now affects 32 per 1,000 eight-year-old children in America. Studies from the 1960s estimated prevalence at around two to four per 10,000. Much of the apparent increase is an artefact of the widening definition of autism over recent decades, increased awareness and improved and earlier detection.
Heritability is estimated to be more than 80%. In a small number of cases, a single variant of a gene—a duplication of a DNA stretch or a mutation that disables the gene—can be sufficient to cause the condition. Children with mutations in these "high-impact" genes, such as SHANK3 and NLGN3, often have a plethora of other diagnoses too, from epilepsy to intellectual disability. These high-impact variants, which can be inherited or arise randomly in the sperm or egg, account for at most a fifth of all autism diagnoses.
Most autism is thought to derive from far more common genetic variants found widely in the general population. Each may only slightly increase a person's risk, but when a child inherits many from both parents the combined effect can cross the diagnostic threshold. Thomas Bourgeron, a geneticist at the Institut Pasteur in Paris, describes this as two people, each carrying some autism-linked variants and perhaps showing some autism-like traits, "pooling" variants in their children.
High-impact variants are often found in genes involved with how neurons send messages to each other. Some directly code for proteins operating at the junctions connecting neurons; others regulate how and when those proteins are produced.
In a study of more than 46,000 Danes, Jakob Grove, a mathematician working in bioinformatics at Aarhus University, flagged genes such as KCNN2 and FEZF2 that are primarily active in the amygdala, the hippocampus and the neocortex.
Autism was first officially recognised in 1980 when it was included in the Diagnostic and Statistical Manual of Mental Disorders (DSM). The initial definition was narrow, focused on young children who seemed not to respond to social engagement. An update in 1994 created five categories: classic autism, Asperger's syndrome, childhood disintegrative disorder, pervasive developmental disorder not otherwise specified, and Rett's syndrome. These were ditched in 2013 and the condition was redefined as the present-day ASD, because the number and severity of symptoms overlapped between the groups and they did not usefully predict how an individual's condition would develop.
Researchers at Princeton University and the Flatiron Institute in New York, looking at genetic and behavioural data from more than 5,000 autistic Americans, showed in Nature Genetics in July 2025 that ASD can be broken into four subcategories, each with its own genetic profile affecting development:
An analysis by epidemiologists at the University of Wisconsin found that between 2000 and 2016 the number of autism diagnoses among children with mild, minimal or no adaptive impairments rose markedly, while moderate to severe cases remained mostly flat. One explanation is that an autism diagnosis unlocks care—and insurance coverage—for other conditions that might otherwise go untreated.
Applied Behaviour Analysis relies on reinforcement to encourage desirable behaviours and reduce disruptive ones. Early forms using shocks, restraints or unpleasant tastes have been discarded. In 2014 federal regulators clarified that Medicaid must cover treatment for autistic children; between 2019 and 2025 visits to ABA therapy centres rose from just under 7m to 28.5m, according to Trilliant Health, an analytics firm. The number of registered behaviour technicians, who administer ABA, jumped from 328 in 2014 to 246,000 in 2025. They are often paid modestly, with high turnover—especially disruptive for autistic children who depend on routine.
Private-equity firms accounted for 85% of all mergers and acquisitions in the ABA sector between 2017 and 2022, according to the Centre for Economic and Policy Research. A study by researchers at the Brown University School of Public Health identified 574 therapy centres owned by PE firms as of 2024.
ABA is often paid per hour, creating incentives for over-treatment. State Medicaid costs have exploded: North Carolina's rose from $121.7m in 2022 to an expected $1.1bn in 2027; Nebraska's from $4.6m in 2020 to $85.6m in 2024. In December 2024 the Department of Health and Human Services began auditing state Medicaid spending on ABA; its report on Colorado found $285m in questionable payments over two years.
The idea of environmental influences became tainted after Andrew Wakefield, a British doctor, claimed wrongly in 1998 that the measles, mumps and rubella (MMR) vaccine was a cause of autism.
One of the most well-supported environmental causes is exposure to valproate, an epilepsy medication, in the womb. Valproate is a known instigator of epigenetic changes. Higher parental age has been linked to a child's chance of being autistic; as people age, their sperm and egg cells accrue new mutations. A meta-analysis of 36 studies from 2021 found that mothers who experience a serious fever during pregnancy more often have autistic children, though a causal link has not been established.
Robert F. Kennedy Jr, America's health secretary, leader of the MAHA movement and himself a campaigner against vaccines, announced a $50m National Institutes of Health programme called the Autism Data Science Initiative (ADSI) to investigate environmental causes. Judith Miller, a psychologist at the Children's Hospital of Philadelphia, is leading one of the ADSI projects, tracking roughly 104,000 children born as early as 2008, of whom around 4,000 are autistic.
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