Autism doesn’t have a single cause. It develops from a combination of genetic factors, prenatal influences, and differences in early brain development that together shape how the brain processes information. Genetics account for the largest share of risk, with heritability estimated at roughly 83%, meaning that the vast majority of what determines whether someone develops autism comes down to their genetic makeup. The remaining risk traces back to environmental factors, most of which act before birth.
Genetics Play the Largest Role
Twin studies provide some of the clearest evidence for how strongly genetics influence autism. When one identical twin has autism, the other is far more likely to also have it compared to fraternal twins, who share only half their DNA. Using data from twins, full siblings, and half-siblings, researchers have placed heritability at about 87% in twin-specific analyses, with earlier studies estimating it as high as 90%.
But “genetic” doesn’t simply mean “inherited from a parent.” Some of the most impactful genetic changes are spontaneous mutations that appear for the first time in the child and aren’t found in either parent’s DNA. These are called de novo mutations, and they can carry substantial individual risk. One type of spontaneous mutation in the coding regions of genes confers about five times more risk than certain inherited variants. Loss-of-function mutations, which disable a gene’s ability to produce a working protein, show up in about 20% of autistic individuals compared to only 10% of their unaffected siblings.
There is no single “autism gene.” Researchers have identified 93 significant genetic markers linked to autism risk through large-scale genome studies, with 53 confirmed in independent groups of participants. Hundreds of genes likely contribute, each adding a small amount of risk that accumulates differently in each person. This is a big part of why autism varies so widely from one individual to the next.
What Happens Differently in the Developing Brain
One of the most concrete biological differences in autism involves how the brain refines its connections during early childhood. In typical development, the brain massively overproduces synapses (the connection points between neurons) during the first two years of life, then aggressively prunes them back. By late childhood, the density of these connections drops by about 50%. This pruning is essential: it sharpens the brain’s ability to distinguish meaningful signals from background noise.
In autistic brains, this pruning process is significantly reduced. Connection density drops by only about 16% instead of 50%, leaving a large number of redundant pathways intact. The result is a brain with far more connections, but less efficient filtering. This helps explain many core features of autism, from heightened sensory sensitivity to difficulty processing complex social cues in real time. When the brain can’t easily separate signal from noise, everyday environments become more intense and harder to navigate.
The cells responsible for pruning are microglia, a type of immune cell in the brain that identifies and removes weak or unnecessary synapses. When this process is disrupted, the balance between excitatory and inhibitory signaling in the brain shifts, which researchers believe is a key mechanism underlying many neurodevelopmental differences in autism. The dramatic increase in dendritic growth during the first year of life, paired with reduced pruning, creates a fundamentally different wiring pattern.
Prenatal Environment and Risk Factors
While genetics set the foundation, conditions during pregnancy can shift the odds. Infection during pregnancy, gestational diabetes, and maternal obesity are all established risk factors for autism. The biological pathways connecting these factors to brain development aren’t fully mapped, but immune system disruption, oxidative stress, and changes to the gut microbiome are leading candidates.
Air pollution exposure during pregnancy has also been linked to increased autism risk in epidemiological studies. Researchers are additionally investigating the effects of certain medications taken during pregnancy, including some antidepressants and antibiotics, though the evidence for these is still emerging and less definitive than for infection or metabolic conditions.
One protective factor stands out clearly. Folic acid supplementation during early pregnancy is associated with a 43% lower risk of autism in offspring. Getting at least 400 micrograms daily from food and supplements was linked to a 45% reduction in risk. This is one of the more actionable findings in autism research, and it aligns with existing recommendations for prenatal vitamins.
Parental Age Matters
Advanced paternal age is a well-documented risk factor. Fathers aged 50 or older are 2.2 times more likely to have an autistic child compared to fathers aged 29 or younger, even after accounting for the mother’s age and other known risk factors. This likely reflects the accumulation of spontaneous mutations in sperm over time. Sperm cells divide continuously throughout a man’s life, and each division introduces a small chance of new mutations. By the time a man reaches his 50s, his sperm carry significantly more de novo mutations than they did in his 20s.
Epigenetics: Changes Beyond the DNA Code
Your DNA sequence isn’t the only thing that matters. How genes are read and expressed can change without altering the underlying code, through a process called epigenetics. The best-studied mechanism involves chemical tags (methyl groups) that attach to DNA and dial gene activity up or down. When these tags land in the wrong places, they can silence genes that should be active or activate ones that should be quiet.
In the context of autism, abnormal patterns of these chemical tags have been found on genes involved in brain development. These changes can affect how proteins are assembled from genetic instructions, sometimes producing altered versions of important brain proteins. One particularly interesting aspect is timing: an epigenetic change acquired during a critical window of fetal development may sit dormant and only produce effects later, when the affected genes are called into action during a specific phase of brain maturation. This could explain why autism becomes apparent at certain developmental stages rather than being immediately obvious at birth. Researchers believe epigenetic changes may not drive autism on their own but instead modify how strongly genetic risk factors express themselves.
Why Males Are Diagnosed More Often
Current CDC data shows autism prevalence at about 1 in 31 children aged 8, and males are diagnosed roughly three to four times more often than females. Two competing but compatible theories explain this gap. The first focuses on something inherent to male biology, possibly the influence of fetal testosterone on brain development, that may push the brain toward patterns associated with autism.
The second theory proposes a “female protective effect,” suggesting that girls require a greater genetic and environmental burden to develop autism than boys do. Research across two nationally representative samples supports this: families of autistic girls tend to carry a higher overall load of autism-related genetic variants than families of autistic boys. In other words, it takes more genetic risk factors to push a female past the threshold into clinical autism, which means fewer girls cross that line. This also implies that when girls do develop autism, they may carry more extensive underlying genetic differences, even if their outward presentation looks similar to or milder than that of autistic boys.
Vaccines Do Not Cause Autism
The World Health Organization reviewed 31 primary research studies published between 2010 and 2025, covering data from multiple countries, and reaffirmed that childhood vaccines do not cause autism. This includes vaccines containing thimerosal (a mercury-based preservative) and vaccines with aluminum-based ingredients. A large cohort study analyzing nationwide registry data from every child born in Denmark between 1997 and 2018 found no association. The WHO has maintained this position consistently since 2002, with reaffirmations in 2004, 2012, and again in 2025. The original study that claimed a link was retracted and its author lost his medical license due to ethical violations and data manipulation.