Autism spectrum disorder (ASD) has no single cause. It arises from a combination of genetic predisposition, prenatal environmental exposures, and factors during birth, with genetics carrying the largest share of risk. About 1 in 31 children in the United States is now identified with ASD, based on CDC surveillance data from 2022.
Genetics Play the Largest Role
Twin studies provide the strongest evidence for a genetic foundation. When one identical twin has autism, the other twin also has it in 60 to 90 percent of cases. For fraternal twins and non-twin siblings, that rate drops to about 20 percent. The gap between those numbers points to a powerful genetic component, since identical twins share nearly all their DNA while fraternal twins share roughly half.
Despite this strong heritability, a specific genetic cause can be identified in only about 20 percent of cases. Those known causes break down into three main categories: chromosomal abnormalities account for up to 5 percent, single-gene mutations for another 5 percent, and copy number variants (small deletions or duplications of DNA segments) for up to 11 percent. The remaining 80 percent of cases likely involve combinations of common gene variations that individually have small effects but together push development in a different direction. Many very rare mutations also remain undiscovered.
What Happens in the Developing Brain
One of the clearest biological patterns in autism involves early brain growth. Children who go on to be diagnosed tend to have lower-than-expected brain weight at birth, followed by unusually rapid growth over the next three years. This overgrowth is most pronounced in the frontal, temporal, and cingulate regions, areas involved in social behavior, language, and attention.
Brain imaging of young children with autism also shows stronger-than-typical connections between neighboring brain areas but weaker long-distance connections. Think of it as a city with excellent local roads but limited highways. This wiring pattern may help explain why some individuals with autism process details intensely while finding it harder to integrate information across different contexts. Researchers believe that the brain’s normal process of pruning unused connections, which typically streamlines neural circuits during childhood, is insufficient in autism, leaving an excess of synapses that can create neural “noise.”
Parental Age
Both maternal and paternal age at conception are independent risk factors. Mothers aged 35 or older have roughly 1.3 times the odds of having a child with autism compared to mothers aged 25 to 29. Fathers aged 40 or older carry about 1.4 times the odds compared to the same reference group. These are modest increases on their own, but they compound. Firstborn children of two older parents (mother 35-plus, father 40-plus) are about three times more likely to develop autism than third-or-later-born children of younger parents.
The likely explanation is that older parents accumulate more spontaneous mutations in their egg and sperm cells over time. Sperm cells in particular divide continuously throughout a man’s life, and each division introduces a small chance of copying errors in DNA.
Prenatal Exposures
Certain medications and environmental exposures during pregnancy are linked to increased risk. The most well-documented is valproate, an anti-seizure medication. Children exposed to valproate in the womb have roughly three times the risk of an autism spectrum diagnosis and five times the risk of a childhood autism diagnosis specifically. This elevated risk persists even when researchers account for the mother’s own neurological and psychiatric conditions, confirming that the drug itself, not the underlying illness, drives the association. The risk is highest when valproate is taken around conception, though it remains significantly elevated even if the medication is stopped six months before pregnancy. Other anti-seizure medications do not show the same pattern.
Air pollution, pesticides, and certain industrial chemicals have also been studied as potential contributors, though their individual effects are harder to isolate. The broader mechanism connecting these exposures involves epigenetics, the biological system that turns genes on or off without changing the DNA itself. Environmental factors like chemicals, diet, and drugs can alter methylation patterns, a process where small molecules attach to DNA and silence or activate specific genes. These epigenetic changes can shift how a genetically predisposed brain develops, essentially acting as a dimmer switch on genes that influence neural wiring.
Complications During Birth
Preterm birth, particularly late-preterm delivery (between 34 and 36 weeks), is associated with higher rates of neurodevelopmental differences including autism. These babies are more vulnerable to oxygen deprivation, low blood sugar, and metabolic disturbances during delivery, all of which can compromise nutrient and oxygen delivery to the developing brain during a critical window of growth. Even early-term births (37 to 38 weeks) carry a slightly elevated risk of neonatal complications and intensive care admissions compared to full-term births at 39 weeks or later.
It is worth noting that birth complications alone rarely “cause” autism. In most cases, they add incremental risk on top of existing genetic vulnerability. A child with a strong genetic predisposition may be tipped toward a diagnosis by a difficult birth, while the same birth complications in a child without that predisposition would not produce the same outcome.
Vaccines Do Not Cause Autism
This is one of the most thoroughly investigated questions in modern medicine, and the answer is unambiguous. At least 16 large, well-designed population-based studies conducted by independent research teams across multiple countries have examined whether the MMR vaccine, the preservative thimerosal, or the number of vaccines given at one time are associated with autism. None found any relationship. The original 1998 paper that sparked the concern was retracted for fraudulent data, and its author lost his medical license. The timing of autism diagnoses often coincides with the childhood vaccination schedule simply because both occur in the same developmental window, not because one causes the other.
Why Most Cases Have No Single Explanation
For the majority of families, there is no one identifiable cause. Autism most often results from a mosaic of common genetic variants, each contributing a small amount of risk, interacting with prenatal conditions and environmental exposures in ways that are difficult to trace backward after the fact. Two children with the same diagnosis can have entirely different underlying risk profiles: one may carry a rare chromosomal deletion, while another may have dozens of common gene variants that converged with an older father and a stressful pregnancy.
This complexity is part of why autism presents so differently from person to person. The spectrum is not just a range of severity. It reflects genuinely different biological pathways arriving at overlapping behavioral patterns, which is why no single treatment works for everyone and why the search for causes remains one of the most active areas in developmental neuroscience.