Autism doesn’t have a single cause, but research consistently points to three major contributing factors: genetics, prenatal environmental exposures, and differences in early brain development. These three forces rarely act alone. In most cases, they overlap and amplify each other, which is why autism looks so different from one person to the next. Current CDC estimates identify about 1 in 31 children (3.2%) with autism spectrum disorder, making it one of the most common neurodevelopmental conditions.
Genetics: The Strongest Factor
Genetics is by far the largest piece of the puzzle. Autism is estimated to be 70 to 90% heritable, making it one of the most genetically influenced neurodevelopmental conditions known. That doesn’t mean a single “autism gene” exists. Instead, hundreds of genetic variations contribute, each nudging risk up by a small amount. These variations fall into two broad categories: inherited changes passed down from parents, and spontaneous (de novo) mutations that arise for the first time in the child.
Some genetic changes are small, involving a single letter of DNA code. Others are larger structural changes where whole chunks of genetic material are duplicated or deleted. In a minority of cases, one of these larger changes is powerful enough on its own to cause autism. More commonly, though, a person carries a collection of smaller-effect variants that individually wouldn’t do much but together push development past a tipping point.
Children with no family history of autism are more likely to carry spontaneous mutations than those who have an autistic sibling. This helps explain why autism sometimes appears “out of nowhere” in a family. Meanwhile, in families where autism does run in multiple members, the genetic architecture tends to involve many inherited common variants adding up over generations.
Prenatal Environmental Exposures
The prenatal environment, meaning everything a developing fetus is exposed to in the womb, is the second major factor. These exposures don’t cause autism on their own in most cases, but they can significantly raise risk, especially in a child who already carries genetic susceptibility.
Infections and Inflammation During Pregnancy
Maternal infections during pregnancy, including rubella, cytomegalovirus, and illnesses that cause prolonged fever, are linked to higher autism rates. The key mechanism appears to be inflammation. When the mother’s immune system mounts a strong inflammatory response, the signaling molecules it releases can cross the placenta and affect fetal brain development during critical windows.
Certain Medications
Valproic acid, an anti-seizure medication, carries one of the most well-documented prenatal risks. Thalidomide and possibly misoprostol have also been linked to increased rates. These medications can alter how genes are expressed in the developing brain, particularly by changing chemical tags on DNA that control which genes turn on or off.
Air Pollution and Toxins
Heavy exposure to fine particulate air pollution (tiny particles small enough to enter the bloodstream through the lungs) during pregnancy has been associated with autism risk. Pesticide exposure and certain toxic metals show similar patterns. Chemicals like bisphenol A and lead can alter the chemical markers on DNA that regulate gene activity, essentially changing how genes behave without changing the genes themselves.
Parental Age
Older parents face modestly higher odds. For fathers, risk increases steadily and linearly across the lifespan. For mothers, autism risk stays very low for births before age 30, then rises more rapidly with each additional year. Even so, the absolute risk remains relatively small: less than 2 in 100 even for mothers up to age 45. The likely explanation is that older parents accumulate more spontaneous genetic mutations in their egg and sperm cells over time.
Premature Birth and Low Birth Weight
Babies born preterm face roughly 3.3 times the odds of an autism diagnosis compared to the general population, and risk climbs as gestational age drops. A large Swedish study broke this down clearly: autism prevalence was 6.1% for extremely preterm babies (born at 22 to 27 weeks), 2.6% for very to moderately preterm (28 to 33 weeks), and 1.4% for full-term births (39 to 41 weeks). Prematurity likely contributes through disrupted brain development during the third trimester, when critical neural wiring normally takes place.
Differences in Early Brain Development
The third major factor involves how the brain physically develops in early childhood. One of the most striking findings is that children with autism have significantly more synapses, the connection points between brain cells, than their peers. This surplus comes from a slowdown in a normal process called synaptic pruning.
In typical development, the brain massively overproduces synapses in early childhood, then trims roughly half of them by late childhood to create efficient, streamlined neural circuits. Research at Columbia University examined brain tissue from children with and without autism and found that by late childhood, synapse density had dropped by about 50% in neurotypical brains but only by about 16% in autistic brains. The result is an excess of connections that can make neural signaling noisier and less organized.
The researchers traced this pruning failure to a specific cellular recycling system. Brain cells normally break down their own old and damaged components through a self-cleaning process. In the autistic brains studied, this recycling system was severely impaired, leaving cells cluttered with worn-out parts. A protein that regulates cell growth was found to be overactive, effectively shutting down the brain’s ability to clean house and trim unnecessary connections. In mouse models, correcting this overactivity restored normal pruning and reversed autism-like behaviors.
This brain overgrowth pattern helps explain some of the sensory and information-processing differences that characterize autism. Too many synapses can mean the brain has difficulty filtering and prioritizing incoming signals, which may contribute to sensory sensitivity, intense focus on details, and differences in social processing.
How These Three Factors Work Together
The most accurate way to think about autism’s causes is as a threshold model. Everyone carries some genetic variants that influence brain development. Most people don’t carry enough of them, or encounter enough environmental triggers, to cross the threshold into autism. But when genetic susceptibility is high and prenatal exposures add further pressure, the likelihood increases substantially.
Environmental exposures can directly influence the expression of neurodevelopmental genes. Valproic acid, for example, changes chemical tags on the proteins that package DNA, altering which genes are active during brain development. Air pollutants and toxic metals do something similar by shifting DNA methylation patterns. These are not changes to the genetic code itself but changes to how that code is read, and they can be just as consequential.
This interplay also works in the other direction. A child’s genotype can determine how vulnerable they are to a given environmental exposure. Two children exposed to the same level of air pollution may respond very differently depending on the genetic variants they carry for detoxification enzymes and neural development. This is why the same environmental risk factor produces autism in some children and not others.
The practical takeaway is that autism rarely has one explanation. For some individuals, genetics alone is powerful enough, particularly those with a single high-impact mutation. For others, a combination of moderate genetic risk and specific prenatal exposures tips the balance. And in all cases, the downstream effect plays out through the physical architecture of the developing brain, where synaptic pruning, connectivity patterns, and neural circuit formation determine how the condition ultimately manifests.