What Causes Autism? A Plain-Language Guide to the Risk Factors

Why there is no single cause

Autism is not caused by one thing. It is a neurodevelopmental condition, meaning a difference in how the brain develops. It comes from many factors working together, and most of them act while the baby's brain is forming in the womb, long before birth.

A simple way to picture it: genes set up the likelihood, and things in the womb sometimes add to it. For many autistic people, genes alone are enough. The environment adds relatively little.

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1. Genes: the biggest factor

What the evidence shows:

• Studies of twins show how much genes matter. When one identical twin is autistic, the other is too in 60 to 90% of pairs. For non-identical twins, who share fewer genes, it is 0 to 30%. That gap tells us genes matter a great deal.
• Across the major reviews, genes explain roughly 50 to 80% of the differences in who becomes autistic. Researchers call this share heritability.
• More than 200 genes have been found that are linked to a higher chance of autism.
• No single gene causes autism. Many genes each add a small amount.
• Some children have a rare gene change with a big effect. These changes happen by chance in the child. They are not passed down from the parents.

What this means for families: If you have one autistic child, the chance that a second child is autistic is about 10 to 20%. Across the whole population it is roughly 1 to 2%. This reflects genes the family shares, not parenting or environment.

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2. Parents' age

Many large reviews that pool studies together agree on this: older mothers and older fathers each, on their own, have a higher chance of having an autistic child.

• A 2024 review that pooled 41 studies looked at parents older than average. For mothers, the odds of an autistic child were 47% higher. For fathers, they were 51% higher. (Odds are one way researchers measure how much more likely something is.)
• Each extra 10 years of a mother's age went with an 18% higher risk. For fathers it was 21%.
• The likely reason: as parents age, more chance gene changes build up in sperm and eggs. These changes can happen with no family history of autism at all.

Important note: These numbers describe large groups of people. For any one family, the actual chance stays low whatever the parents' age.

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3. Things in the womb

A large 2024 review in BMC Medicine brought together the evidence on many risk factors during pregnancy. These have the most consistent support:

Infections during pregnancy

An infection during pregnancy, especially a viral one in the first three months, is one of the more solid findings. The idea is that the mother's immune system, busy fighting the infection, can disrupt how the baby's brain develops.

Diabetes in pregnancy and the mother's weight

Diabetes that starts in pregnancy (gestational diabetes) and obesity in the mother are both established risk factors, with consistent evidence across studies. The likely reason is inflammation and disrupted metabolism, the way the body handles energy, during a key window for the baby's brain.

Medicines in pregnancy

• Valproate (a medicine for epilepsy): one of the strongest environmental risk factors found. Taking it in pregnancy is linked to a clearly higher chance of autism. Women who could become pregnant are advised to talk with their doctor about alternatives.
• SSRIs (a common type of antidepressant): the link has been studied, but the results are mixed. Much of the apparent risk may come from the mother's depression itself rather than the medicine.

Air pollution

The evidence is growing, and it is consistent. Breathing polluted air in pregnancy, especially the tiniest particles of dust and soot, is linked to a higher chance of autism:

• A 2025 study that followed children across all of Denmark found that pollution in pregnancy went with a higher chance of autism.
• A 2025 study in California found risks from several pollutants together, shaped by social and economic factors.
• Reviews that pool studies suggest these tiny particles in pregnancy are linked to up to 15% higher autism risk.
• The likely reason is wear on cells and inflammation in the baby's developing brain.

Pesticides

A study of nearly 3,000 children found a higher chance of autism linked to farm pesticides in pregnancy, including one called chlorpyrifos. Reviews that pool studies support this link. The risk seems concentrated in areas where farms use a lot of pesticide.

Heavy metals and hormone-disrupting chemicals

Some chemicals interfere with the body's hormones, including one called BPA. Exposure in the womb to these, or to lead or mercury, has been linked to a higher chance of differences in brain development, including autism. The evidence is still building.

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4. Around birth

Being born early (preterm) is consistently linked to a higher chance of autism. A 2025 review that pooled studies of whole populations confirmed this across several countries. The likely reasons include too little oxygen, inflammation, and disruption to brain growth that normally happens in the last three months of pregnancy.

Other birth factors with some supporting evidence:

• Low birth weight
• Complications at birth where the baby showed signs of distress
• Cesarean section (some studies find a link; the reason is unclear, and it may reflect the mother's own health)

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5. What does NOT cause autism

• Vaccines. The 1998 study that claimed a link was withdrawn by its journal. Its lead author lost his medical license for data fraud. Dozens of large studies involving millions of children have found no link. This is settled.
• Parenting style. The "refrigerator mother" theory from the 1950s blamed cold mothers. It has been thrown out and has no scientific basis whatsoever.
• Screen time. Screens do not cause autism. Autistic children are drawn to screens, not the other way around.
• Sugar. Sugar does not cause autism or make its main traits worse. Those are the differences in social communication and repeated behaviors. A lot of sugar may make other difficulties that often come alongside autism worse.

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Autism is not a disease caused by something that went wrong. In most cases it reflects a way of developing that is shaped mainly by genes. The environment plays a smaller part that can nudge things, mostly before birth.

The factors above are patterns of risk seen across large groups of people. They do not explain any one child's autism, and they carry no suggestion that anyone is at fault. The vast majority of autistic children were born to parents with no known risk factors at all.

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