What the Strongest Science Says About Autism Causes.

When families first hear the word autism, one of the most common questions that follows is: What causes it?

Over decades of research, scientists have learned a lot about this question. The short version: genetics is the main driver, while a smaller set of prenatal and perinatal factors can also contribute. Here’s what the evidence really shows.

Genetics: The Main Source of Autism Risk

Family and twin studies consistently show that autism is highly heritable. Large national registry studies, including one from Sweden, estimate that around 80–90% of the overall risk comes from genetic factors, with the best estimate near 83%.

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It’s important to understand that genetic risk isn’t a single thing. It comes from two main sources that add together:

1. Rare, High-Impact Variants

These are unusual genetic changes that can strongly affect brain development. Some arise de novo (new in the child, not inherited from parents), while others are rare variants passed down through families.

Large genetic studies have now identified more than 100 high-confidence autism risk genes, many of which are involved in synapse function, chromatin regulation, and early brain development.

2. Common Variants of Small Effect

Each person also carries thousands of common DNA differences. Individually, each has a tiny effect — but together they add up. This cumulative influence is called polygenic risk.

Genome-wide association studies (GWAS) show that these common variants explain a meaningful share of autism risk across the population.

How Rare and Common Variants Work Together

These two sources don’t compete — they combine. A well-known study demonstrated that even when a child has a rare, high-impact mutation, they also tend to carry higher-than-average polygenic risk.

This means that the broader genetic background can shape how a rare variant expresses itself and why autism looks different from person to person.

A Note on the June 2025 Genetics Study

A major international study published in June 2025 looked at how genetics relates to the wide range of traits seen across the autism spectrum.

The results showed that both rare variants and polygenic scores help explain differences in language, cognition, and other developmental features. Common and rare variations often act together, reinforcing the additive model mentioned above.

This helps explain why two people with the same diagnosis can have such different profiles: their individual mix of genetic influences differs.

Takeaway: Most autism risk comes from genetics, and both rare and common genetic variants contribute in an additive way.

Environmental and Perinatal Factors: What the Evidence Really Shows

Before we go further, let’s clear up one important point:

“Environmental factors” does not mean parenting.

The outdated “refrigerator mother” theory from the 1960s — which wrongly blamed cold or unloving parenting — has been completely disproven. Autism is not caused by anything a parent does.

In science, “environmental factors” means influences outside our DNA that can affect early development. Here are some of the best-supported examples:

• Parental age: Risk rises modestly with age, about 18% higher per 10 years for mothers and 21% higher per 10 years for fathers.
• Valproic acid (Depakote) use during pregnancy: This antiseizure and mood-stabilizing medication is linked to roughly a threefold increase in risk, or an absolute risk of around 4–5%. Clinical guidelines recommend avoiding it in pregnancy when possible.
• Very preterm birth or very low birth weight: Earlier gestational age and lower birth weight are both linked to higher autism odds, with the strongest risk seen in very preterm births.
• Serious maternal infections: Conditions like congenital rubella have been associated with increased risk, another reminder of why vaccination remains crucial.

Other factors, such as certain air pollutants and agricultural pesticides, are being studied. Current data suggest small effects (typically a 5–20% increase in odds) that vary by study design and may be influenced by confounding factors. These associations are real but modest.

Takeaway: Genetics explains most of autism’s overall risk. Environmental and perinatal factors can add smaller contributions, usually modest in size.

Practical Numbers Families Often Ask About

Here are some well-established figures that can help families understand relative risk:

• Sibling recurrence: About 20% of younger siblings of autistic children are diagnosed with autism by age 3. Risk is higher when more than one older sibling is autistic and somewhat higher for boys.
• Advanced parental age: Each 10-year increase in age raises risk by roughly 18% for mothers and 21% for fathers. In the oldest age groups, risk can be 40–50% higher than average.
• Valproate exposure during pregnancy: Absolute risk for autism is about 4.4% in exposed children, with a roughly threefold increase compared to unexposed peers.
• Very preterm birth: Autism risk rises with earlier gestational age, especially in births before 32 weeks.

Takeaway: The clearest and most actionable risk factors are sibling recurrence and valproate exposure. Parental age and prematurity also show consistent, moderate effects.

The Bottom Line

Autism arises primarily from genetic influences, with smaller contributions from specific prenatal and perinatal factors.

These influences combine additively, meaning no single gene or exposure “causes” autism on its own. Instead, many small and large factors interact to shape a child’s development.

This scientific understanding helps explain the tremendous diversity within the autism spectrum — and underscores that autism reflects biology, not blame.