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

Creator: Bhavin Solanki
Published: 2026-07-03

Autism does not have a single cause. It emerges from a complex interaction between genetic predisposition and environmental influences — mostly acting before birth. Genetics is the dominant driver, accounting for roughly half of the risk, but prenatal exposures, parental age, birth complications, and environmental toxins each contribute. None of these factors act in isolation.

Published 3 July 2026 · 6 min read

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At a glance

Key outcome: ASD arises from gene–environment interactions; heritability is estimated at ~50–80%; over 200 susceptibility genes identified; prenatal environmental exposures (air pollution, maternal health, toxins, parental age) contribute meaningfully but none are deterministic
Methodology: Multiple systematic reviews and meta-analyses; large cohort studies; umbrella reviews of genetic and environmental risk; 2024–2025 BMC Medicine comprehensive review of prenatal factors
Key finding: No single cause — genetics sets the predisposition; the environment, largely prenatal, shapes whether and how it expresses
Institutes: PMC/NCBI, BMC Medicine/Springer Nature, Journal of Clinical Psychiatry, Nature/Pediatric Research, Wiley/Autism Research, ScienceDirect

Limitations:

Most environmental risk studies are observational — they show association, not proof of cause
Heritability estimates vary widely across studies (50–90%) depending on methodology
Gene–environment interactions are poorly understood — we know both matter but not precisely how they interact
Many studies are conducted in specific populations and may not generalise
ASD is heterogeneous — what drives one person's autism may differ from another's

Why there is no single cause

Autism spectrum disorder is not caused by one thing. It is a neurodevelopmental condition that emerges from the interaction of many factors — most of which act during prenatal brain development, long before birth.

The clearest way to understand this: genetics loads the gun, and the prenatal environment (sometimes) pulls the trigger. But for many autistic people, genetics alone is sufficient — the environment adds relatively little.

1. Genetics — the dominant factor

What the evidence shows:

Twin studies find concordance rates of 60–90% in identical twins and 0–30% in fraternal twins — the gap tells us genes matter enormously
Heritability is estimated at approximately 50–80% across major reviews
More than 200 susceptibility genes have been identified and linked to ASD
No single gene causes autism — it is polygenic, meaning many genes each contribute a small amount
Some cases involve rare, high-impact mutations (de novo variants) that arise spontaneously and are not inherited from parents

What this means for families: Having one autistic child raises the probability of a second autistic child to approximately 10–20%, compared to roughly 1–2% in the general population. This reflects shared genetic background, not parenting or environment.

2. Parental age

A consistent finding across multiple meta-analyses: both advanced maternal and paternal age increase autism risk independently.

A 2024 meta-analysis of 41 studies found maternal age above average was associated with 47% increased odds; paternal age with 51% increased odds
Each additional 10 years of maternal age was associated with an 18% higher risk; paternal age with 21% higher risk
The mechanism is thought to involve accumulation of de novo (spontaneous, non-inherited) genetic mutations in sperm and eggs as parents age — these mutations do not require a family history of autism

Important note: These are population-level statistics. The absolute risk for any individual family remains low regardless of parental age.

3. Prenatal environmental exposures

A comprehensive 2024 review in BMC Medicine synthesised evidence across multiple prenatal environmental risk factors. The most consistently supported include:

Maternal infections during pregnancy

Immune activation during pregnancy — particularly from viral infections in the first trimester — is one of the more robust environmental findings. The proposed mechanism involves maternal immune activation disrupting fetal brain development.

Gestational diabetes and maternal obesity

Both are established risk factors with consistent evidence across studies. The mechanism likely involves inflammatory pathways and metabolic disruption during a critical window of fetal brain development.

Medication exposures

Valproate (an anti-epileptic medication): one of the strongest environmental risk factors identified — prenatal exposure is associated with significantly elevated autism risk. Women of childbearing age are advised to discuss alternatives with their doctor
SSRIs (antidepressants): association has been studied but the evidence is inconsistent; much of the apparent risk may reflect the underlying maternal depression rather than the medication itself

Air pollution

Growing and consistent evidence links prenatal exposure to air pollutants — particularly fine particulate matter (PM2.5) — with elevated autism risk:

A 2025 Danish nationwide cohort study found prenatal air pollution exposure was associated with increased ASD risk
A 2025 California study found multipollutant risks with sociodemographic influences
Meta-analyses suggest PM2.5 exposure during pregnancy is associated with up to 15% increased autism risk
The mechanism involves oxidative stress and neuroinflammation during fetal brain development

Pesticide exposure

A study of nearly 3,000 children found increased ASD risk associated with prenatal exposure to agricultural pesticides, including chlorpyrifos. Meta-analyses support this association. Risk appears concentrated in areas of high agricultural pesticide use.

Heavy metals and endocrine disruptors

Prenatal exposure to lead, mercury, and endocrine-disrupting chemicals (including bisphenol A / BPA) has been associated with increased neurodevelopmental risk including ASD, though evidence is still accumulating.

4. Birth and perinatal factors

Preterm birth is consistently associated with elevated ASD risk — a 2025 systematic review and meta-analysis of population-based studies confirmed this across multiple countries. The proposed mechanisms include hypoxia (oxygen deprivation), inflammation, and disruption to late-stage brain development that occurs in the third trimester.

Other birth factors with supporting evidence:

Low birth weight
Birth complications involving foetal distress
Caesarean section (association found in some studies, mechanism unclear, may reflect underlying maternal health factors)

5. What does NOT cause autism

Vaccines. The original 1998 study claiming a link was retracted, the lead author lost his medical licence for data fraud, and dozens of large-scale studies involving millions of children have found no link. This is settled.
Parenting style. The discredited "refrigerator mother" theory from the 1950s has no scientific basis whatsoever.
Screen time. Does not cause autism — autistic children are drawn to screens, not the other way around.
Sugar. Does not cause or worsen core autism traits, though high sugar intake may worsen co-occurring difficulties.

The bigger picture

Autism is not a disease caused by something that went wrong. In most cases, it reflects a neurodevelopmental profile shaped primarily by genetics, with environmental factors playing a secondary, modulating role — mostly during prenatal development.

For caregivers: the factors above are population-level risk associations. They do not explain any individual child's autism, and they carry no implication of fault. The vast majority of autistic children were born to parents with no known risk factors at all.

Sources

Reviewed by Bhavin Solanki, caregiver parent. This is informational content, not medical advice — see our disclaimer.