What Causes Autism? What the Evidence Really Shows

What causes autism? There is no single cause. Genes play the biggest known role: studies of millions of families estimate that around 80% of the differences in who is autistic are due to inherited genetic factors. Some events before or around birth add smaller risks. Vaccines do not cause autism, and neither does parenting [1][2].

What causes autism? The short answer

Autism is a neurodevelopmental condition, meaning it is linked to how the brain develops from early life [3]. So when parents ask what causes autism, scientists look mainly at genes and at influences during pregnancy and early development. The American Academy of Pediatrics (AAP) explains that research points to multiple influences, much of them genetic, plus some environmental factors that are still being studied [2]. In most autistic children, no single identifiable cause is found [2].

What causes autism most often? Genes

Evidence from twins and families

Identical twins share all their genes, while non-identical twins share about half. A meta-analysis of twin studies found autism is far more often shared by identical twins, with heritability estimates of 64% to 91% [4]. A study of two million people in five countries estimated heritability at about 80% [1].

Autism also runs in families. In a large international study, about 20% of babies with an older autistic sibling were autistic by age 3 [5].

Many genes, many routes

There is no single “autism gene.” Large studies have found both common genetic variants, each with a tiny effect [6], and rarer changes with bigger effects. One study of more than 42,000 autistic people identified 60 genes strongly linked to autism [7]. Some rare genetic changes are new (not inherited from either parent), which is one reason autism can appear in families with no history of it.

Autism is also more common in some genetic conditions, such as fragile X syndrome and tuberous sclerosis complex [2]. That is why the AAP recommends offering genetic testing after diagnosis [8].

Factors before and around birth

Reviews of the research have found that some factors are linked to a somewhat higher chance of autism [9][10]:

• Older parents, especially a mother aged 35 or over or an older father.
• Being born very early (preterm) [2], and some birth complications involving low oxygen.
• Some health conditions in pregnancy, such as high blood pressure, pre-eclampsia, or being overweight.

It is important to understand what “linked” means here. These factors raise the chance only modestly, most children exposed to them are not autistic, and some links may reflect shared genes rather than direct causes [9].

Valproate

One medicine has a clearer link. In a Danish study of more than 650,000 children, those exposed to the epilepsy medicine valproate in pregnancy had an absolute risk of autism of about 4.4%, compared with about 1.5% overall [11]. Anyone taking valproate who could become pregnant should talk to their doctor before making changes, and should not stop it suddenly.

Vaccines do not cause autism

The idea that vaccines cause autism began with a 1998 paper that was later retracted and found by the BMJ to be fraudulent [12]. Since then, the question has been studied extensively:

• A meta-analysis of studies including more than 1.2 million children found no link between vaccines, the MMR vaccine, thimerosal, or mercury and autism [13].
• A Danish study of 657,461 children found MMR did not increase autism risk, including in children with an autistic sibling [14].
• A US study of children with older siblings found MMR was not linked to autism, even in families where an older sibling was autistic [15].
• A Cochrane review found no association between MMR and autism [16].
• A 2025 Danish study of aluminum in childhood vaccines found no increase in autism or other neurodevelopmental conditions [17].

In November 2025, a CDC webpage on autism and vaccines was changed in ways that the AAP describes as misleading. The AAP’s position remains that decades of high-quality, large-scale studies show vaccines do not cause autism, and that autistic children should be vaccinated on the recommended schedule [2][18][8].

Acetaminophen (Tylenol) in pregnancy

Some studies have reported a small association between acetaminophen use in pregnancy and autism. The strongest test is to compare brothers and sisters, which accounts for shared genes and family factors. A Swedish study of nearly 2.5 million children found a tiny association in the overall group that disappeared completely when siblings were compared, suggesting the earlier link was due to family factors, not the medicine [19]. A 2026 umbrella review concluded that the evidence does not support a robust causal relationship, and that acetaminophen remains an appropriate first-line treatment in pregnancy when needed [20]. The AAP states that studies do not show a causal link [2].

As with any medicine in pregnancy, use it only when needed, at the right dose, and ask your doctor or midwife if you are unsure.

Why are more children diagnosed now?

Rising numbers lead many parents to wonder whether what causes autism has changed.

About 1 in 31 US eight-year-olds was identified as autistic in 2022 [21]. The AAP explains that the rise in diagnoses over recent decades reflects greater awareness, more screening, earlier identification, more school services, and a broader definition of autism that now includes children who were once given other labels or none [2]. This does not rule out other influences, but it explains much of the change.

What you can do

Knowing what causes autism, and what does not, can ease worry and help you make good decisions for your family.

• Let go of blame. Autism is not caused by parenting, and the main known causes are outside anyone’s control.
• Ask about genetic testing. It can sometimes identify a specific cause, guide health checks, and inform family planning. A genetic counselor can help [8].
• Watch younger siblings’ development closely and ask for early screening [5].
• Be wary of tests or treatments that claim to find and remove a “cause,” such as hair-metal tests or chelation. The AAP says there is no evidence for routine testing of hair, blood, or urine for toxins or heavy metals, other than standard lead screening [8]. See unproven autism treatments.
• Keep vaccinations up to date for your autistic child and their siblings.

To learn more about the condition itself, read what is autism.

When to talk to a professional

Talk to your child’s doctor or a genetic counselor if you have questions about family history, testing, or future pregnancies. If you are pregnant or planning a pregnancy and take regular medicines, especially for epilepsy or mood, discuss them with your doctor before making any changes.

Frequently asked questions

Is autism genetic?

Largely, yes. Twin and family studies estimate that around 80% of the variation in who is autistic is due to inherited genetic factors. Many genes are involved, each usually adding a small amount, and some children have a single rare genetic change with a larger effect. Genes are not the whole story, and some factors around pregnancy and birth also play a smaller role.

Can vaccines cause autism?

No. Large studies involving millions of children in several countries, along with systematic reviews, have found no link between vaccines, including MMR, and autism. The 1998 paper that started the concern was retracted and judged fraudulent. The AAP continues to state that vaccines do not cause autism and recommends autistic children receive all routine vaccines.

Did something I did in pregnancy cause my child’s autism?

Almost certainly not. Most autism is linked to genes. A few pregnancy factors, such as preterm birth or certain medicines like valproate, are associated with a higher chance, but most children exposed are not autistic, and these are not things parents should blame themselves for. The best current evidence does not show that acetaminophen in pregnancy causes autism.

If I have one autistic child, will my next child be autistic?

The chance is higher than average but most siblings are not autistic. In a large international study, about 1 in 5 younger siblings of autistic children were diagnosed by age 3. The chance was higher for boys and for families with more than one autistic child. Genetic testing and counseling may give more personal information.

References

1. Bai D, Yip BHK, Windham GC, Sourander A, Francis R, Yoffe R, et al. Association of Genetic and Environmental Factors With Autism in a 5-Country Cohort. JAMA Psychiatry. 2019;76(10):1035-1043. Link
2. American Academy of Pediatrics Council on Children With Disabilities Autism Subcommittee. Understanding Autism: Information for Families. HealthyChildren.org. Last updated February 20, 2026. Link
3. Lord C, Brugha TS, Charman T, Cusack J, Dumas G, Frazier T, et al. Autism spectrum disorder. Nat Rev Dis Primers. 2020;6(1):5. Link
4. Tick B, Bolton P, Happé F, Rutter M, Rijsdijk F. Heritability of autism spectrum disorders: a meta-analysis of twin studies. J Child Psychol Psychiatry. 2016;57(5):585-595. Link
5. Ozonoff S, Young GS, Bradshaw J, Charman T, Chawarska K, Iverson JM, et al. Familial Recurrence of Autism: Updates From the Baby Siblings Research Consortium. Pediatrics. 2024;154(2):e2023065297. Link
6. Grove J, Ripke S, Als TD, Mattheisen M, Walters RK, Won H, et al. Identification of common genetic risk variants for autism spectrum disorder. Nat Genet. 2019;51(3):431-444. Link
7. Zhou X, Feliciano P, Shu C, Wang T, Astrovskaya I, Hall JB, et al. Integrating de novo and inherited variants in 42,607 autism cases identifies mutations in new moderate-risk genes. Nat Genet. 2022;54(9):1305-1319. Link
8. Hyman SL, Levy SE, Myers SM; Council on Children With Disabilities, Section on Developmental and Behavioral Pediatrics. Identification, Evaluation, and Management of Children With Autism Spectrum Disorder. Pediatrics. 2020;145(1):e20193447. Link
9. Modabbernia A, Velthorst E, Reichenberg A. Environmental risk factors for autism: an evidence-based review of systematic reviews and meta-analyses. Mol Autism. 2017;8:13. Link
10. Kim JY, Son MJ, Son CY, Radua J, Eisenhut M, Gressier F, et al. Environmental risk factors and biomarkers for autism spectrum disorder: an umbrella review of the evidence. Lancet Psychiatry. 2019;6(7):590-600. Link
11. Christensen J, Grønborg TK, Sørensen MJ, Schendel D, Parner ET, Pedersen LH, et al. Prenatal valproate exposure and risk of autism spectrum disorders and childhood autism. JAMA. 2013;309(16):1696-1703. Link
12. Godlee F, Smith J, Marcovitch H. Wakefield’s article linking MMR vaccine and autism was fraudulent. BMJ. 2011;342:c7452. Link
13. Taylor LE, Swerdfeger AL, Eslick GD. Vaccines are not associated with autism: an evidence-based meta-analysis of case-control and cohort studies. Vaccine. 2014;32(29):3623-3629. Link
14. Hviid A, Hansen JV, Frisch M, Melbye M. Measles, Mumps, Rubella Vaccination and Autism: A Nationwide Cohort Study. Ann Intern Med. 2019;170(8):513-520. Link
15. Jain A, Marshall J, Buikema A, Bancroft T, Kelly JP, Newschaffer CJ. Autism occurrence by MMR vaccine status among US children with older siblings with and without autism. JAMA. 2015;313(15):1534-1540. Link
16. Di Pietrantonj C, Rivetti A, Marchione P, Debalini MG, Demicheli V. Vaccines for measles, mumps, rubella, and varicella in children. Cochrane Database Syst Rev. 2020;4(4):CD004407. Link
17. Andersson NW, Bech Svalgaard I, Hoffmann SS, Hviid A. Aluminum-Adsorbed Vaccines and Chronic Diseases in Childhood: A Nationwide Cohort Study. Ann Intern Med. 2025;178(10):1369-1377. Link
18. American Academy of Pediatrics Section on Infectious Diseases. Vaccine Safety: Examine the Evidence. HealthyChildren.org. Last updated April 3, 2026. Link
19. Ahlqvist VH, Sjöqvist H, Dalman C, Karlsson H, Stephansson O, Johansson S, et al. Acetaminophen Use During Pregnancy and Children’s Risk of Autism, ADHD, and Intellectual Disability. JAMA. 2024;331(14):1205-1214. Link
20. Tayebi-Hillali H, Blanco-Suárez M, Rivas-Mundiña B, Limeres-Posse J, Diniz-Freitas M, Diz-Dios P. Prenatal exposure to paracetamol and the risk of autism and attention-deficit/hyperactivity disorder: An umbrella review and meta-meta-analysis. Dev Med Child Neurol. 2026 (online ahead of print). Link
21. Shaw KA, Williams S, Patrick ME, Valencia-Prado M, Durkin MS, Howerton EM, et al. Prevalence and Early Identification of Autism Spectrum Disorder Among Children Aged 4 and 8 Years – Autism and Developmental Disabilities Monitoring Network, 16 Sites, United States, 2022. MMWR Surveill Summ. 2025;74(2):1-22. Link

Professional review: this guide is waiting for review by a qualified clinician. It was written and checked against the published sources listed above. How we check our guides.

This guide is general education, not medical advice. Always talk to your child’s doctor or care team. Last checked: October 2026.