When Autism Has No Known Cause

Causes of autism remain mysterious

While autism is increasingly common, its cause is usually unknown. In a general way, researchers believe that there is a strong genetic component to autism and that there are environmental "triggers" that may cause certain individuals to develop symptoms; for any individual, however, the precise nature of the genetic and environmental triggers is unknown.

When autism is of known origin (caused by a known genetic anomaly or exposure), it is referred to as secondary autism. When autism is of unknown origin, it is called idiopathic autism.

Family in session with a child psychiatrist

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Known and Unknown Causes of Autism

While there are over a dozen established causes of autism, most are very rare genetic disorders or prenatal exposures. As a result, approximately 85% of autism cases are idiopathic.

In other words, in the vast majority of cases:

  • A child is born to parents who are not autistic
  • Autism is not a known part of the child's family history
  • The child was not premature
  • The parents were under 35 years old
  • Tests did not uncover genetic anomalies (such as Fragile X syndrome) that might cause autism in the child
  • The mother was not exposed to or taking any of the drugs known to increase the risk of autism while she was pregnant (rubella, valproic acid, and thalidomide are known to cause autism in unborn children)

Heredity, Genetics, and Autism Risk

Heredity does play a role in autism: having one child with autism does increase the likelihood that your next child may also be autistic. This is a concern to bear in mind when planning your family's future.

According to the National Human Genome Research Institute, "The risk that a brother or sister of an individual who has idiopathic autism will also develop autism is around 4% plus an additional 4% to 6% risk for a milder condition that includes language, social or behavioral symptoms. Brothers have a higher risk (about 7%) of developing autism, plus the additional 7% risk of milder autism spectrum symptoms, over sisters whose risk is only about 1% to 2% percent."

While we know that heredity plays a role in autism, we don't know exactly how or why. Dozens of genes seem to be involved in autism, and research is ongoing. There is no existing genetic test to determine whether a parent "carries" autism or whether a child (or fetus) is likely to develop autism.

Genetic mutation may also cause autism. Genetic mutation can occur for many different reasons and may or may not be related to parents' genetics. Genetic mutation occurs frequently but does not always result in physical or developmental challenges. 

Because we know so little (so far) about genetics and autism, it's rare that a diagnostician can draw a direct line between a particular genetic anomaly and a particular person's autism.

Theories About Environmental Exposures

There has been a rise in cases of autism, and theories about why abound. While there are certainly people who believe there is a connection between some of the following and autism, there is no solid evidence to support this:

  • Ultrasound used to monitor fetal growth
  • Vaccines given to young children
  • Cell phone usage among parents
  • Allergies to peanuts and gluten
  • Prevalence of Lyme disease

Autism presents itself differently in different people. This suggests a variety of causes and, perhaps, a variety of syndromes with some (but not all) symptoms in common.

A Word From Verywell

The reality is that, for most autism parents, there will never be a clear answer to the question "why did my child develop autism?" While this can be terribly frustrating, the good news is that causes don't really matter when it comes to taking action for your child's future. Whether your child's autism was the result of a genetic difference, a prenatal exposure, a mutation, or heredity, the same therapies and treatments are likely to be helpful. Rather than spending a great deal of time and money seeking reasons, in most situations the better route is to spend that time, money, and energy on helping your child reach their potential.

11 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control. Data and Statistics on Autism Spectrum Disorder.

  2. Casanova M, Casanova E, Frye R, et al. Editorial: Secondary vs. Idiopathic Autism. Front. Psychiatry, 14 April 2020 doi:10.3389/fpsyt.2020.00297

  3. National Human Genome Research Institute. About Autism.

  4. Krupp DR, Barnard RA, Duffourd Y, et al. Exonic Mosaic Mutations Contribute Risk for Autism Spectrum DisorderAm J Hum Genet. 2017;101(3):369-390. doi:10.1016/j.ajhg.2017.07.016

  5. Hodges H, Fealko C, Soares N. Autism spectrum disorder: definition, epidemiology, causes, and clinical evaluation. Transl Pediatr. 2020 Feb;9(Suppl 1):S55-S65. doi: 10.21037/tp.2019.09.09

  6. Centers for Disease Control. Screening and Diagnosis of Autism Spectrum Disorder.

  7. Rosman NP, Vassar R, Doros G, et al. Association of Prenatal Ultrasonography and Autism Spectrum Disorder. JAMA Pediatr. 2018 Apr 1;172(4):336-344. doi: 10.1001/jamapediatrics.2017.5634

  8. Centers for Disease Control. Autism and Vaccines.

  9. Davidovitch M, Shrem M, Golovaty N, Assaf N, Koren G. The role of cellular phone usage by parents in the increase in ASD occurrence: A hypothetical framework. Med Hypotheses. 2018 Aug;117:33-36. doi:10.1016/j.mehy.2018.06.007

  10. Hyman S, Stewart P, Foley J, et al. The gluten-free/casein-free diet: A double-blind challenge trial in children with autism. J Autism Dev Disord 46, 205–220 (2016). doi:10.1007/s10803-015-2564-9

  11. Ajamian M, Kosofsky BE, Wormser GP, Rajadhyaksha AM, Alaedini A. Serologic markers of Lyme disease in children with autismJAMA. 2013;309(17):1771–1773. doi:10.1001/jama.2013.618

By Lisa Jo Rudy
Lisa Jo Rudy, MDiv, is a writer, advocate, author, and consultant specializing in the field of autism.