Mental Health Neurodevelopmental Disorders Autism How Often Does Autism Have No Known Cause? By Lisa Jo Rudy Lisa Jo Rudy Facebook LinkedIn Twitter Lisa Jo Rudy, MDiv, is a writer, advocate, author, and consultant specializing in the field of autism. Learn about our editorial process Updated on May 11, 2023 Medically reviewed by Ripal Shah, MD, MPH Medically reviewed by Ripal Shah, MD, MPH Ripal Shah, MD, MPH, is a board-certified psychiatrist and clinical professor at Stanford, specializing in care for women and BIPOC communities. Learn about our Medical Expert Board Print While autism is increasingly common, its cause is usually unknown on an individual level. 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 the condition; 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. NoSystem images / Getty Images 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: Neither of a child's biological parents are not on the autism spectrum Autism is not a known part of the child's family history The child was not born prematurely The biological father was under 40 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 that is idiopathic may mean there is a genetic component; if so, the chances that a sibling may also be on the autism spectrum can be higher than someone who does not have a brother or sister on the autism spectrum. 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%, in addition to an already 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 are screening tests for phenotypic traits, but 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 have don't have all the details down about genetics and autism, a diagnostician can only attempt to 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 growthVaccines given to young children. This theory has been disproven in numerous studies.Cell phone usage among parentsAllergies to peanuts and glutenPrevalence 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. Centers for Disease Control. Data and Statistics on Autism Spectrum Disorder. Casanova M, Casanova E, Frye R, et al. Editorial: Secondary vs. Idiopathic Autism. Front. Psychiatry, 14 April 2020 doi:10.3389/fpsyt.2020.00297 National Human Genome Research Institute. About Autism. Krupp DR, Barnard RA, Duffourd Y, et al. Exonic Mosaic Mutations Contribute Risk for Autism Spectrum Disorder. Am J Hum Genet. 2017;101(3):369-390. doi:10.1016/j.ajhg.2017.07.016 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 Centers for Disease Control. Screening and Diagnosis of Autism Spectrum Disorder. 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 Centers for Disease Control. Autism and Vaccines. 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 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 Ajamian M, Kosofsky BE, Wormser GP, Rajadhyaksha AM, Alaedini A. Serologic markers of Lyme disease in children with autism. JAMA. 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. See Our Editorial Process Meet Our Medical Expert Board Share Feedback Was this page helpful? Thanks for your feedback! What is your feedback? Other Helpful Report an Error Submit