# What causes autism? What the research actually says > Genes, development before birth, and the myths that won't go away, explained calmly with sources. URL: https://theautismkit.com/guides/what-causes-autism By The Autism Kit team · Reviewed by [BCBA NAME], BCBA · Updated 2026-09-27 **Short answer:** There isn't one cause of autism. Research points mainly to genes, which shape how the brain develops before birth, sometimes together with factors during pregnancy. Twin and family studies suggest genes explain most of the likelihood. Parenting doesn't cause autism, and large studies of millions of children have found no link between vaccines and autism. **Key takeaways** - There's **no single cause**. Autism comes mainly from **genes** and how the brain develops **before birth**. - **Parenting doesn't cause autism.** Nothing you did or didn't do made your child autistic. - Large studies have found **no link between vaccines and autism**. - The best next step isn't finding a cause. It's **finding support** that fits your child. ## Is autism genetic? Mostly, yes. Researchers agree that genes are one of the main causes of autism.[1] A study of more than 2 million people in Sweden estimated that about **80%** of the likelihood of autism comes from genetic factors.[2] That doesn't mean there's one "autism gene." Hundreds of genes are involved, each adding a little. Many are common in the general population. Some children have a rare genetic change that neither parent has. This is why autism often runs in families. After a child's diagnosis, many parents recognize some traits in themselves or relatives. ## What else raises the likelihood of autism? Researchers have found some factors that make autism more likely, though most children with them are not autistic:[3] - Having a **brother or sister** who is autistic - Having **older parents** - Certain **genetic conditions**, such as fragile X syndrome or Down syndrome - Being born with a **very low birth weight** Some researchers are also studying how genes and conditions during pregnancy may interact, such as infections or certain medications.[1] **Why are more kids diagnosed now?.** Much of the rise comes from better awareness, broader criteria and more screening. Children who once went unnoticed, especially girls and kids who talk well, are now being identified. ## Do vaccines cause autism? No link has been found. A Danish study followed more than **650,000 children** and found no increase in autism after the MMR vaccine, including in children with autistic siblings.[4] Dozens of other studies, covering millions of children, show the same result. In 2025, some federal health messaging in the U.S. changed to question this. Major medical organizations, including the American Academy of Pediatrics, disagreed and continue to recommend routine vaccines. If you have questions, your pediatrician is the right person to ask. ## Does Tylenol in pregnancy cause autism? In 2025, U.S. federal officials announced plans to add a possible link to the acetaminophen (Tylenol) label. The evidence is mixed, and the best-designed studies don't show that acetaminophen causes autism. A 2024 study of about **2.5 million children** in Sweden compared brothers and sisters. When siblings were compared, there was no link between acetaminophen use in pregnancy and autism.[5] The American College of Obstetricians and Gynecologists says acetaminophen remains an appropriate choice for pain and fever in pregnancy, used as needed.[6] **If you're pregnant now.** Talk to your own doctor or midwife before starting or stopping any medicine. Untreated high fever in pregnancy has risks too. ## What doesn't cause autism? - **Parenting style**, discipline or attachment - **Too much or too little attention** as a baby - **Screens.** Research hasn't shown that screen time causes autism (more in [Can screen time cause autism?](https://theautismkit.com/guides/screen-time-and-autism)) - **Food or diet** after birth ## Does knowing the cause change what helps? Usually not. For most children, testing won't find a single cause. What helps is the same either way: understanding your child's strengths and needs, and supports like speech therapy, visual routines and help at school. Some doctors recommend **genetic testing** after a diagnosis, because it can find related conditions and help with family planning. Ask your pediatrician whether it makes sense for your child. **Talk to your doctor about....** genetic testing, your family's health history, or any worries about vaccines or medicines in pregnancy. They can talk through what the evidence means for your family. Free printable: [Free printable: appointment notes](https://theautismkit.com/printables/appointment-notes.pdf) ## What can I do next? - [ ] Let go of the idea that you caused this - [ ] Ask your pediatrician whether genetic testing makes sense - [ ] Write down questions for your next appointment - [ ] Focus on one routine that would help this week New to all of this? Start with [What is autism?](https://theautismkit.com/guides/what-is-autism), or take the [2-minute quiz](https://theautismkit.com/start) for one plan to try this week. ## Common questions ### Is autism genetic? Mostly, yes. A large study of over 2 million people in Sweden estimated that about 80% of the likelihood of autism comes from genes. That doesn't mean a single gene, and it doesn't mean a parent is autistic. ### Did I cause my child's autism? No. Autism isn't caused by parenting, discipline, how much you played with your child, or anything you did or didn't do. ### Is autism caused by vaccines? Large studies following millions of children have found no link. Major medical groups, including the American Academy of Pediatrics, continue to recommend routine vaccines. ## Sources 1. NICHD (NIH): What causes autism? https://www.nichd.nih.gov/health/topics/autism/conditioninfo/causes 2. Sandin S, et al. The heritability of autism spectrum disorder. JAMA, 2017. https://doi.org/10.1001/jama.2017.12141 3. National Institute of Mental Health: Autism spectrum disorder (risk factors). https://www.nimh.nih.gov/health/publications/autism-spectrum-disorder 4. Hviid A, et al. Measles, mumps, rubella vaccination and autism: a nationwide cohort study. Annals of Internal Medicine, 2019. https://doi.org/10.7326/M18-2101 5. Ahlqvist VH, et al. Acetaminophen use during pregnancy and children's risk of autism, ADHD, and intellectual disability. JAMA, 2024. https://doi.org/10.1001/jama.2024.3172 6. American College of Obstetricians and Gynecologists: Acetaminophen use in pregnancy and neurodevelopmental outcomes (2025). https://www.acog.org/clinical/clinical-guidance/practice-advisory/articles/2025/09/acetaminophen-use-in-pregnancy-and-neurodevelopmental-outcomes General guidance, not a diagnosis or treatment plan. Talk with your pediatrician or a developmental specialist about your child's care.