What Causes Autism? Genetics, Environment and the 90% Figure
Is autism genetic or environmental? Mostly genetic, with environmental factors playing a smaller supporting role. Twin studies estimate that 64% to 91% of the differences in autism between people are explained by genes, which is where the "90%" figure comes from (Tick et al., 2016). Autism is not caused by parenting, and the evidence does not show that vaccines cause it.
If you're reading this late at night, you may be wondering whether something you did, or didn't do, led to your child's autism. That's one of the most common worries parents have. The short answer is no: autism develops before birth, mostly because of genes, and it is not your fault.
This guide explains what the research really says, what the "90%" means, which environmental factors matter, and what does not cause autism.
Is Autism Genetic or Environmental?
Both play a part, but genes play the biggest one.
Where the "90%" Figure Comes From
The best evidence comes from twin studies. Identical twins share nearly all of their genes. Fraternal twins share about half, like any brothers and sisters.
If autism is mostly genetic, identical twins should be much more likely to both be autistic. That's exactly what researchers found. A 2016 meta-analysis that pooled twin studies found that autism traits in identical twins were almost perfectly matched. It estimated heritability at 64% to 91%, depending on the assumptions used (Tick et al., 2016).
A larger 2019 study followed more than 2 million people across five countries. It estimated autism heritability at about 80% (Bai et al., 2019).
What "Heritable" Really Means
Heritability is a number about groups of people, not about one child. It doesn't mean 90% of your child's autism came from genes and 10% from something you did. It means that, across a whole population, most of the differences in who is autistic are explained by genetic differences.
How Genes Are Involved
There is no single "autism gene." Scientists have found rare gene changes, as well as many small common genetic variations, in autistic people (NIEHS). Many genes work together, each adding a small amount.
Some of these genes are passed down from parents. Others are new changes that happen in the egg, sperm or early in development, so they don't show up in either parent. Our article on which parent carries the autism gene explains how inheritance works.
Environmental Factors Research Supports
"Environmental" doesn't mean things in your home or your parenting. In autism research, it usually means things that happen before or around birth.
According to the National Institute of Environmental Health Sciences, the clearest evidence involves events before and during birth, such as (NIEHS):
- Older parents at the time of conception
- Exposure to air pollution or certain pesticides during pregnancy
- Maternal obesity, diabetes or immune system disorders
- Extreme prematurity or very low birth weight
- Birth difficulties that lead to periods of low oxygen to the baby's brain
How Strong Is the Evidence?
These factors are linked with a somewhat higher chance of autism. But NIEHS notes that they are unlikely to cause autism on their own. Instead, they appear to raise the likelihood when combined with genetic factors.
In other words, most children exposed to these factors are not autistic, and many autistic children had none of them. They are pieces of a bigger picture, not causes you could have spotted or stopped.
What May Lower the Likelihood
Some research points to things that may help. NIEHS-funded studies found that taking prenatal vitamins around the start of pregnancy, and folic acid in particular, was linked with a lower likelihood of autism (NIEHS). Our guide to healthy pregnancy and brain development covers more ways to support a baby's development.
What About Acetaminophen (Tylenol)?
You may have seen news about acetaminophen in pregnancy. In September 2025, federal health officials announced an FDA label change saying acetaminophen use during pregnancy may be linked to a higher risk of autism and ADHD. The American College of Obstetricians and Gynecologists and the American Academy of Pediatrics disagreed, saying the full body of evidence doesn't support that claim (HCPLive).
One of the largest studies, of nearly 2.5 million children in Sweden, compared brothers and sisters. It found no link between acetaminophen in pregnancy and autism once family factors were taken into account (Ahlqvist et al., 2024). If you're pregnant, talk to your doctor before taking or stopping any medicine.
What Does NOT Cause Autism
Many myths about autism causes have spread online. Here's what the evidence says.
Vaccines
Large studies have found no link between vaccines and autism. A Danish study of 657,461 children found no increased risk of autism after the MMR vaccine, including in children with autistic siblings or other risk factors (Hviid et al., 2019). NIEHS also states that no link has been found between autism and vaccines, including those containing thimerosal.
You may hear that this has changed. In November 2025, the CDC updated its website to say studies have not ruled out a link between infant vaccines and autism. Many doctors and scientists said the change was not supported by the research (PBS NewsHour). The studies themselves have not changed. If you have questions about vaccines, talk them through with your child's pediatrician.
Parenting
Autism is not caused by parenting style, how much affection you show, discipline or stress at home. This old idea was disproved decades ago. Autism starts with how the brain develops, mostly before birth. Our article on whether bad parenting causes autism explains why this myth persists.
Screen Time
There's no evidence that screen time causes autism. Some studies have found that heavy screen use in babies is linked with more autism-like symptoms later, but these studies show a connection, not a cause (Drexel University, 2020). It's also possible that babies who are already developing differently use screens differently.
Limiting screen time and spending lots of face-to-face play time is good for every child. It just isn't a cause, or a fix, for autism.
Diet, Sugar and Other Myths
Autism is not caused by sugar, gluten, food dyes or anything a child ate after birth. It isn't contagious, and it isn't caused by anything you did in your everyday life as a parent.
What This Means for Your Family
Knowing the causes can bring real relief. It also helps you focus on what matters most now.
It's Not Your Fault
Autism comes mainly from genes and from development that happens before birth. Nothing you did as a parent caused it. Letting go of guilt frees up energy for supporting your child.
Siblings May Have a Higher Chance
Because autism is strongly genetic, younger brothers and sisters of autistic children have a higher chance of being autistic too. If you have younger children, mention it to your pediatrician so they can watch development closely and screen early.
Should We Have Genetic Testing?
Some families are offered genetic testing after an autism diagnosis. It doesn't diagnose autism, but it can sometimes find a related genetic condition that affects your child's health care. Ask your child's doctor whether it makes sense for your family.
When Family Members Have Opinions
Relatives and friends sometimes share theories about what caused your child's autism, from vaccines to "too much screen time." Even when they mean well, it can hurt.
You don't have to argue every point. A simple reply can help: "Doctors say autism is mostly genetic and starts before birth. Right now we're focused on helping her learn." You can also share this article.
Questions to Ask Your Child's Doctor
If you want to understand more about your child's autism, these questions can help:
- Would genetic testing be useful for my child?
- Should my other children be screened, and when?
- Are there related health conditions we should watch for, like sleep problems or seizures?
- What support should we start now?
Focus on What Helps Now
You can't change your child's genes, and you don't need to. What makes the biggest difference is support: early help with communication, social skills and daily routines, plus a family that understands your child.
If you're noticing signs but don't have a diagnosis yet, our guide explains how to get your child tested for autism, step by step.
How ABA Helps, Whatever the Cause
Whatever caused your child's autism, the right support helps them learn and grow. Applied Behavior Analysis (ABA) is a therapy that teaches skills in small, manageable steps.
A Board Certified Behavior Analyst (BCBA) starts by getting to know your child and your family's goals. Then they build a plan that may include:
- Communication: asking for what they want, using words, pictures, signs or a device
- Social skills: taking turns, playing with others and making friends
- Daily living skills: routines like dressing, mealtimes and bedtime
- Emotional regulation: handling big feelings and coping with change
- Reducing behaviors that get in the way, by teaching better ways to meet the same need
Inclusive ABA offers in-home ABA therapy and school-based ABA, so your child practises skills where they actually use them. We also offer parent training, so you can use the same strategies every day.
Noticing early signs, or just got a diagnosis? We'll help you with the next steps.
Talk to our teamThe Bottom Line
Autism is mostly genetic. Twin studies put heritability at 64% to 91%, and some factors before and around birth can add a small amount of extra likelihood. Autism is not caused by parenting, and large studies have found no link with vaccines.
The cause matters less than what comes next. With understanding and the right support, your child can keep learning and growing at every age.
Frequently Asked Questions
Is autism genetic?
Mostly, yes. Twin studies estimate that 64% to 91% of the differences in autism between people are explained by genes. Many genes are involved, and some are new changes that don't appear in either parent.
What environmental factors cause autism?
No environmental factor causes autism on its own. Research links some factors before and around birth, like older parents, maternal diabetes or obesity, extreme prematurity and some prenatal exposures, with a somewhat higher likelihood. These seem to matter mainly when combined with genetic factors.
Do vaccines cause autism?
Large studies have found no link. A Danish study of more than 650,000 children found no increased risk of autism after the MMR vaccine. If you have questions, talk them through with your child's pediatrician.
Did I cause my child's autism?
No. Autism comes mainly from genes and from development before birth. It is not caused by parenting, discipline or anything you did in your child's daily life.
Can autism be prevented?
There's no known way to prevent autism. Some research links prenatal vitamins and folic acid around the start of pregnancy with a lower likelihood, but this isn't prevention. What helps most is early support once you notice signs.
Sources
- Tick B, Bolton P, Happé F, Rutter M, Rijsdijk F. Heritability of autism spectrum disorders: a meta-analysis of twin studies. Journal of Child Psychology and Psychiatry, 2016. https://pmc.ncbi.nlm.nih.gov/articles/PMC4996332/
- Bai D, et al. Association of genetic and environmental factors with autism in a 5-country cohort. JAMA Psychiatry, 2019. https://pmc.ncbi.nlm.nih.gov/articles/PMC6646998
- National Institute of Environmental Health Sciences. Autism. https://www.niehs.nih.gov/health/topics/conditions/autism
- Hviid A, Hansen JV, Frisch M, Melbye M. Measles, mumps, rubella vaccination and autism: a nationwide cohort study. Annals of Internal Medicine, 2019. https://www.acpjournals.org/doi/10.7326/M18-2101
- Ahlqvist VH, et al. Acetaminophen use during pregnancy and children's risk of autism, ADHD, and intellectual disability. JAMA, 2024. https://jamanetwork.com/journals/jama/fullarticle/2817406
- HCPLive. ACOG, AAP respond to White House autism announcements (September 2025). https://www.hcplive.com/view/acog-aap-respond-white-house-autism-announcements-acetaminophen-leucovorin
- PBS NewsHour. CDC vaccine safety webpage changed (November 2025). https://www.pbs.org/newshour/health/cdc-vaccine-safety-webpage-changed-to-contradict-scientific-conclusion-that-vaccines-dont-cause-autism
- Drexel University. Screen time for babies linked to higher risk of autism-like symptoms (2020). https://drexel.edu/news/archive/2020/april/screen-time-for-babies-linked-with-autism-like-symptoms
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