If you spend even five minutes on the internet searching for answers, you'll find a massive, often contradictory pile of theories about what can be the cause of autism. It’s overwhelming. One person tells you it’s definitely "leaky gut," another blames heavy metals, and a third insists it’s just evolution in action.
The reality is way more complex. And frankly, a lot more interesting.
Autism Spectrum Disorder (ASD) isn't like a broken leg where you can point to the exact moment the bone snapped. We aren't looking for a "smoking gun." Instead, researchers are looking at a massive, tangled web of biology, environment, and timing. Honestly, the more we learn, the more we realize that "autism" isn't just one thing. It's likely dozens of different biological paths that all lead to a similar way of experiencing the world.
The Genetic Blueprint: It’s Not Just One Gene
Most experts agree that genetics play the biggest role. But it’s not as simple as having an "autism gene."
Think of it like building a house. If one brick is slightly off, the house is fine. If a hundred bricks are slightly off, or if the foundation is poured during a rainstorm, the whole structure changes. Studies on twins have been the gold standard here. Research from institutions like the Simons Foundation Autism Research Initiative (SFARI) has shown that if one identical twin has autism, there is a very high probability the other will too—far higher than with fraternal twins. This tells us DNA is the heavy lifter.
We’ve identified hundreds of genes linked to ASD. Some are "de novo" mutations, which basically means they appear spontaneously in the child and weren't passed down by the parents. Others are inherited variations that have been in the family for generations.
Rare vs. Common Variants
Sometimes, a single, rare genetic mutation is powerful enough to cause autism on its own. These are often seen in conditions like Fragile X Syndrome or Rett Syndrome. But for the vast majority of people, it's the "common" variants. These are tiny genetic quirks that many people have, but when you get a specific combination of hundreds of them at once, you cross a threshold.
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It’s about the "burden" of these variations.
Environmental Triggers and the Womb
This is where things get controversial and often misunderstood. When we talk about what can be the cause of autism in an environmental sense, people often jump to vaccines. Let's be incredibly clear: the science has debunked that link over and over. The real environmental factors are much subtler and usually happen before a child is even born.
The environment of the uterus is everything.
Maternal immune activation is a big area of study right now. If a mother gets a severe infection—the kind that requires hospitalization—during pregnancy, the resulting inflammation might affect the developing fetal brain. It’s not the germ itself, usually; it’s the mother’s immune response. Researchers like those at the UC Davis MIND Institute have been tracking how these immune markers might nudge brain development toward an autistic profile.
Then there’s parental age. This is a hard truth for many, but older parents (both mothers and especially fathers) have a statistically higher chance of having a child with ASD. As men age, the sperm accumulates more of those "de novo" mutations I mentioned earlier. It’s just biology.
Other Prenatal Factors
- Valproate exposure: This is a medication used for epilepsy. We know for a fact that taking it during pregnancy significantly increases the risk of the child being autistic.
- Premature birth: Babies born very early, specifically before 26 weeks, have a higher incidence of ASD. Their brains are doing a lot of heavy lifting outside the womb during a critical window.
- Birth spacing: Some evidence suggests that having pregnancies less than 12 months apart might increase risk, possibly because the mother’s body hasn't fully replenished certain nutrients like folate.
Brain Connectivity: The "Local vs. Long-Distance" Problem
If you look at an autistic brain under a high-powered microscope or an fMRI, you don't see "damage." You see a different wiring diagram.
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In a typical brain, there's a balance between "local" connections (short wires) and "long-distance" connections (long wires). In many autistic individuals, there seems to be an overgrowth of local connections. Basically, certain areas of the brain are talking to themselves too much, while the long-distance lines that connect the front of the brain to the back are a bit thin.
This explains why someone might be brilliant at recognizing patterns or focusing on details (local processing) but struggle to integrate social cues, tone of voice, and body language all at once (global processing).
The Gut-Brain Axis: More Than a Stomach Ache
You've probably heard that many autistic kids have "tummy issues." For a long time, doctors dismissed this as a side effect of picky eating or anxiety. Now, we're realizing the gut might be part of the "why."
The microbiome—the trillions of bacteria in your intestines—produces chemicals that talk directly to the brain via the vagus nerve. Some studies have found that the gut bacteria in autistic children are significantly different from those in neurotypical children. Is this a cause? Or is it a result of the restricted diets many autistic people have due to sensory issues?
It’s a "chicken or the egg" problem that hasn't been fully solved yet. But researchers like Dr. Sarkis Mazmanian at Caltech are looking into whether specific probiotics could actually change behavior by altering the signals sent from the gut to the brain. It’s a wild, frontier science area.
Epigenetics: The "Light Switch" Effect
This is the bridge between genes and the environment. You have your DNA, which is the "hardware." But epigenetics acts like the software—it tells the genes when to turn on and when to turn off.
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Certain environmental factors—pollution, stress, nutrition—don't change your DNA, but they can flip the switches. This might explain why one twin has autism and the other doesn't, even though their DNA is identical. Something in their individual experience in the womb or early life flipped a switch that changed how their genes expressed themselves.
What It Isn't: Moving Past the Myths
We can't talk about what can be the cause of autism without clearing the air on what definitely isn't a cause.
Bad parenting doesn't cause autism. In the 1950s, the "Refrigerator Mother" theory suggested that cold, unloving mothers caused their children to retreat into themselves. This was devastating, wrong, and has been completely dismantled by modern science.
Screens don't cause autism. While heavy screen time might affect a child's development in other ways, it doesn't re-wire the brain into an autistic profile. Neither does "heavy metal toxicity" from modern living, despite what some unregulated wellness "gurus" might claim.
The Path Forward: Actionable Insights for Parents and Adults
Understanding the causes of autism isn't just about satisfying curiosity. It’s about knowing how to support people. Since we know that brain plasticity is highest in early childhood, the "cause" matters less than the "intervention" once the child is here.
- Prioritize Prenatal Health: If you're planning a pregnancy, focus on high-quality prenatal vitamins with folic acid and manage chronic conditions like diabetes or high blood pressure, as these are linked to lower risks.
- Early Monitoring: Because we know autism starts in the womb, the signs can show up early. Don't "wait and see" if a child isn't hitting social milestones like eye contact or responding to their name by 12 months. Early intervention (like speech or occupational therapy) takes advantage of that early brain wiring.
- Genetic Counseling: If you have one child with autism and are planning another, or if you are an autistic adult curious about your own biology, genetic testing can sometimes provide specific answers. It can identify those "rare variants" and help you understand the broader family picture.
- Focus on Co-occurring Conditions: Often, the "cause" of the most difficult autism symptoms isn't the autism itself, but things like GI distress, sleep disorders, or anxiety. Treating the gut or the sleep cycle can dramatically improve the quality of life for an autistic person.
The search for what can be the cause of autism is ongoing. We are moving away from seeing it as a "mystery to be solved" and toward seeing it as a complex biological reality. The goal isn't necessarily to "fix" the cause, but to understand the unique way an autistic brain processes the world so we can build a world that actually fits them.