What Causes Autism? What the Evidence Actually Says

Research consistently shows autism is largely heritable — it runs in families. Studies of twins and families find that when one identical twin is autistic, the other very often is too. Hundreds of different genes have been linked to autism, which is part of why it looks so different from child to child.

For most children there isn't one "autism gene" to point to. Instead, many small genetic factors combine. Sometimes autism is linked to a specific genetic condition (such as Fragile X syndrome or tuberous sclerosis), and a doctor may discuss genetic testing — but for the majority of children, no single genetic cause is found, and that's completely normal.

Autism is connected to differences in how the brain develops and forms connections, mostly very early — often before a baby is born. This is why signs can appear so young and why autism is lifelong. These are differences in development, not damage, and not something caused by how a child is raised.

Alongside genetics, researchers have studied a number of factors that may slightly raise the likelihood of autism. These don't cause autism on their own, and most children with these factors are not autistic:

• Being born very prematurely or at a very low birth weight
• Older parental age
• Certain conditions or medications during pregnancy (always discuss medication with your doctor — never stop prescribed medicine on your own)
• Having a sibling who is autistic

These are associations seen across large groups of people, not predictions about any one child.

Some ideas have been studied carefully and firmly ruled out. These do not cause autism:

• Vaccines, including the MMR vaccine. The original claim was based on fraudulent, withdrawn research, and many large, high-quality studies since have found no link. Vaccines protect your child from serious illness.
• Parenting. The old "cold parent" theory was wrong and has been completely rejected. Love, warmth and parenting style do not cause autism.
• Screen time, sugar, or diet. These don't cause autism.
• Anything you did during pregnancy out of normal life.

Be especially wary of products, diets, supplements or clinics that promise to cure or reverse autism. Some are expensive, and a few are genuinely unsafe.

It's natural to want a reason. But for most families, there is no clear single cause to find — and knowing the cause usually wouldn't change the support a child needs anyway. Once the search for blame is set down, parents often feel freed up to focus on what genuinely helps: understanding their child, supporting communication, and building a calmer, more predictable day.

If you'd like a practical next step, see what to do if you're worried or explore the free Parent Toolkit.