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What causes autism?

Autism is genetic. Most autistic people are autistic because of genes they got from their parents. This is why autism often runs in families. Much less commonly, other factors that change or influence genes can play a role, but usually in combination with genes from parents.

Genes from parents

Most autistic people are autistic because of the genes they got from their parents. This is why autism often runs in families. 
 

What are genes? 

Genes are sections of DNA that contain instructions for how our bodies grow, develop and function. 

Each person has around 20,000 to 25,000 genes. They are found on long strands of DNA called chromosomes. We have 46 chromosomes (23 pairs) in most cells of our bodies. 

You get half of your chromosomes (and the genes within) from your mum, and half from your dad. The genes we get from our parents determine things like our hair and eye colour, and whether we are autistic or not. 

Because of this, we can think of autism as a natural variation in humans. This is known as the neurodiversity approach. 

Questions and answers

Other factors

A person is much more likely to be autistic because of genes they got from their parents than any other factor. But in some cases, there are other factors that affect genes and contribute to a person being autistic.

There are two types of these factors:

  • ‘Environmental’ factors that can influence genes
  • Random genetic changes.

‘Environmental’ factors that can influence genes

It can be confusing when scientists talk about ‘environmental’ factors. Generally speaking, it doesn’t mean that things in a person’s environment made them autistic.

There are only two environmental factors that make it much more likely a baby will be born autistic even without getting genes from their parents. They are:

  • the mum having rubella (‘German measles’) in the first trimester of pregnancy, which can cause ‘congenital rubella syndrome’ in the baby
  • the mum taking the anti-seizure medication valproate while pregnant.

These factors are now very rare because most people in the UK are vaccinated against rubella and strict prescribing guidelines mean valproate isn’t given to people who could become pregnant. They will only be a factor for a very small number of babies; these babies are more likely to be autistic and have health problems and learning disabilities.

Other ‘environmental’ factors cannot make a person autistic on their own. Generally, the person must already have genes from their parents that could make them autistic, and these factors make it more likely that they will be autistic. But the impact is small and unpredictable: these factors may make a difference for some people but not for others.

When scientists talk about ‘environmental’ factors that make it slightly more likely that a person is autistic, they mean things like:

  • the dad being older than 40-50 when the mum got pregnant (this may be due to changes in the dad’s genes that are more common as men age, but we don’t know for sure)
  • the mum being older than 30 when they got pregnant (this is less impactful that the dad being older, but the potential impact increases as the mum gets older)
  • the mum getting serious or frequent infections with a fever while pregnant
  • the baby being born ‘prematurely’ (too early)
  • the baby having a low weight at birth
  • the baby not getting enough oxygen.

Confusion about environmental factors has led to misinformation about the causes of autism. For more information about this, visit our page Myths about the causes of autism.


Random genetic changes

Less commonly, even if a person didn’t get genes from their parents that made them autistic, random changes can happen in their genes that make them autistic.

Scientists call these changes ‘de novo mutations’. They can happen in the sperm or egg, or in the early development of the baby in the womb (before the baby is born).

Some of these random changes make it very likely that the person will be autistic. People with more of these random changes are also much more likely to have learning disabilities and health problems.

Questions and answers

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