What causes autism spectrum disorder?

We don’t know exactly what causes autism spectrum disorder (ASD). There might be several causes, including brain development and genetic factors.

We do know that ASD isn’t caused by anything that parents do or don’t do while raising their child.

Brain development and autism spectrum disorder

In children with autism spectrum disorder (ASD), the brain develops differently from typically developing children.

The brains of these children tend to grow too fast during early childhood, especially during the first three years of life. And the brains of babies with ASD appear to have more cells than they need, as well as bad connections between the cells.

Too many connections between brain cells
A young child’s brain is developing all the time. Every time a child does something or responds to something, connections in the brain are reinforced and become stronger. Over time, the connections that aren’t reinforced disappear – they are ‘pruned’ away as they’re not needed.

This ‘pruning’ is how the brain makes room for important connections – those needed for everyday actions and responses, like walking, talking or understanding emotions. This pruning doesn’t seem to happen as much as it should in children with ASD – so information might be lost or sent through the wrong connections.

The lack of pruning might also explain why the brain seems to be growing faster in children with ASD than in children with typical development.

Genetics and autism spectrum disorder

In some families, there seem to be more people with autism spectrum disorder (ASD) than in others. This suggests that genes might be one cause of ASD. Evidence for this idea has been increasing over the past few decades.

It’s unlikely that there’s one specific gene that causes ASD. Rather, it might be that several genes combine and act together. Researchers have found many possible genes that might play a role in the development of ASD. But they don’t yet understand what causes ASD or increases children’s risk of developing it.

It’s also possible that different gene combinations might explain the differences seen in ASD – for example, why one child is more sensitive to sounds than another.

ASD can also happen together with other genetic conditions. This is called comorbidity.

One example of a gene identified in people with ASD is ‘neurexine 1’. This is a gene we all have, and it’s important for communication within the brain. Problems (or ‘disruptions’) in this gene are a known problem in ASD. But because the disruption by itself is not enough to cause ASD, this is an example of how multiple factors might be involved in causing ASD.

Other factors and autism spectrum disorder

Recently several factors have been linked to an increased risk of autism spectrum disorder (ASD):

  • older parents
  • poor growth of the foetus in the womb
  • lack of oxygen when a baby is born
  • exposure to air pollution during pregnancy.

Also, external factors might trigger ASD in a child who is already genetically prone to developing the condition