What causes autism?

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

Brain development and autism

The brain develops differently in autistic children compared with typically developing children.

In young children, the 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’re ‘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.

In autistic children, the brain tends to grow faster than average during early childhood, especially during the first three years of life. The brains of autistic babies appear to have more cells than they need, as well as poor connections between the cells.

Also, pruning doesn’t seem to happen as much in autistic children. This means that 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 autistic children than in typically developing children.

It’s not yet clear what causes this difference in brain development.

Genetics and autism

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

It’s unlikely that there’s one specific gene that causes autism. 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 autism. But they don’t yet understand what causes autism or increases children’s chances of developing it.

Autism can also happen together with other genetic conditions.

‘Neurexine 1’ is an example of a gene identified in autistic people. 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 factor in autism. But because the disruption by itself is not enough to cause autism, it means there must be other factors involved in causing autism.

Other factors and autism

Several other factors have been linked to an increased likelihood of autism:

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

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