Causes - Autism spectrum disorder (ASD)

The exact cause of autism spectrum disorder (ASD) is currently unknown.

It's a complex condition and may occur as a result of genetic predisposition (a natural tendency), environmental or unknown factors.


Most researchers believe that certain genes a child inherits from their parents could make them more vulnerable to developing ASD.

Cases of ASD have been known to run in families. For example, younger siblings of children with ASD can also develop the condition, and it's common for identical twins to both develop ASD.

No specific genes linked to ASD have been identified, but it may be a presenting feature of some rare genetic syndromes, including Fragile X syndrome, Williams syndrome and Angelman syndrome.

Environmental triggers

Some researchers believe that a person born with a genetic vulnerability to ASD only develops the condition if they're exposed to a specific environmental trigger.

Possible triggers include being born prematurely (before 35 weeks of pregnancy), or being exposed in the womb to alcohol or to certain medication, such as sodium valproate (sometimes used to treat epilepsy during pregnancy).

No conclusive evidence has been found linking pollution or maternal infections in pregnancy with an increased risk of ASD.

Other health conditions

Below are some other conditions known to be associated with ASD:

  • muscular dystrophy – a group of inherited genetic conditions that gradually cause the muscles to weaken
  • Down's syndrome – a genetic condition that typically causes a learning disability and a range of physical features
  • cerebral palsy – conditions that affect the brain and nervous system, causing problems with movement and co-ordination
  • infantile spasms – a type of epilepsy that develops while a child is still very young (usually before they're one year old)
  • neurofibromatosis – a number of genetic conditions that cause tumours to grow along the nerves (the main types are neurofibromatosis type 1 and neurofibromatosis type 2)
  • the rare genetic conditions fragile X syndrome, tuberous sclerosis and Rett syndrome 

Misconceptions about the causes of ASD

In the past, a number of things were linked to ASD, but extensive research has found no evidence to suggest that any of these contribute to the condition. These include:

  • the MMR vaccine
  • thiomersal – a compound that contains mercury, which is used as a preservative in some vaccines
  • the way a person has been brought up
  • diet, such as eating gluten or dairy products

Page last reviewed: 06/01/2016
Next review due: 06/01/2019