Autism spectrum disorder - Causes 

Causes of autism spectrum disorder 

NHS Change Day: Lucy's story - understanding autism

Lucy has Asperger's syndrome. On a visit to her local NHS hospital, the triage nurse told her to wait outside after the initial examination, which she took literally, rather than taking a seat in the waiting room. This resulted in Lucy waiting outside for hours in the cold before she was found and brought inside. This video recreates Lucy's experience and shows that even health professionals need a better understanding of the challenges patients with autism experience within the NHS.

Media last reviewed: 26/03/2014

Next review due: 26/03/2016

Misconceptions about the causes of ASD

There are a number of things that have been linked to ASD in the past, but extensive research has found no evidence to suggest any of these contribute to the condition.

These include:

  • the MMR (mumps, measles and rubella) vaccine
  • a compound containing mercury called thiomersal, 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

The exact causes of autism spectrum disorder (ASD) are unknown, although it is thought that several complex genetic and environmental factors are involved.

The causes of ASD can be described in two ways:

  • Primary ASD (also known as idiopathic ASD) – where no underlying factors can be identified to explain why ASD has developed.
  • Secondary ASD – where an underlying medical condition or environmental factor thought to increase the risk of ASD is identified.

About 90% of cases of ASD are primary, and about 10% are secondary.

Risk factors

Factors thought to increase the risk of developing ASD, known as ‘risk factors’, can usually be divided into five main categories:

  • Genetic factors – certain genetic mutations may make a child more likely to develop ASD.
  • Environmental factors – during pregnancy, a child may be exposed to certain environmental factors that could increase the risk of developing ASD.
  • Psychological factors – people with ASD may think in certain ways that contribute towards their symptoms.
  • Neurological factors – specific problems with the development of the brain and nervous system could contribute to the symptoms of ASD.
  • Other health conditions – certain health conditions associated with higher rates of ASD.

Each of these factors is described in more detail below.

Genetic 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 have an increased risk of developing the condition themselves and it is common for identical twins to both develop ASD.

At present, however, no specific genes linked to ASD have been identified and there are currently no tests that can screen for 'ASD genes'. However, the specialist seeing your child may screen them for genetic conditions that have similar features to ASD, depending on any additional symptoms your child has.

Environmental factors

Some researchers have argued that ASD is not primarily caused by genes, but also by environmental factors. The theory is that a person is born with a vulnerability to ASD, but the condition develops only if that person is exposed to a specific environmental trigger.

Some suggested environmental factors include being born before 35 weeks of pregnancy (premature birth) and exposure to alcohol or medications such as sodium valproate (a medication 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.

Psychological factors

Much of the research into the possible psychological factors behind ASD is based on a concept known as 'theory of mind' (TOM). This is a person's ability to understand other people's mental states, recognising that each person they meet has their own set of intentions, beliefs, emotions, likes and dislikes. To put it simply, it's seeing the world through another person's eyes.

It is thought that most children without ASD have a full understanding of theory of mind by around the age of four. Children with ASD develop a limited understanding or no understanding at all of theory of mind.

This may be one of the root causes of their problems with social interaction and may explain some of the psychological features of ASD, such as a tendency towards needing an order or routine and getting lost in detail rather than seeing the bigger picture.

Neurological factors

In people without conditions affecting the brain and nervous system (see below), medical theories and brain imaging studies carried out in people with ASD suggest that the connections between parts of the brain called the cerebral cortex, the amygdala and the limbic system may have become scrambled or ‘over connected’.

As a result, people with ASD may suddenly experience an extreme emotional response when seeing a trivial object or event. This may be a reason why people with ASD are fond of routines, as they have found a set pattern of behaviour that does not provoke an extreme emotional response. It may also explain why they often become very upset if that routine is suddenly broken.

This confusion of emotional responses may also explain why children with ASD are interested in topics that most children would find boring, such as train timetables or have altered or exaggerated responses to sensory stimulation such as tastes, sounds, noises, smells.

Other health conditions

Some conditions that are known to increase the risk of ASD include:

  • Fragile X syndrome – an uncommon genetic condition that usually causes certain facial and bodily characteristics, such as a long face, large ears and flexible joints. 
  • Tuberous sclerosis – a rare genetic condition that causes multiple non-cancerous tumours to grow throughout the body, including the brain.
  • Rett syndrome – a rare genetic condition that mostly affects girls. It causes symptoms of ASDs, and difficulties with physical movement and development.
  • Neurofibromatosis – a number of genetic conditions that cause tumours to grow along your nerves. The main types are neurofibromatosis type 1 and neurofibromatosis type 2.
  • Muscular dystrophy – a group of inherited genetic conditions that gradually cause the muscles to weaken, leading to an increasing level of physical disability.
  • Down’s syndrome – a genetic condition that typically causes some level of learning disability and a characteristic range of physical features.
  • Cerebral palsy – conditions that affect the brain and nervous system, causing problems with a child's movement and coordination.
  • Infantile spasms – a type of epilepsy that develops while a child is still very young (usually before they are one year old).
  • Intellectual disability – about half of those diagnosed with autism have an IQ below 70 (the average IQ is 100).


Page last reviewed: 18/12/2013

Next review due: 18/12/2015

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The 3 comments posted are personal views. Any information they give has not been checked and may not be accurate.

tiggerrmummy said on 11 March 2014

I disagree with this page. I think that a lot of the information is a blanket and that the change from invividual conditions and lumping them all into ASD is wrong. My child has AS - Aspergers Syndrome, lets call it what it actually is. I do not consider it a disorder, what a lot of rubbish, its another part of who they are and a facet of their character. Disorder implies that something is wrong, Autistic Spectrum Disorders appears to imply that the people with ASD are themselves disorders and therefore can be fixed. I refuse to believe that disorder is the correct terminology here. And any parent should feel the same way. Its going backwards and causing a stigma where there shouldnt be one and doing the amazing scientists who have researched these conditions a massive diservice to put very disparate conditions under one heading without any real explanation of what each one is. So, change it NHS Its wrong and as a parent I find it offensive.

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rolando49 said on 07 October 2011

I disagree partly with the above post.There are many causes of autisim.After 35 years of complaining to the nhs about physical symptoms of aspergers syndrome/high functioning autisim all I can say is I'm so lucky not to be misdiagnosed by the NHS's crappy shallow mental health philosophy.Whats more disturbing for me is that I was dumped in care because of High Functioning Autisim,then left with no family/freinds /support.In my early 30's I became a single dad,since then I've had two partners,both who were unconciously abusive and manipulative because of my 'learning disabilities',both of them midwives.I'm 48 now and after a year of realising what all the mental confusion/physical pain/severe dietry problems are about I discover that the NHS has a basic fill em up with drugs policy to people who are on the Autisim Spectrum.Last march 2010 I had a severe motorcycle accident,injuring my soine,neck /legs and a brain injury,I was left to die by the NHS in Rochdale and Manchester.i lived out in woodland during last winter and was placed in a care home for people with severe mental health problems.i was treated like an idiot and the Manchester NHS were in complete denial about my autisim.This summer I spent every penny I had on an osteopath this has prevented me being crippled for life and has helped my brain injury heal.Now after 7 months in the mental care home they know I dont have a mental illness and want to make me homeless again.I have still not recived any treatment but luckily went homeless in Sheffeild where the NHS appears to be outstanding and seems to be at the forefront of understanding Autisim Spectrum Disorder,unlike manchester NHS and Rochdale quote 'we dont do autisim here'.While many people may be fustrated by the lack of awareness and help with ASD's ,we must remember that its early days so blame does'nt help anyone,awareness does,so be patient.it's a very creative thing to have,just refuse to take drugs for it,diet helps alot

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alfieraysautismawareness said on 18 March 2011

You know what this page is a waste of time there is no known cause ASD is not discriminitive and does not matter if you are young/old/rich/poor/addict or infact the second coming. It knows no boundries it just happens.Lots of conspiricy theories on this subject and we have our own beliefs. One things for sure our son is no longer getting any more immunisations and our next sure aint either. Our son hit every single target milestone. Was talking etc saying mum, dad, dog, no laughing and holding good eye contact. Post MMR his words left him and he went into his own world... We have had same story from a lot of parents.. Now I do not believe for one minute that MMR causes Autism, I think children are born with the capability to have it. I feel immunisation is the trigger. It overloads an already weak immune system.

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