Causes and Risk Factors of Autism Spectrum Disorder (ASD)

Newborn boy sleeping

Thanasis Zovoilis / Getty Images

If you are a parent or loved one of someone with autism spectrum disorder (ASD), you might have wondered what causes it. This question, however, has no straightforward answer because there is no single cause of ASD.

Research points to certain risk factors that put some people at a higher risk of developing the condition than others. Risk factors include having a family member who has also been diagnosed with the condition, environmental factors like severe pollution, and being born preterm. 

Brain & Body Risk Factors 

Let's take a look at some risk factors that might increase the likelihood of having autism.

Brain Shape and Brain Growth During Development

Brain scans of autistic people show that there are differences in the way their brains are shaped and structured as compared to neurotypical people.

During early childhood, the brains of children with autism grow faster than usual and typically develop more cells than they need.

They might also have poor connections between these cells. In children without autism, a process called pruning (where the brain gets rid of connections that aren’t needed to make room for important connections) doesn’t happen in children with autism. This means that sometimes information could pass through wrong connections and cause it to be lost.

Infection During Pregnancy

Some research points at pregnancy being a point at which a child might be susceptible to developing ASD. Bacterial infections during pregnancy have been thought to increase the risk of the unborn child developing ASD, but this is only slightly and in most cases play no role in the development of this condition. 

Family History & Genetics

Research shows that family history and genetics play a role in the development of the condition. Complications during pregnancy or having children later in life may also play a role.

Families With a History of ASD

Children with families who have a history of the disorder are at a higher risk of developing it. Even if both parents of a child do not have ASD, they might be carriers of gene changes that cause autism and can be passed on to the child.

When a child is diagnosed with ASD, the next child could have up to a 20% chance of developing the condition. And if the first two children in a family have ASD, the third child has about a 32% risk of developing the condition.

The way the disorder is transmitted down generational lines is not completely understood. It is impossible to predict who will or won’t get ASD in a family who has a history of the condition. A meta-analysis of studies into how genetics factor into the development of ASD in twins found that if one twin had ASD, the other twin was at a 64% to 91% risk of developing the condition.

Neurexin 1 is a gene we all have that plays a vital role in communication in the brain. Some research shows that disruptions with this gene play a factor in the development of autism. However, there is no evidence to support that disruptions with this gene alone are capable of causing autism. Therefore, only a small number of autism cases can be attributed to genetic factors. 

There is some evidence to suggest that as many as 12 or more genes could be involved with ASD. About 10% of children with ASD have genetic conditions like Down’s syndrome and Fragile X Syndrome. Also, the Reelin gene, which plays an important role in the lamination of the brain during pregnancy, has been associated with autism.

Having Children Later in Life

Parents who have no family history of the condition but start to have children at an advanced age also put their children at risk of developing ASD. Some research notes that this risk is usually heightened if someone has children in their mid-40s. 

Pregnancy Complications

Pregnancy complications such as multiple pregnancies and preterm pregnancies are also contributing factors. In addition, there has been some research indicating that pregnancies spaced less than a year apart could also put a child at risk of developing ASD.

Environmental Risk Factors 

Certain environmental influences could increase the risk of a person developing ASD. In addition, people who are already genetically predisposed to the condition are at an even higher risk when exposed to these environmental factors.

Environmental factors also tend to include events that occur after a person is pregnant with a child. For example, some evidence shows that anticonvulsants taken during pregnancy might lead to the development of ASD in the child.

A vast majority of the research into the causes of ASD focuses on its connection to family history and genetics. However, young children are exposed to thousands of toxic materials in early childhood and during pregnancy. For example, studies have linked the development of autism to exposure in early pregnancy to chemicals like valproic acid, misoprostol, and thalidomide.

Lifestyle Risk Factors 

It used to be believed that certain vaccines could cause ASD. However, extensive research has been put into this theory, and it has been proven that no vaccines can cause ASD.

ASD starts to show as early as 18 months, and because many children are getting different types of vaccines in those early years, the development of ASD has been erroneously linked to taking vaccines. 

Before more research was done into the causes of ASD, parents of children with ASD were often blamed for causing the condition. Some people even claimed that a lack of attention and care from the parents of a child with ASD could cause the condition. This is untrue, and decades of research have continued to disprove these theories.

This theory was prevalent between the 1950s and 1970s when people had very little understanding of ASD and mistook the condition as a psychological disorder instead of a developmental challenge. 

No one can point out a single cause for ASD, but it’s never the parents' fault when the condition develops. 

Was this page helpful?
Article Sources
Verywell Mind uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Ha S, Sohn I-J, Kim N, Cheon HJS and K-A. Characteristics of brains in autism spectrum disorder: structure, function and connectivity across the lifespan. Experimental Neurobiology. 2015;24(4):273-284.

  2. Shen MD, Piven J. Brain and behavior development in autism from birth through infancy. Dialogues Clin Neurosci. 2017;19(4):325-333.

  3. Columbia University Irving Medical Center. Children with autism have extra synapses in brain. August 21, 2014

  4. Zerbo O, Qian Y, Yoshida C, Grether JK, Van de Water J, Croen LA. Maternal infection during pregnancy and autism spectrum disorders. J Autism Dev Disord. 2015;45(12):4015-4025.

  5. Cleveland Clinic. Autism spectrum disorder (Asd): Causes, symptoms, treatment & outlook. December 29, 2020

  6. Tick B, Bolton P, Happé F, Rutter M, Rijsdijk F. Heritability of autism spectrum disorders: a meta-analysis of twin studies. J Child Psychol Psychiatr. 2016;57(5):585-595.

  7. Rylaarsdam L, Guemez-Gamboa A. Genetic causes and modifiers of autism spectrum disorder. Front Cell Neurosci. 2019;13:385.

  8. Durkin MS, Maenner MJ, Newschaffer CJ, et al. Advanced parental age and the risk of autism spectrum disorder. American Journal of Epidemiology. 2008;168(11):1268-1276.

  9. Szatmari P. The causes of autism spectrum disorders: Multiple factors have been identified, but a unifying cascade of events is still elusive. BMJ. 2003;326(7382):173-174.

  10. Landrigan PJ. What causes autism? Exploring the environmental contribution. Curr Opin Pediatr. 2010;22(2):219-225.

  11. Plotkin S, Gerber JS, Offit PA. Vaccines and autism: a tale of shifting hypotheses. Clinical Infectious Diseases. 2009;48(4):456-461.