Research has not found one single cause of autism. However, genetic factors and environmental factors — such as exposure to pesticides before birth or maternal immune system disorders — may play a role in the development.

Genetic factors include gene variants, which some people inherit from their parents. This does not guarantee that a child will have autism, but it may raise the likelihood.

Environmental factors may include exposure to certain infections or medications during pregnancy, very low birth weight, maternal obesity, and older parental age at the time of conception. However, more research is necessary to confirm these.

This article discusses what autism is, the ongoing research into its development, some of the more widespread myths about its causes, and more.

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Autism is a neurodevelopmental condition that affects how a person communicates, learns, thinks, behaves, and interacts with others. People of all ages, sexes, and races can have autism.

Autism is a “spectrum” disorder, meaning that autistic people can have a wide range of symptoms, strengths, skills, and support needs. The healthcare community now uses the umbrella term autism spectrum disorder (ASD) to refer to anyone who falls on this spectrum.

Typically, symptoms of ASD present in early childhood. They can include communication difficulties, challenges with social interaction, and repetitive behaviors. Some autistic people are able to perform the everyday activities they need to do independently, while others may need substantial support.

There is no medical test to determine whether a person is autistic. Doctors use developmental monitoring and screening to make a diagnosis, and they are often able to diagnose a child by age 2 years. However, some autistic people do not receive a diagnosis until they reach adolescence or even adulthood.

Researchers have not found one single cause of autism. Several factors could play a role in its development. They include:

  • having a twin or older sibling with autism
  • older parental age at the time of conception
  • brain growth disruptions in early development
  • preterm birth

Much of the research on autism suggests that a combination of genetic and environmental factors cause it.

Over the last few decades, there has been a significant increase in autism genetics research. During that time, scientists have found that ASD is highly heritable. This means that genetics play a large role in its development.

For example, a 2019 study of more than 2 million people across five countries estimated ASD’s heritability to be approximately 80%.

Researchers have also found that specific gene variants can contribute to autism risk. A gene variant is a permanent change in the DNA sequence that makes up a gene. Some gene variants come from a person’s parents, while others can occur during a person’s lifetime.

There is no clinical reason to carry out routine testing for common gene variants, as many people have them, and they do not reliably predict that a person will be autistic.

However, there is a growing number of rare gene variants that doctors could look for. Testing for them could potentially:

  • give people a better understanding of why they, or their child, is autistic
  • enhance early recognition, allowing people to gain support sooner
  • increase the quality of healthcare a person receives

However, genetic testing also comes with some potential difficulties, including:

  • Difficulty interpreting findings: The rare gene variants that have links to autism are not a guarantee a person will be autistic nor a way to predict the severity of symptoms.
  • False negatives: This can happen if a doctor fails to detect a known variant. This may mean a child does not receive a correct diagnosis.
  • Ethical concerns: There are significant fears among members of the autistic community that scientists could use genetic testing for autism as a first step toward erasing it. This is known as eugenics.

Researchers have identified several environmental factors that may play a role in developing autism, including:

It is important to note that some studies on these factors have somewhat confusing results.

For example, an older 2010 study from Denmark found no association between maternal infection and ASD diagnosis when looking at the total period of pregnancy.

However, researchers did find an association between ASD diagnosis and pregnant people who were admitted to the hospital for viral infections during the first trimester and pregnant people who had bacterial infections in the second trimester.

Similarly, in 2012, a study found “little evidence” of an association between ASD and common infectious diseases or fevers during pregnancy. However, it did find an increased risk of an ASD diagnosis if a parent had influenza (flu) or prolonged fever during their pregnancy.

More research is necessary to better understand the role that environmental factors might play.

While researchers are still learning what causes autism, they do know that a number of myths about its causes are false. They include:

Parenting and neglect

During the mid-20th century, both the public and the medical community widely believed that child neglect was the cause of autism.

Leo Kanner introduced neglectful parenting as the cause of autism in his 1943 paper Autistic Disturbances of Affective Contact. Cold, distant parenting — especially on the part of mothers — became the go-to explanation for why some children develop behavioral and emotional issues.

During his time as director of the Orthogenic School for Troubled Children, Bruno Bettelheim supported this “refrigerator parenting” theory and even went on to compare autistic children to concentration camp prisoners in his 1967 book Empty Fortress: Infantile Autism and the Birth of the Self.

The medical community abandoned this theory when, in the 1970s and 1980s, researchers began discovering genetic and environmental explanations.

MMR vaccination

Another myth about autism is that the measles, mumps, and rubella (MMR) vaccine can cause it. This idea came about when The Lancet published a now widely criticized paper by Dr. Andrew Wakefield and his colleagues in 1998.

The paper highlighted 12 children with intestinal symptoms and regressive developmental disorder, which is when a child acquires skills but then loses them again. The parents of eight of the children attributed this loss of skills to the MMR vaccine.

Wakefield then implied that the MMR vaccine was to blame by suggesting that “environmental triggers” were responsible for the changes in health and behavior.

This resulted in many parents and caregivers losing trust in the MMR vaccine, meaning many children did not gain the protection it offers against three potentially fatal illnesses. The United Kingdom’s Health Protection Agency linked the drop in MMR vaccination to large measles outbreaks in 2008 and 2009.

However, there are a number of flaws in Wakefield’s paper. Firstly, eight is a very small number of children to base this hypothesis on. There is also evidence that Wakefield carefully selected the children he focused on, and that some of the funding for the research came from lawyers who were helping parents sue vaccine manufacturers.

This suggests that Wakefield and his colleagues may have been biased. The funding sources also had a vested interest in proving that vaccines were responsible for the children’s symptoms. As a result, The Lancet retracted Wakefield’s paper in 2010.

Since then, several studies have found no evidence of a link between MMR vaccination and autism, even in children with a high chance of developing it.

According to the Centers for Disease Control and Prevention (CDC), the reported prevalence of ASD has increased over the years. However, an increase in reported prevalence does not necessarily mean that autism itself is becoming more common.

More diagnoses could be the result of:

  • changes in autism’s clinical definition
  • increased awareness of autism
  • increased emphasis on early diagnosis so people can receive support sooner

These factors may mean doctors are better equipped to identify autism, resulting in an apparent increase in cases. It is also possible that there has been a true increase in the number of autistic people, but it is difficult to know if that is the case.

The Autism and Developmental Disabilities Monitoring (ADDM) Network, a CDC-funded program that tracks data to better understand the number and characteristics of children with autism, reports that:

  • Approximately 1 in 44 children (aged 8 years) received an ASD diagnosis in 2018, which is an increase from the previous report of 1 in 59 children in 2014.
  • Children born in 2014 were 50% more likely to receive an ASD diagnosis or an ASD special education classification by age 4 years compared with children born in 2010.

Current research suggests that a combination of genetic and environmental factors may increase the chances a person will meet the criteria for ASD. These may include specific genetic variants, older parental age, preterm birth, and others. However, researchers are still learning about the possible causes.

There are many myths about what causes autism. Over the years, scientists have debunked many of these myths. The claim that MMR vaccines or childhood neglect are responsible for autism is not based on medical science.