What causes Autism? Is it genetic and environmental?
11 min · Special Needs
July 26, 2023

What causes Autism? Is it genetic and environmental?

The most common question asked after autism diagnosis is what is the cause of autism.

Until this day, there is no single known cause of autism spectrum disorder.

There are several studies that autism is a combination of genetic and non-genetic factors as well as environmental influences.

Autism is a complex disorder. Its symptoms and severity can vary on a wide spectrum. This suggests that there may be many causes for it.

Both genetics and environmental factors have found to play a role in development of the disorder.

These risks, which we will be discussing in this article, seem to increase the risk of a child developing autism spectrum disorder.

But it is important to note here that increased risk does not necessarily mean the actual cause. Those with the risks may not develop the disorder at all.

Through advancements in technology and medicine, we have learned over the years, from the beginning of the disorder, that ASD is NOT caused by;

  • Vaccines

  • An infection that can be spread around

  • Bad parenting

What are the Genetic Causes of Autism?

Studies have shown that autism may run in families and several different genes seem to be involved in ASD.

ASD can be associated with a genetic disorder like Rett syndrome or fragile X syndrome for some children.

For others, genetic mutations could increase risk of the disorder. Also, other genes may be at play in affecting the brain development.

Is Autism Hereditary?

When genes are involved, one may ask to themselves if autism is hereditary or not. Research after research concluded that autism tends to run in families. Although we don’t specifically know what causes autism during pregnancy, certain gene mutations during pregnancy can change the chance of a child being born on the autism spectrum.

Autism is not the only disorder to be passed from parents to a child. Even depression and alcoholism are proven to be passed to the child because of gene mutations. Studies have shown that families that have children with autism have a greater chance of prevalence of the disorder. These gene mutations and chances also alter the child’s risk of having autism.

Genes may also be impacting the way brain cells communicate, or may be determining the symptoms or severity. Research indicates that some genetic mutations are likely to be inherited, while others occur spontaneously.

When a parent is the carrier of these genes, they may pass it onto the child. In this case, the parent does not have to have autism. Other times, the genetic mutations could happen in an early embryo or the sperm and/or egg that combine to create the embryo.

However, these gene mutations do not cause autism by themselves, but increase the risk of developing the disorder.

How is the Brain Affected by Autism?

Autism is a developmental disorder that affects how a person interacts with their surrounding.

The brains of people with autism spectrum disorder are known to process information differently to those without the disorder.

It is found that the brains of individuals with autism as a whole is less coordinated in terms of activity.

However, it is not clear whether or not the individual brain regions work differently in autism.

In a study conducted by Watanabe et al., it was found that sensory areas of the brain in autistic individuals showed more random activity compared to individuals without the disorder:

  1. Those with the most severe forms of autism showed the most random activity. This suggests that brains of people with autism can’t hold and process sensory input for as long as those with neurotypical development.

  2. On the other hand, a brain region called the caudate was shown to be more predictable in individuals with autism.

  3. People with the most rigid and repetitive behaviors exhibited the most predictable caudate. The difference in this neural randomness stems from the changes in the structure of the individual brain regions.

  4. The results of this study indicate that the changes in the structure and activity of brain regions may give rise to complex symptoms in autism.

In another study, Dr. Jeff Anderson, a professor in Radiology at the University of Utah Health in Salt Lake City, and his team looked into the way autism works in the brain.

They suggest that the symptoms may be linked to persistent connections in the brain.

Dr. Anderson and his team investigated the reason why people with ASD often dislike exposure to unexpected stimuli.

This research takes a look at what happens in the brain. And its purpose was to understand how this process relates to a person’s ability to tolerate exposure to stimuli.

Researchers used a novel fMRI method to explore the brain activity in the participants and looked at the duration of connections established across the brain regions.

They were able to find that connections in the brains of people with autism spectrum disorder persist for extended periods compared to the brains of neurotypical individuals.

This means that the brains of the autistic individuals find it more difficult to switch between processes.

The study’s findings also indicate that brain connections in people with autism remained synchronized for up to 20 seconds. However, these disappeared faster in individuals without ASD.

Also, symptom severity seemed to increase the duration of the connectivity for those with autism spectrum disorder.

Cambridge neuroscientists have researched the fetal testosterone since ASD affects boys more than it affects girls.

They wanted to examine the effects of ASD on brain development and postnatal behavior.

In this study, researchers analyzed the effects of prenatal testosterone levels which are produced by the fetus on autistic behavior.

Fetal testosterone shapes brain development, creating the individual’s cognitive profile.

Cambridge researchers found out that higher parental testosterone levels are associated with reduced social skills and superior attention to details in infants.

Is Autism Considered a Behavioral Disorder?

Autism spectrum disorder is a developmental disability. This developmental disability can cause severe impairment in social, communication, and behavioral challenges.

The reason that autism can be considered a behavioral disorder is these challenges seen in terms of behavior.

CDC lists autism one of the developmental and behavioral disorders, which is a group of conditions caused by impairment in learning, behavior, physical and language areas.

They may begin early during developmental periods, and affect daily life.

Oftentimes, there is nothing about how people with autism spectrum disorder look that is different from other people.

However, the way that people with ASD communicate, interact, learn and behave differs from most other people.

What Causes Autism During Pregnancy?

Studies are still looking into the factors that may contribute to the occurrence of autism.

Recent studies indicate that mutation and changes during conception and pregnancy, and even after delivery could increase risk of developing autism in children who are genetically predisposed to the disorder.

A study showed that the differences in the brains of the children could be detected as early as the second trimester of the pregnancy.

Although we do not have a definitive cause for autism, we know that it is developed through a combination of factors, including genetic and environmental factors.

It is not possible to change genetics. However, there are ways to limit exposure to certain environmental factors that have been shown to contribute to development of autism.

Although these are worth trying, it is not certain that lowering exposure will lower the child’s risk for developing ASD.

The evidence we have about the environmental risk during pregnancy is still at its infancy. There are many ways to go.

A study published in the American Journal of Epidemiology in 2014 found that children who were born to mothers with iron deficiency are five times more likely to have autism.

This risk also increases if the mother is aged 35 or older. Metabolic conditions like obesity, high blood pressure or diabetes also contribute to the increase of the risk.

Various studies have shown connection between pregnancy exposure to air pollution and risk of developing autism.

One Harvard School of Public Health study found that the risk of ASD doubled for children born to women exposed to high levels of pollution, especially in the third trimester.

University of Utah published a research in 2013 issue of Pediatrics suggesting a potential link between excess pregnancy weight and autism risk.

5-pond increases incrementally above the American Congress of Obstetricians and Gynecologists’ (ACOG) recommendation were shown to have a link to a slightly higher risk for autism.

Previous studies also found a possible connection between pre-pregnancy body mass index, pregnancy weight gain, and the risk of developing autism.

A theory suggests that excess body fat may change hormone levels or cause inflammation that affects fetal brain development.

Potential links have been found between the medication a mother takes during pregnancy and the risk for autism.

For instance, antidepressant use has shown to be associated with autism in various studies.

However, it is not clear that this link is specifically related to the drugs or the mother’s depression.

It is important that the mother works with her doctor and determine whether the benefits of the medications outweigh the risk, as sometimes the disorder like epilepsy may have a greater damage on the fetus.

Genetic Risk Factors

In light of studies, researchers think that ASD is the result of hereditable genetic differences and/or mutations.

Studies that showed a genetic link between developing autism state that ASD is more common in boys than in girls.

This is most likely linked to the genetic differences associated with the X chromosome.

Another genetic risk factor found is in identical twins. Their rate of concordance was found to be higher compared with the rate in fraternal twins.

Moreover, around 20% of children with an older sibling with autism spectrum disorder developed ASD.

The risk of developing the disorder was found to be greater if there was more than one older sibling with ASD.

Neurobiological Factors

Differences and abnormalities in the genetic code could cause certain mechanisms of the brain development to be abnormal.

This could result in structural and functional brain abnormalities. Also, cognitive and neurobiological abnormalities and symptomatic behaviors could be experienced.

In the frontal and temporal lobes, increased gray matter could be an example for structural and functional abnormalities in the developing brain.

Also, by adolescence, white matter was found to be decreased compared with gray matter.

Anatomical and functional differences were found in the cerebellum and in the limbic system.

Moreover, synaptic deficits affecting anatomical structures and neuronal circuitry were observed.

Several factors make it difficult to determine the pathological nature of ASD and understand the relationship between genetic mutations and neurobiological outcomes. The following are some of these factors:

  • Brain development having a dynamic nature

  • Single genes affecting multiple traits

  • Genetic heterogeneity underlying ASD

Environmental Factors

There have been many studies on the pre- and postnatal environmental risk factors for developing ASD.

These studies inquired about the interaction between various environmental factors like diet, exposure to drugs, and environmental toxicants with genetic susceptibility to autism spectrum disorder.

Studies have found a couple of environmental exposures. These include lead, polychlorinated biphenyls (PCBs), automotive exhaust, and flame retardants. But to this day, no specific environmental factors have been identified to certainly cause ASD.

Other environmental risk factors for autism before and during birth are as follows:

  • Advanced parental age

  • Extreme prematurity

  • Low birth weight

  • Maternal obesity, diabetes, immune system disorders

However, these factors by themselves do not cause autism. They increase the risk of developing autism in combination with other genetic factors.

Do Vaccines Cause Autism?

The subject of whether or not vaccines cause autism has been a heated debate for a long time.

The rumor started back when Dr. Andrew Wakefield and his colleagues published an article in 1998 linking measles, mumps and rubella vaccines to autism.

The article has since been retracted due to the article not being correct.

People have had concerns that autism could be linked to the vaccines children get. However, an abundance of studies have shown that there is no link between getting vaccinated and developing ASD.

CDC has conducted [a study][20] in 2013 that showed that vaccines do not cause autism spectrum disorder.

The study investigated various substances in vaccines that cause the body’s immune system to produce antibodies within the vaccines in the first two years of life.

The study’s results stated that the total amount of antigen from vaccines was the same between children with autism spectrum disorder and those who did not have ASD.

[Thimerosal][21], a controversial vaccine ingredient that has been researched specifically, is a mercury-based preservative.

It is used to prevent contamination of vaccines. Studies showed that thimerosal does not cause autism.

CDC has [funded or studied][21] nine studies on this matter and none of them found any link between vaccines containing thimerosal and autism spectrum disorder.

And these studies also failed to find any connection between measles, mumps, and rubella vaccine and autism spectrum disorder in children.

[20]: https://www.jpeds.com/article/S0022-3476(13 [21]: https://www.cdc.gov/vaccinesafety/pdf/cdcstudiesonvaccinesandautism.pdf

Last Updated: November, 2021


This article is examined by Clinical Child Psychologist and Ph. D. Researcher Kevser Çakmak, and produced by Otsimo Editorial Team.

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This post does not provide medical advice. See Additional Information.