What Causes Autism Spectrum Disorder?

In recent months, we’ve seen several stories in the news – on television and online – about what causes autism spectrum disorder (ASD).

In this article, we’ll review the current state of knowledge on ASD, including the primary symptoms and the latest information about what causes autism spectrum disorder, sometimes called autism spectrum disorders, with an ‘s’, and sometimes known simply as autism.

We’ll focus on verified, peer-reviewed research, and share facts confirmed by experimental replication, meta-analysis, and independent third-party, peer-reviewed publications in reputable medical and scientific journals.

What is Autism Spectrum Disorder?

Here’s an evidence-based definition of autism spectrum disorder (ASD):

“Autism spectrum disorder is a neurodevelopmental disorder characterized by impaired social interaction and communication, repetitive and stereotyped patterns of behavior, and uneven intellectual development often with intellectual disability.”

In most cases, individuals receive an ASD diagnosis early in life, during the toddler years. However, in some cases, individuals may receive an ASD diagnosis during late childhood, adolescence, or adulthood.

What Are the Symptoms of Autism Spectrum Disorder?

Regardless of when ASD is diagnosed, experts recognize two primary categories of ASD symptoms: social/communication symptoms and repetitive behavior symptoms.

Social/communication symptoms include:

• Deficits, delays or differences in social-emotional reciprocity, such as:
  • Limited ability to engage in conversational back and forth
  • Limited/no initiation of social contact/conversations
• Deficits, delays, or differences in nonverbal social communication, such as
  • Little/no eye contact during interactions
  • Deficits in understanding gestures, body language, facial patterns

Repetitive behavior symptoms include:

• Repetition itself:
  • Repetitive movements
  • Repetitive use of objects
  • Repeating words, phrases, and/or sounds
• Repeating routines:
  • Inflexibility around schedules and routines
  • Habitual patterns of thought
  • Habitual pattern of interaction
• Interests:
  • Focused attention on specific objects
  • Focused, strong attention on specific activities
  • Immersion in specific things/ideas
• Interaction:
  • Overreaction to external stimuli
  • Underreaction to external stimuli
  • Atypical interest/focus on sensory input

The nature of ASD symptoms explains why it’s most often diagnosed early in life. When a child does not meet specific developmental milestones associated with interacting with the outside world – beginning at around 15 months – they typically receive a professional screening for developmental disorders such as ASD. In addition, the early years, including toddlerhood, overlap with a period when many children interact with components of the outside world for the first time. They experience new places, see new people, and encounter new things, including things in the environment that may have a negative effect on development: that’s why there’s an interest in identifying any environmental factors that may contribute to or increase risk of autism.

What Causes Autism?

As of 2025, we – meaning medical researchers, the scientific community, child development experts, parents, and mental health professionals – cannot identify one single cause of autism spectrum disorder (ASD).

Let’s repeat that:

We don’t know what causes autism spectrum disorder.

We do, however, understand what increases individual risk of developing autism spectrum disorder.

Here’s what we know as of 2025:

1. Genetic Factors Can Increase Risk. Specific genetic factors increase risk of developing an autism spectrum disorder (ASD). Read the source: Identification, Evaluation, and Management of Children With Autism Spectrum Disorder – PubMed
   • For parents with one child with ASD, risk of a second child with ASD is 5-10%. Read the source: Recurrence Risk of Autism in Siblings and Cousins: A Multinational, Population-Based Study – PubMed
     • 7% risk if first child is female
     • 5% risk if first child is male

2. Disease Can Increase Risk. Some cases of ASD are associated with:
   • Congenital rubella syndrome
   • Cytomegalic inclusion disease
   • Phenylketonuria
   • Tuberculosis sclerosis
   • Fragile X Syndrome
3. Environmental Factors May Increase Risk. Some researchers suggest environmental factors, such as pesticides, maternal illness, low birth weight/premature birth, social determinants of health, and vaccines may increase risk of ASD, but these hypotheses are not supported by experimental data or evidence and are therefore unproven.

We’ll close this article by addressing a topic in that last bullet point parents may be interested in: speculation about a relationship between vaccines and autism.

Do Vaccines Cause Autism?

First, as we mention above, we currently do not know what causes autism: we know what increases risk. Second, the American Academy of Pediatrics published this excellent primer for parents on vaccines and vaccine safety:

Vaccine Safety: Examine the Evidence

Here’s a brief summary:

1. Before a vaccine can be recommended, it must be tested and found safe and effective.
2. Any vaccine on this Child and Adolescent Immunization Schedule  is safe and effective.

The independent non-profit organization Autism Speaks recognizes parental concern about the impact of vaccines on overall health, including the impact on developmental disorders such as ASD. To help parents understand the relationship between vaccines and autism, they published the following article:

Do Vaccines Causes Autism?

Here’s the takeaway from that article:

“Vaccines do not cause autism. It is possible that the timing of an autism diagnosis or onset of autism symptoms might coincide with the recommended vaccine schedule for children, but this is a coincidence, not a cause.”

To support this claim, scientific and medical experts at Autism Speaks reviewed and published this list of fourteen (14) peer-reviewed studies on the relationship between vaccines and autism.

The first five examine the relationship between autism and the MMR vaccine:

MMR Vaccine and Autism: Five Studies

1. Autism occurrence by MMR vaccine status among U.S. children with older siblings with and without autism (2015)

1. • Data from 95,000 children showed no increased risk of ASD associated with the MMR vaccine at any age, regardless of older sibling status.

2. Measles, mumps, rubella vaccination and autism: A nationwide cohort study (2019)

1. • Data from 650,000 children in Denmark showed no association between MMR vaccines and autism.

3. A population-based study of measles, mumps, and rubella vaccination and autism (2002)

1. • Data from 537,000 children born between 1991 and 1998 in Denmark showed no association between MMR vaccination and autism.

4. Autism and measles, mumps, and rubella vaccine: no epidemiological evidence for a causal association (1999)

1. • Data from over 500 children with autism showed no increase in autism prevalence after introduction MMR vaccine.

5. MMR-vaccine and regression in autism spectrum disorders: Negative results presented from Japan (2006)

1. • In 904 children with autism, evidence showed no difference between children who received the MMR vaccine and those who didn’t.

Summary: there is no association between the MMR vaccine and autism spectrum disorders.

The second set of five examine the relationship between autism and the vaccines containing thimerosal:

Thimerosal-containing Vaccines and Autism: Five Studies

1. Association between thimerosal-containing vaccine and autism(2013)
   • Data from close to half a million children in Denmarkshowed no difference in autism rates between children who received thimerosal-containing vaccines and those who received vaccines without thimerosal.
2. Pervasive developmental disorders in Montreal, Quebec, Canada: prevalence and links with immunizations (2006)
   • Study analyzed the impact of the addition of thimerosal to the MMR vaccine autism on autism prevalence in Montreal, Canada between 1987 and 1998.
   • ASD rates increased after removing thimerosal removed from vaccines in 1996
   • The study identified changes in diagnostic criteria, increased awareness, and better identification as reasons for increased prevalence.
3. Thimerosal exposure in infants and developmental disorders: A prospective cohort study in the United Kingdom does not support a causal association (2004)
   • Data from over 14,000 children showed no association between thimerosal and negative developmental outcomes
4. Autism and thimerosal-containing vaccines: lack of consistent evidence for an association (2013)
   • Study examined association between autism prevalence and presence of thimerosal in vaccines.
   • Data showed increase in autism prevalence after removing thimerosal from vaccines.
5. Continuing increases in autism reported to California’s developmental services system: mercury in retrograde (2008)

• • Study examined the effect of removing thimerosal from vaccines between 1999 and 2001 and found that after removal, autism rates continued to increase.

Summary: there is no association between vaccines containing thimerosal and autism spectrum disorders.

The final set includes four studies on the overall relationship between vaccines and autism:

Additional Analyses and Meta-Analyses: Four Studies

1. Increasing exposure to antibody-stimulating proteins and polysaccharides in vaccines is not associated with risk of autism(2013):
   • Study examined the impact of immune products – i.e. antigen – in vaccines on autism risk.
   • Data from 256 children with ASD and 752 children without ASD showed no association between immune product in vaccines – cumulative or otherwise – and autism risk.
2. Number of antigens in early childhood vaccines and neuropsychological outcomes at age 7–10 years(2013)
   • Data from over 1,000 children with high vaccine exposure at 7, 12 and 24 months showed no negative association between high vaccine exposure and cognition, behavior, memory, attention, or executive function.
3. Vaccines are not associated with autism: An evidence-based meta-analysis of case-control and cohort studies(2014)
   • Data from over one million children (1,200,000) showed no association between vaccines and autism.
4. Safety of vaccines used for routine immunization in the United States: An updated systematic review and meta-analysis (2021)
   • Review of over 300 studies showed no association between the MMR vaccine and autism.

Summary: evidence shows no association between high vaccine risk and autism spectrum disorders, with two meta-analyses involving over a million children showing no association between vaccines and autism spectrum disorders.