How Common Is Autism in Siblings?

This blog post addresses the question, “How common is autism in siblings?” It explores factors that contribute to autism symptoms in unaffected siblings and lists scientific studies regarding other related findings that research has provided us. The article outlines the underlying risk factors of ASD, how you may feel about becoming pregnant after having a child with ASD, and what to do if you suspect autism in your child. 

Autism spectrum disorder (ASD) is a developmental condition that affects the behaviors and social skills of the individual with ASD. It is considered a spectrum as it is based on the severity and impairment of the disorder.

How Common Is Autism in Siblings?

The probability of a child developing autism spectrum disorder (ASD) increases fourteenfold when they have an older sibling with ASD. Children with older siblings on the spectrum had a risk of 11.3% to have ASD, while children with siblings without ASD had a chance of 0.92% (Xie et al., 2016). 

This finding is an excerpt from a longitudinal study conducted between 2001 and 2010 by a Southern Californian organization, Kaiser Permanente. The investigators analyzed more than fifty thousand children’s medical records. Among these children, almost 600 had ASD. Other findings from the study include:

  • When children with an older sibling with ASD were born at term (i.e., between the 37th and 42nd week of pregnancy) were at 15 times the higher risk for being on the spectrum. Those children born preterm and had an older sibling on the spectrum were at ten times increased risk of having the disorder.
  • For a male child, having an older brother increased ASD risk compared to female children with an older sister.  

Factors that Contribute to Autism Symptoms in Unaffected Siblings

Frazier and colleagues conducted a study in 2010, wherein they investigated the severity of autism symptoms in almost 3,000 children with autism and more than 2,500 of their siblings without ASD (Frazier et al., 2015). They found the following results. 

  • Children with more than one sibling with ASD are more likely to exhibit autism traits when compared to kids with only one sibling with ASD;
  • In boys, if one of the siblings with ASD is a sister, they are more likely to have the disorder; and
  • Children without ASD but with siblings with the condition are likely to show autism symptoms in the form of language problems, including language developmental delay, using strange or made-up words or phrases, and other unconventional speech characteristics.

These findings corroborated other studies that indicated an increased risk of ASD in children with more than one sibling with ASD and boys exhibiting stereotypical behaviors when they have sisters instead of brothers on the spectrum.

Other Research Findings

The following list is a compilation of research studies and statistics related to autism and family.

  • Identical twins are at the highest risk of ASD in both children. Fraternal or non-identical twins have a higher risk of both of them having ASD than non-twin siblings (Zaidman-Zait et al., 2017);
  • Mothers with an adolescent or older child with autism and a younger child with autism or another disability were more like to suffer from depression and anxiety than mothers with only one affected child. Apart from depression and anxiety, they found it challenging to establish emotional connections and adjust to novel circumstances or requirements (Orsmond et al., 2007); 
  • Between three and five percent of children with an aunt or uncle with ASD are likely to have the disorder instead of 1.5% of children in the general population. This study also found that the risk of a child with a maternal uncle or aunt with ASD developing ASD was not different from that of a child with a paternal uncle or aunt with ASD. This finding challenged the theory of a female protective effect to explain a lower prevalence of ASD among girls (Bai et al., 2020);
  • Parents using structure and schedules can help their children with ASD cope with anxiety and changes more effectively (Losh et al., 2008); and
  • Families of children with ASD are likely to deal with stress better when they have appropriate support systems than those without such networks (Rosenberg et al., 2009).  

Risk Factors 

Various factors increase the risk of the development of ASD, such as:

  • Birth Complications
  • Genetic Factors
  • Familial Factors
  • Prenatal Factors
  • Chromosomal Disorders
  • Environmental Challenges

Birth Complications

A review in 2017 mentioned significant associations between autism spectrum disorder and birth complications, like oxygen deficiency (hypoxia) and insufficient blood supply (ischemia). Children with such difficulties had an eightfold risk of developing ASD. 

Children experiencing fetal stress due to inadequate oxygen, making the fetus take in unwanted remains in the uterus (known as meconium aspiration), are seven times more likely to develop ASD.  

Genetic Factors

Researchers have not tracked down a single “autism gene” to attribute to the condition. However, studies indicate the role of several genes and their mutations to an increased risk of ASD. 

They have identified more than 60 genes with a strong link to autism and more than 200 with a feebler link. There are genetic mutations that impair early brain development. Every child with ASD presents a unique set of factors that contribute to their condition. Experts are beginning to understand ASD better with every breakthrough in this field.

Familial Factors

Apart from genetic factors, biological or familial influences affect the probability of ASD in children. As we have already noted, siblings with ASD is a significant contributor to the risk of children developing ASD. Moreover, paternal age is a strong determinant; the older the father, the greater the risk.  

Prenatal Factors

Chromosomal Disorders

Children with specific chromosomal disorders, like fragile X syndrome and tuberous sclerosis, have an increased risk of being on the spectrum, according to the Centers for Disease Control and Prevention (CDC).

Environmental Challenges

A majority of children with ASD have vitamin D deficiency and other dietary issues. Additionally, children exposed to infectious instruments, chemicals, pesticides, and heavy metals are also likely to suffer from ASD. Physical and psychological stress also increases such risk. 

How Having a Child with ASD can Affect Your Decision to Become Pregnant Again?

If you have a child with ASD, you may hesitate to become pregnant again due to several reasons, including:

  • Feel guilty about your hope to have a child without ASD;
  • Worry about not having adequate support to bring up another child with ASD;
  • Fear how another child with ASD might affect your familial relationships;
  • Be concerned about insufficient time to care for your child with ASD when you have to tend to a newborn; and
  • Feel excited yet guilty about potentially having a typically developing child

If You Suspect ASD in Your Child

Firstly, remember that early diagnosis implies early intervention, which is crucial in managing autism spectrum disorder. If you notice any issues with your child’s behavior or how they speak, learn, and play, contact your family physician and address these concerns.

Depending on your physician’s suggestions, get a referral to a specialist in this field to understand your child’s difficulties better. Specialists can make a thorough assessment and provide a diagnosis for the problem. These specialists include:

  • Child Psychologists or Psychiatrist;
  • Child Neurologist; and
  • Developmental Pediatrician

Upon receiving a diagnosis, contact your state’s public early childhood system to know if your child is eligible for availing intervention services after a free assessment. It is commonly known as a Child Find evaluation. 

If your child is below three years of age, call the local early intervention services. Visit the Early Childhood Technical Assistance (ECTA) website or call them at 919-962-2001 to acquire your state’s communication information for evaluation.

If your child’s age is three years or more, you can call the local public-school services, the board of education, or local elementary school to request an evaluation. Otherwise, contact ECTA or visit their website. 


This blog article answered the question, “How common is autism in siblings?” It further analyzed factors that contribute to autism symptoms in children without ASD but with siblings with ASD, other crucial research findings, the risk factors for ASD, how you may feel about becoming pregnant after having a child with ASD, and what to do if you suspect ASD in your child.

Frequently Asked Questions (FAQs): How Common Is Autism in Siblings?

What are the odds of a child developing ASD if their sibling is on the spectrum?

The risk of having a child with ASD increases to 20% if their sibling has ASD. Otherwise, generally, the probability of having a child with ASD is 1.5%. 

How does having siblings with ASD affect a child’s probability of having ASD?

If a child has more than one sibling with autism, they are more likely to have autistic traits than those with only one sibling with autism. Moreover, if a male child has more than one sibling on the spectrum, one of whom is a sister, their chance of displaying autistic traits increases.

Is autism likely to run in families?

Yes, autism is likely to run in families, but the pattern of hereditary is still unknown. Individuals with genetic alterations related to autism are likely to inherit a higher risk of developing ASD instead of the condition itself. 

Is ASD genetically transmissible?

Dan Bai and colleagues found that genes determine 80% of the risk of having ASD. This study explored the causes of ASD with more than 2 million participants from five countries, including Denmark, Israel, Sweden, Finland, and Western Australia, and is the most extensive investigation into the causes of ASD yet. 


Bai, D., Marrus, N., Yip, B. H. K., Reichenberg, A., Constantino, J. N., & Sandin, S. (2020). Inherited Risk for Autism Through Maternal and Paternal Lineage. Biological Psychiatry, 88(6), 480–487.

Frazier, T. W., Youngstrom, E. A., Hardan, A. Y., Georgiades, S., Constantino, J. N., & Eng, C. (2015). Quantitative autism symptom patterns recapitulate differential mechanisms of genetic transmission in single and multiple incidence families. Molecular Autism, 6(1).

Orsmond, G. I., Lin, G. Y., & Seltzer, M. M. (2007). Mothers of adolescents and adults with autism: Parenting multiple children with disabilities. Intellectual and Developmental Disabilities, 45(4).[257:MOAAAW]2.0.CO;2

Rosenberg, R. E., Law, J. K., Yenokyan, G., McGready, J., Kaufmann, W. E., & Law, P. A. (2009). Characteristics and concordance of autism spectrum disorders among 277 twin pairs. Archives of Pediatrics and Adolescent Medicine, 163(10), 907–914.

Sarris, M. (2019, January 08). AUTISM PARENT, TIMES TWO: WHEN MORE THAN ONE CHILD IN THE FAMILY HAS ASD. Interactive Autism Network. Retrieved from  

Xie, F., Peltier, M., & Getahun, D. (2016). Is the Risk of Autism in Younger Siblings of Affected Children Moderated by Sex, Race/Ethnicity, or Gestational Age? Journal of Developmental & Behavioral Pediatrics, 37(8), 603–609.

Zaidman-Zait, A., Mirenda, P., Duku, E., Vaillancourt, T., Smith, I. M., Szatmari, P., Bryson, S., Fombonne, E., Volden, J., Waddell, C., Zwaigenbaum, L., Georgiades, S., Bennett, T., Elsabaggh, M., & Thompson, A. (2017). Impact of personal and social resources on parenting stress in mothers of children with autism spectrum disorder. Autism, 21(2), 155–166.

Zeliadt, N. (2015, November 26). Three sets of traits in siblings signal autism’s heritability. Spectrum. Retrieved from