Autism spectrum disorder (ASD) is a complex developmental condition that affects how a person communicates with and relates to other people, and how they experience the world around them. ASD is a spectrum condition, which means that while all people with ASD share certain difficulties, the condition affects each person differently. In recent years, autism diagnoses have become more common. Understanding who is most likely to have ASD, and why, can help us better support individuals on the spectrum.
What is autism spectrum disorder?
Autism spectrum disorder refers to a range of conditions characterized by challenges with social skills, repetitive behaviors, speech and nonverbal communication. According to the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), people with ASD have persistent deficits in social communication and social interaction across multiple contexts. This may include:
- Deficits in social-emotional reciprocity
- Deficits in nonverbal communicative behaviors used for social interaction
- Deficits in developing, maintaining and understanding relationships
In addition to social communication deficits, people with ASD tend to have restricted, repetitive patterns of behavior, interests or activities. These may include:
- Stereotyped or repetitive motor movements or speech
- Insistence on sameness and inflexible adherence to routines
- Highly restricted, fixated interests
- Hyper- or hyporeactivity to sensory input
The symptoms of autism spectrum disorder can range from mild to severe, and often vary depending on the person’s developmental level. Some people with ASD are able to live independently, while others require life-long care and support.
Prevalence of autism
In recent decades, the number of autism diagnoses has increased dramatically. For example, data from the Centers for Disease Control and Prevention’s Autism and Developmental Disabilities Monitoring Network shows that in 2000, around 1 in 150 children were identified with ASD. By 2016, this had increased to 1 in 54 children.
It is unclear how much of this increase is due to better awareness and screening, broader diagnostic criteria, and other social factors. However, the higher prevalence also suggests that autism may be more common than previously thought.
Who is most likely to have autism?
Although autism occurs in all racial, ethnic and socioeconomic groups, research shows that certain factors place some people at higher risk. Here is an overview of who is most likely to have autism spectrum disorder.
Sex
ASD is significantly more common in boys than girls. According to the CDC, 1 in 34 boys were identified with autism in 2016, compared to 1 in 145 girls. Some key facts about sex differences include:
- Boys are about 4 times more likely to be diagnosed than girls
- Among children diagnosed with autism, the male to female ratio is approximately 4:1
- However, this ratio may be closer to 3:1 among those with average or above average cognitive ability
Scientists are investigating the reasons for this difference. Research suggests biological factors may play a role, including differences in brain anatomy and hormones. Genetic factors are also being studied, as are female “camouflaging” behaviors that may mask symptoms.
Genetics
Studies with twins have identified a strong genetic component in ASD. If one identical twin has autism, chances are extremely likely the other twin will too. Some key facts about autism’s genetic links include:
- ASD has high heritability – studies estimate 50-95% can be attributed to genetic factors
- In families with one child with ASD, estimated recurrence risk for future siblings is 18.7%
- At least 100 genes have been associated with increased ASD risk
However, most cases of ASD are complex, involving variations in many genes. Other familial factors, like shared environment, also contribute. Overall though, individuals with a sibling or parent with ASD are at substantially higher risk.
Fragile X syndrome
Fragile X syndrome is the most common known single-gene cause of autism spectrum disorder. It is caused by mutations in the FMR1 gene on the X chromosome. Key facts about Fragile X include:
- About 2-6% of children with ASD have Fragile X syndrome
- Approximately 30% of individuals with Fragile X also meet ASD diagnostic criteria
- Fragile X is the leading genetic cause of intellectual disability
Because Fragile X is X-linked, it affects males more frequently and severely. Girls with Fragile X have about a 20-50% chance of also having ASD.
Preterm birth
Several studies show an association between preterm birth (before 37 weeks gestation) and increased ASD risk. Key facts include:
- Children born preterm are about twice as likely to be diagnosed with ASD
- Very preterm infants have 3-4 times higher risk
- ASD prevalence is higher in preemies with very low birth weight
The reasons are still being investigated, but may involve atypical brain development in the third trimester or complications requiring intensive neonatal care.
Older parents
Advanced parental age has been linked to increased autism risk in multiple studies. Some key findings on parental age include:
- Children of parents over age 35 are at higher risk for ASD
- With each 10-year increase in father’s age, risk may increase by about 10%
- High maternal age mainly influences risk when combined with advanced paternal age
Possible reasons include age-related mutations and epigenetic alterations to sperm, reduced ovarian reserve and dysregulation, and social/environmental factors.
Siblings with autism
Having a sibling with autism spectrum disorder significantly increases an individual’s risk of also having ASD. Key facts include:
- Recurrence risk for siblings is estimated at 18.7%, much higher than general population
- Later-born siblings are more likely to be diagnosed than firstborns
- Differences in severity between siblings in 50% of cases
ASD has a strong genetic component, so shared genes likely account for the higher sibling risk. Environmental factors may also be a factor.
Minority backgrounds
Research on differences in ASD prevalence across racial, ethnic and socioeconomic groups has had mixed results. However, some key patterns have emerged:
- Black and Hispanic children tend to be diagnosed at later ages than white children
- Diagnostic disparities may contribute to observed differences in prevalence
- Children of disadvantaged backgrounds face barriers accessing quality screening and intervention
Overall, strong evidence suggests autism occurs in all groups. However, gaps in recognition, diagnosis and support must be addressed.
Why are these groups at higher risk?
In many cases, the reasons certain groups are more likely to have autism are not fully understood. However, researchers have identified some potential biological and social factors that may explain the differences in risk.
Biological factors
Several biological factors may account for some of the differences in autism risk and prevalence between groups:
- Genetics – Gene mutations and variants conferring risk likely contribute to sibling recurrence and sex differences.
- Hormones – Fetal testosterone exposure may increase susceptibility in males.
- Brain structure – Sex differences in brain anatomy, connectivity may play a role.
- Prenatal environment – Inflammation, maternal antibodies, medications and complications may impact fetal brain development.
However, biology does not fully explain group differences. Broader social and cultural factors likely also contribute.
Social factors
Some patterns in autism diagnosis may stem from social and cultural influences:
- Awareness – More public knowledge about autism likely contributes to rising prevalence.
- Screening practices – Increased and earlier screening accounts for more diagnoses.
- Diagnostic practices – Potential racial, ethnic and gender bias may affect who gets diagnosed.
- Access barriers – Disadvantaged groups face more barriers to obtaining high-quality diagnostic services.
- Stigma – Cultural stigma may deter some families from seeking help.
Addressing these social factors will help close gaps and ensure all groups have access to timely, unbiased identification and support.
Key Takeaways
In summary, key points about autism risk include:
- Boys are around 4 times more likely to have ASD than girls.
- Children with a sibling or parent with ASD are at substantially increased risk.
- Premature infants have higher rates of autism later in childhood.
- Advanced parental age is linked to increased ASD risk.
- Minority groups may be underdiagnosed and underserved.
While progress has been made, the causes of these group differences are still not fully understood. Ongoing research to uncover genetic and neurobiological factors, as well as efforts to address social inequities, will help provide answers.
Conclusion
Autism spectrum disorder is complex condition that manifests differently in each individual it affects. However, research has revealed some patterns in who is most likely to have ASD. Male sex, family history, premature birth, older parental age and minority background all confer increased risk. A mix of genetic, neurobiological and social/cultural factors likely contribute to these vulnerabilities. Understanding the influences on autism risk can help guide future research and ensure equitable access to services. While our knowledge has advanced greatly, there is still much more to learn about the causes of ASD and how we can best support individuals across the spectrum.