Autism spectrum disorder (ASD) is a complex neurological and developmental condition that impacts how individuals interact, communicate, learn, and behave. It involves persistent challenges with social communication, along with restricted interests and repetitive behaviors. ASD’s origins are multifactorial, stemming from a combination of influences.
Genetic Factors in Autism
Genetics play a significant role in autism. Heritability, the proportion of trait differences explained by genetic variation, is substantial. Studies consistently estimate autism’s heritability to range from approximately 60% to 90%.
Autism is often described as polygenic, indicating that many genes contribute to its development rather than a single “autism gene.” Each common genetic variant typically confers only a small increase in an individual’s chances of having the condition. However, when combined, these variants can collectively raise the overall likelihood. Only about 10% of autistic individuals have an identifiable genetic cause.
Inherited Genes and New Genetic Changes
Autism’s genetic contributions can originate from either parent. These can include common genetic variants or rare inherited mutations. For instance, children with autism spectrum disorder (ASD) tend to have a higher average polygenic risk score, indicating they have inherited more contributing common variants from their parents.
Beyond inherited genes, many autism cases involve de novo mutations. These new genetic changes occur spontaneously in the child and are not present in either parent’s DNA. De novo mutations are a key reason why autism can appear in families with no prior history. For example, in families with a single individual with autism (simplex families), de novo mutations may contribute to a substantial percentage of cases, with some estimates suggesting a range of 52% to 67%.
Advanced parental age is associated with an increased risk of de novo mutations. Both paternal and maternal age can influence the number of these new mutations in offspring, though paternal age generally has a larger effect. For instance, fathers accumulate de novo mutations at a faster rate than mothers, with one study indicating men gain approximately 1.51 de novo mutations per year compared to 0.37 for women. While these mutations contribute to an increased risk, the absolute risk of autism remains relatively small, even for children born to older parents.
Environmental and Interacting Influences
While genetics are a primary driver, environmental factors also play a role in autism, often interacting with an individual’s genetic predispositions. Environmental factors encompass a range of non-genetic influences. These can include prenatal exposures, such as certain infections during pregnancy, maternal health conditions like obesity or diabetes, and exposure to certain medications.
These environmental factors do not typically cause autism in isolation. Instead, they can increase the risk in individuals who are already genetically predisposed. For example, studies indicate that exposure to air pollution or certain pesticides before birth, or maternal immune system disorders, may increase autism risk. The interplay between these external influences and an individual’s genetic makeup is a complex area.
Understanding the Complex Picture of Autism Causes
Autism spectrum disorder is rarely due to a single cause or inherited simply from one parent. This intricate interplay means that while there is an increased likelihood of recurrence for siblings of an autistic child, it is not guaranteed.
For instance, a recent study reported a 20.2% recurrence rate of ASD in children who have a younger sibling with autism, which is significantly higher than the general population rate of 2.8%. Male infants and those with multiple affected older siblings have an even higher likelihood of developing ASD. Ongoing research continues to unravel these complexities, aiming to better understand the factors that contribute to autism.