There appears to be a strong genetic component to autism. For example, almost 20% of younger siblings of a child with ASD will receive a diagnosis themselves, which is much higher than the CDC-reported 1% to 2% risk among the general population. Researchers continue trying to determine both the genetic and non-genetic environmental factors that contribute to autism—about 15% of children with ASD have been identified as having a genetic disorder, such as Fragile X syndrome, tuberous sclerosis, Down syndrome, or other chromosomal abnormalities, copy number variants, and single-gene mutations.
It is expected that as genetic testing becomes more sensitive, the percentage of individuals with a genetic cause will increase, however, non-genetic factors (such as exposure to a maternal immune response in the womb or complications during birth) have also been found to play a role.
To better understand the similarities and differences between Fragile X syndrome and autism, read this.
You may also want to read our treatment recommendation document, Understanding Autism Spectrum Disorder in Fragile X Syndrome.