Autism is more common in males than females. For those individuals diagnosed with autism, males are four times more likely to receive the diagnosis. The symptoms and characteristics of autism may present themselves in a wide variety of combinations, ranging from mild to severe. Although autism is defined by the presence of a certain set of behaviors, individuals with autism may exhibit any combination of these behaviors, varying in degree of severity. Therefore, two individuals with a diagnosis of autism may act very differently from one another.
Autism is more common in some families than in others.
Studies have shown that the prevalence of autism among siblings of individuals with autism is substantially higher than in the general population. Particularly in twin studies, if one identical twin has autism, it is likely that the other one does as well. Additionally, even when a history of autism is not found within a family, there may be a higher than expected prevalence of cognitive and social impairments within the family. These findings strengthen the hypothesis that some characteristics of autism may have a genetic base. It is conservatively estimated that 400,000 individuals in the United States are diagnosed with autism.