Explain the two main types of studies that showed the genetic basis of ASDs.

First, family studies involving non-twin siblings reveal a genetic basis. These family studies show higher than expected rates of autism among family members. For example, the rate of recurrence of autism in siblings of autistic individuals is 2% to 6%, which is 10 to 60 times greater than the prevalence of autism in the general population. Second, researchers studying the genetics of autism also have compared the incidence of autism in monozygotic (identical) and dizygotic (fraternal) twins. If one monozygotic twin has autism, then there is a 75% chance that the other twin will have autism. In contrast, if one dizygotic twin has autism, then there is a 3% chance that the other twin will have autism. (Note that this falls within the 2% to 6% rate of recurrence among non-twin siblings.) These comparative findings indicate a genetic basis for autism because the incidence among twins is higher when they are nearly genetically identical.