Over the past two decades, research has supported the idea that autism has a strong genetic component. The strongest evidence comes from studies of twins that show that identical twins have a 90 percent chance of sharing a diagnosis of autism. Unfortunately, these studies have been too small to demonstrate the genetic risk for autism in fraternal twins. In addition, these studies have not been able to establish whether people with autism inherit susceptibility for a general diagnosis of autism or whether they inherit susceptibility for specific autism traits and behaviors. This funding will provide additional support to Dr. Hallmayer's group to conduct the largest, most detailed, population-based study of twins with autism, examining 300 pairs of twins across California. They will measure autistic-like behaviors; repetitive and stereotyped behaviors; general cognitive ability; and more specific neurocognitive abilities. This funding will allow Dr. Hallmayer's team to increase the number of twin pairs studied. In addition, it will allow them to measure social and communication abilities with the Social Responsiveness Scale and to collect additional blood samples from family members that can be used to search for specific genes. Such a large sample, together with detailed assessments, will allow the researchers to verify earlier estimates about the heritability of autism. It will also help them tease apart which traits and symptoms of autism are genetic and which are more likely influenced by the environment. What this means for people with autism:This study will not only vastly improve estimates of the heritability of autism, but will also allow researchers to determine which behavioral and cognitive traits are specific to autism, which are mostly determined by genes, and which are more influenced by the environment. Such data will help genetics researchers by providing them with unambiguous genetic subgroups in which to search for genes for specific traits.