In summary, there are a few specific genetic conditions that can be associated with autism. Among cases of unknown etiology, there is ample evidence for a higher genetic liability to autism in siblings of autistic probands than expected from the population prevalence. It appears likely that both parents and siblings have a higher liability for social and cognitive deficits that are milder but conceptually similar to those found in autism. Others factors may alter this underlying genetic liability such as sex, IQ, and prenatal and perinatal injury. In the future, genetic analyses and genetic linkage studies will need to consider using a broader definition of the autism phenotype to include not only autism but severe cognitive and social deficits. The exact genetic mechanisms and genes involved are the subject of current investigations by several research groups. Investigations in this area are likely to continue to provide important information about the causes of autism.