TWIN AND FAMILY STUDIES
In the first systematic and detailed autism twin study, conducted by Dr. Susan Folstein and Dr. Michael Rutter, the rate of concordance was compared between identical twins and fraternal twins. Concordance in this instance refers to the likelihood that if one twin has a diagnosis of autism, the second twin will also have a diagnosis of autism. Because identical twins share 100% of their genes, whereas fraternal twins share on average 50% of their genes, a higher concordance rate among identical twins is evidence for genetic influence. Dr. Folstein and Dr. Rutter found that the concordance rate for autism was significantly higher among the identical twins they studied, and subsequent twin studies have confirmed this finding. In general, the concordance rate for fraternal twins is similar to the 5-8% recurrence rate observed among non-twin siblings. Concordance rates among identical twins are estimated to be approximately 60%, but have been reported to be as high as 95%. The fact that identical twins are not always concordant for autism indicates that there may be non-genetic factors that are important as well, but the high concordance rates are strong evidence for significant genetic influence. The results of family studies, which have shown increased rates of autism among siblings and first degree relatives, are also an indication of the role that inherited factors play in the development of autism.