Autism is a neurodevelopmental disorder characterized by limitations in social interaction and communication in combination with stereotypic, repetitive behavior and restricted interest [APA, 2013]. The symptoms usually emerge in early childhood, before the age of three. The prevalence of the disorder appears to be on the rise over the last decades, with as many as 1 in 68 individuals affected according to the most recent estimates [Surveillance, 2014]. Consistent among all prevalence estimates, more boys than girls are affected. All population studies indicate a significant contribution of genetic components underlying the disorder. While the heritability—the proportion of phenotypic variation explained by genetic factors—was once estimated to be as high as 90% in the first reported twin studies [Folstein and Rutter, 1977; Steffenburg et al., 1989], there is now a consensus that these initial studies may have overestimated the genetic contribution of the disorder. The largest population based study so far reports a heritability of 50% with an increased risk of recurrence of about 10-fold for a first degree relative and of about 2-fold for cousins [Sandin et al., 2014].
