focus was on the incidence of autism spectrum disorder in the twin population in comparison to the non twin population. The following two studies, explored the risk of Autism spectrum disorder in the twin population. The conclusion that the twinning process alone is not a risk factor was drawn. In these studies the data exemplified that both MZ twins will have autism spectrum disorder, but only one of the DZ twins will have autism spectrum disorder with an incidence rate of 90% in MZ twins compared to 0% in DZ twins. The high symmetry in MZ twins can explain the high symmetry of autism spectrum disorder in MZ twins outcome compared to DZ twins and non twin siblings. Proposed models Twin and family studies show that autism is a highly heritable condition, but they have left many questions for researchers, most notably Why is fraternal twin concordance so low considering that identical twin concordance is high? Why are parents of autistic children typically non-autistic? Which factors could be involved in the failure to find a 100% concordance in identical twins? Is profound intellectual disability a characteristic of the genotype or something totally independent? Clues to the first two questions come from studies that have shown that at least 30% of individuals with autism have spontaneous de novo mutations that occurred in the father's sperm or mother's egg that disrupt important genes for brain development. These spontaneous mutations are likely to cause autism in families where there is no family history of the disorder. The concordance between identical twins isn't quite 100% for two reasons, because these mutations have variable 'expressivity' and their effects manifest differently due to chance effects, epigenetic, and environmental factors. Also spontaneous mutations can potentially occur specifically in one embryo and not the other after conception. The likelihood of developing intellectual disability is dependent on the importance of the gene to brain development and how the mutation changes this function, also playing a role is the genetic and environmental background upon which a mutation occurs. The recurrence of the same mutations in multiple individuals affected by autism has led Brandler and Sebat to suggest that the spectrum of autism is breaking up into quanta of many different genetic disorders. Single genes The most parsimonious explanation for cases of autism where a single child is affected and there is no family history or affected siblings is that a