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