Previous studies have suggested that among affected sib pairs with autism there is an increase in the frequency of twins over what would be expected in comparison to the prevalence of twins in the general population. In this study we sought to determine whether sub-threshold autistic traits were more pronounced in twins than in non-twins. The Social Responsiveness Scale (SRS) was administered in an epidemiologic twin sample (n=802) and in a separate population-based sample of non-twins ascertained from a local school district (n=255). For males (but not females), the mean SRS score was significantly higher among twins than among non-twins. As has been suggested for autism, twin status may incur increased liability to subthreshold autistic symptomatology, particularly in males. (Ho, A., Todd, R. D. & Constantino, J. N. (2005). Brief report: Autistic traits in twins vs. non-twins–a preliminary study. Journal of Autism and Developmental Disorders, 35(1), 129-33.)
Higher incidence of autism among twins, of course, suggests genetic origins, though embronic environmental influences come into play. A couple things come to mind reading this passage.
Studies seeking genetic causes of autism point to twins studies as integral to their conclusion that autism is largely a genetic condition. Nevertheless, the relationship between genetics and environment is still very much in flux. Evolutionary developmental biology is just beginning to open those doors.
Second, is there a smaller percentage of identical female twins with at least one twin exhibiting autism or Aspergers than the normal percentage of females to males? In other words, if it is a 4/1 ration male/female, and the number of males to females that are autistic that are identical twins is different that 4/1, then perhaps the etiology of female autism is significantly different than male autism.
