There has been much discussion of twin studies in autism research for a long time. The reason is that if is found that “identical” (monozygotic) twins are often both autistic, that points to genetics as a major influence on the development of autism. For many years it was thought that this rate, the concordance, was about 90%. In other words, if one child is autistic, 90% of the time the other child is autistic. This was based on a number of older, small studies. More recently, a relatively large study showed a lower concordance: about 77% for ASD and 60% for autism. From this the authors claimed that the genetic contribution to autism risk was lower than previously thought, and that the environmental contribution was higher (about 55% environmental contribution).
A study just out from Denmark claims a concordance more in line with the older studies–95%. In A Danish population-based twin study on autism spectrum disorders., the authors write: