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Comment on: A Danish population-based twin study on autism spectrum disorders.

Posted May 12 2013 2:39am

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:

Genetic epidemiological studies of Autism Spectrum Disorders (ASDs) based on twin pairs ascertained from the population and thoroughly assessed to obtain a high degree of diagnostic validity are few. All twin pairs aged 3-14 years in the nationwide Danish Twin Registry were approached. A three-step procedure was used. Five items from the “Child Behaviour Checklist” (CBCL) were used in the first screening phase, while screening in the second phase included the “Social and Communication Questionnaire” and the “Autism Spectrum Screening Questionnaire”. The final clinical assessment was based on “gold standard” diagnostic research procedures including diagnostic interview, observation and cognitive examination. Classification was based on DSM-IV-TR criteria. The initial sample included 7,296 same-sexed twin pairs and, after two phases of screening and clinical assessment, the final calculations were based on 36 pairs. The probandwise concordance rate for ASD was 95.2 % in monozygotic (MZ) twins (n = 13 pairs) and 4.3 % in dizygotic (DZ) twins (n = 23 pairs). The high MZ and low DZ concordance rate support a genetic aetiology to ASDs.

This study is relatively small with only 13 “identical” twin pairs. Also, the concordance for “fraternal” (dizygotic) twins is relatively low at 4.3%. Sibling concordance is estimated at about 20%, so 4.3% raises a bit of a red flag. Of course the recent larger twin study is not without some controversy itself.

In the end, I doubt this new study will have much influence on the online parent community discussions (which are in themselves far from the most productive or important discussions on the topic. Just the apparently most vocal). We are left with there being some genetic contribution and some environmental contribution to autism risk. In other words, it remains important to put effort into both areas of research.

By Matt Carey

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