What causes autism? What we know, don’t know and suspect
To examine the influences of nature (genetics) and nurture (environment) on a given human quality, scientists study twins.
To appreciate how these studies work, it’s first important to understand there are two types of twins. Identical twins share all of their DNA and, assuming they grow up in the same household, they will also share all of their environment. Fraternal twins also share all of their environment, but only around half of their DNA, just like non-twin siblings.
Twin studies start by defining a clear population, say the metropolitan area of a city, and finding as many sets of twins as possible in that area where one or both of the twins have the given trait of interest – in this case, autism.
Scientists then look at the “concordance” of that trait – that is, the percentage chance that if one twin has autism, the other twin will also have autism. If the concordance is higher for identical twins than fraternal twins, then we can say the difference is due to the increased amount of genetic material shared by the identical twins, and that autism is influenced by genetics.
The first twin study of autism was conducted in 1977 on 11 identical and ten fraternal twins across Great Britain, where at least one of the twins had autism. Concordance for identical twins was 36%, compared to 0% for the fraternal twins.
While the study was only small in size, it provided the first evidence that autism may be genetic in origin. Since this pioneering study, more than a dozen further twin studies have confirmed this original observation.