Scientists have known that autism is a combination of genes and the environment for a long time. But the focus has been on genes because early twin studies suggested that autism was mostly genetic.
Scientists looked at sets of identical and fraternal twins to see how often both twins in a pair had autism. Remember, identical twins have the exact same DNA whereas fraternal twins only share as much DNA as any other siblings.
If autism were purely genetic, then both identical twins in a pair should either have it or not. It should be very rare for one twin in a pair to have autism and the other to not have autism. Fraternal twins should have it at about the same rate as any other siblings.
But if autism were purely environmental, then both twins in an identical or fraternal twin pair should get it at about the same rate. Depending on what part of the environment is causing the problem, this rate might be higher than that of siblings.
A study back in the 1970’s found that both twins in an identical pair had autism 72% of the time and that both fraternal twins never had it at the same time. This is where the 90% heritability for autism number came from.
The fact that fraternal twins never both had autism was weird from the start. Scientists knew that if one sibling had autism, the risk for the other siblings was anywhere from 3-14% which is higher than the general risk. Fraternal twins are siblings and so there should probably be some increased risk too.
In a new study, scientists did a more extensive study on 192 twin pairs and arrived at very different results. In this study, male identical twins both had autism 58% of the time and male fraternal twins both had autism 21% of the time. (Female numbers were similar.) These numbers suggest that genetics accounts for about 37% of autism. Still significant but nowhere near 90%!
If this study holds up, it means is that scientists can start looking at environmental effects. They’ve ruled out vaccines as a cause but there are lots of other possibilities. And many of these may happen before the child is even born.
For example, it may be that like Down syndrome or schizophrenia, parents’ age is a factor. Or it may be that diseases mom might have had or chemicals she might have been exposed to while pregnant could increase chances for autism. Or a host of other possibilities might be responsible.
What is important to keep in mind is that if scientists can identify an environmental cause, they can try to keep expectant mothers away. Or try to ameliorate the effects. In many cases, this will be much easier to deal with than genes.
For more, read
Sometimes autism that looks environmental can be genetic from Undestanding Genetics.