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If environmental factors can lead to autism, why does only one of my twin boys have autism?

This week’s “Got Questions?” response comes from Alycia Halladay, PhD, Autism Speaks’ director of research for environmental science.
Today’s question came in response to my last blog post. In it, I explained that when scientists talk about the “environmental factors” that increase the risk of a disorder, they’re referring to pretty much any influence beyond genetics.
In the case of autism, the clearest evidence of environmental influence seems to surround very early events such as conception, pregnancy and birth. Those with the strongest link include parental age at time of conception (both mom and dad), maternal nutrition or illness during pregnancy, and certain birth complications.
The commenter’s question is a great one that scientists are actively exploring. The short answer is that inherited genes (DNA) and environmental factors seem to interact to influence whether an infant goes on to develop autism. So if the commenter’s twins are fraternal (meaning they share about half their DNA), the difference in their genetic makeup might explain why only one developed autism.
But what if the boys are identical twins–meaning they share exactly the same DNA? In this case, something beyond genes likely accounts for the different outcomes. Comparing the rates of autism among identical and fraternal twins provides clues.
In July, researchers used our Autism Genetic Resource Exchange (AGRE) to complete the largest autism twin study to date. They found a 70 percent overlap in autism among identical twins and a 35 percent overlap among fraternal twins. That overlap between fraternal twins is much higher than the estimated 19 percent overlap between different-age siblings.
These numbers tell us that it’s not always genes alone that determine whether a child develops autism. If it were, two identical twins would always share the same outcome, and the rate of a shared autism among fraternal twins would look more like that for different-age siblings. So we conclude that shared environmental influences are also at play.
Although twins share very similar pregnancy and birth environments, those environments aren’t exactly the same. For example, twins can have different positions in the womb or different placentas, and this can affect such environmental influences as blood and oxygen flow. Indeed, twins often have different birth weights, a known risk factor for autism.
It’s important to remember that “environmental” influences such as these don’t cause autism by themselves. Rather, if a child has a genetic predisposition for developing autism, these influences may further increase the risk.
Autism Speaks continues to fund and otherwise support research on both genetic and nongenetic risk factors for autism. EARLI is a network of researchers who follow mothers of children with autism beginning at the start of another pregnancy. IBIS is a study of early brain development in the younger siblings of children with autism. These studies depend on the participation and support of the autism community.  Please visit our Participate in Research page to learn more.
Importantly, these studies provide insights into the underlying biology of different types of autism. This in turn becomes a basis for developing ways to treat and possibly prevent autism. As always, the goal of the research we support is to improve the lives of all on the autism spectrum.
And thanks for the question. Please keep them coming.