As head of clinical programs at Autism Speaks, I oversee a number of vital resources for researchers studying the causes and treatment of autism. Today brought the publication of a new and revealing study made possible by Autism Speaks’ Autism Genetic Resource Exchange (AGRE).

Autism researchers have been studying twins for years for insights into the genetic and nongenetic factors that influence the development of autism. One of the most powerful ways to do so is to study twins (both identical and non-identical) where at least one of the pair has autism.

This approach allows us to look at how often both twins receive a diagnosis of autism.  The study of identical twins, who share 100 percent of their genes, then helps us determine the degree to which autism is inherited or genetic; and comparison to fraternal twins, who share around 50 percent of their DNA, allows us to understand how environmental influences add to the risk of autism spectrum disorder (ASD).

But until now, we’ve had only three small twin studies, which together looked at just 66 twin pairs–a number too small to produce reliable conclusions. Still, these studies were the best we had, and they suggested that when one identical twin develops an ASD, the chance of the other twin developing the disorder is as high as 90 percent. These same studies showed little to no overlap among fraternal twins – leading to the conclusion that inherited genes alone produced the risk.

Now comes the game changer. The California Autism Twins Study (CATS) is the largest ever study of twins with ASD, with scientifically reliable information on 192 twin pairs, both identical and fraternal. It was conducted by a group of renowned researchers in collaboration with the AGRE team. AGRE clinical staff collected DNA and helped perform the home-based diagnostic and cognitive testing on many of the participants, using scientifically validated research measures for diagnosing ASD.

So what were its dramatic findings?

It found that when one identical twin develops autism, the chance of the other twin developing the disorder is 70 percent. More surprisingly, it documented a whopping 35 percent overlap among fraternal twins. This is strong evidence that environmental influences are at play. Moreover, the 35 percent “both twins affected” rate is higher than the 3 percent to 14 percent overlap between different age siblings. (i.e. If one child in a family has autism, there is a 3 percent to 14 percent chance that a younger sibling will develop it.) This suggests that there are environmental influences uniquely shared by twins–for instance, in the womb and perhaps during birth.

In other words, we now have strong evidence that, on top of genetic heritability, a shared prenatal environment may have a greater than previously realized role in the development of autism in twins

This has important implications for future research. For instance, is there a particular time period during the pregnancy when a child’s brain development is particularly vulnerable to environmental influences? And what might these influences be? Already we have evidence implicating such factors as advanced parental age, maternal nutrition, maternal infections (especially flu) during pregnancy, and premature and/or underweight birth. Indeed, multiple-birth pregnancies are themselves associated with increased risk of developmental disorders such as cerebral palsy and autism.

Only by further studying these issues can we begin to provide parents and parents-to-be with the reliable guidance they seek and need.  Autism Speaks is currently investing in several studies that are exploring how environmental factors increase the risk for ASD.  As we go forward in these endeavors, we greatly value your input.

Originally posted on Autism Speaks.