New research from Stanford indicates that scientists have been underestimating the influence of a child's environment on autism risk.
The new study, led by Joachim Hallmayer, MD, appears this week in the Archives of General Psychiatry. It used pairs of twins to tease apart the relative contributions of genes and non-genetic environmental factors to autism risk. Whereas older studies said autism risk was 90% genetic, the new work estimates autism risk is 38% genes and 62% environment. In this context, "environmental" risk factors are any non-genetic contributors to a child's disease risk from conception onwards.
The new study is the largest of its kind, is better controlled than prior research and draws on a more diverse population than has ever been examined in the past.
From a press release I wrote about the research:
"Our research shows us that we need to be studying both genetic and environmental factors as well as how they interact with each other," Hallmayer said. "We need to explore areas of environmental risk that are shared by both twin individuals and impact the development of the child."
To get some perspective on the implications of this study, I turned to another Stanford scientist, Antonio Hardan, MD. Hardan was not directly involved in the research, but he did discuss the study's results with Hallmayer during the research process. Hardan also treats patients with autism at Lucile Packard Children's Hospital. Here's our Q&A:
What was your reaction to these findings?
The surprise is that we're seeing less impact of genetics in autism risk compared to what we've believed so far. That's the main finding from the study.
You can always say, this needs to be replicated, but I think it might be the reality that we've all along overestimated the true heritability of autism.
Some people might say that this study provides fresh evidence for the reputed link between vaccines and autism, even though other research has not been able to support such a link. What would you say to that claim?
There is no evidence to date to show that vaccines are linked to autism – this is supported by 10 to 20 different studies that looked at this relationship quite extensively. The way I think about it is that when you're debating with someone, you can provide the evidence, but that may not be enough to convince them. We have evidence beyond reasonable doubt to say that vaccines and mercury exposure are not linked to autism, but some people still might not believe those findings.
Was the prior research on environmental contributors to autism too heavily slanted toward vaccines?
If you look at how many papers were published on the vaccine issue, this is manpower and financial support that was not spent wisely. Instead of doing 20 papers on vaccines, it would have been nice if we had five studies on vaccines, five studies looking at the effect of multiple birth, some studies on maternal infection during pregnancy, and so on. That's why we should move ahead cautiously - there is this potential for overreaction that will lead to efforts that are not very wisely focused.
I know the present study is agnostic with respect to which specific environmental risk factors are important, but, based on your knowledge of other research, what environmental risk factors do you think hold promise as plausible contributors to autism?
Other people have looked at, for instance, mothers' between-pregnancy intervals – the shorter the interval, the higher the risk of autism in the second child. There was another paper by the MIND Institute that showed that taking folate early in pregnancy is linked to decreasing autism risk and, conversely, not having prenatal care or not taking folate early in pregnancy is linked to higher risk for autism. These are the kind of things we have to be thinking of.
Some of the other environmental factors that have been suggested include low birth weight, older parental age, multiple birth or obstetric complications. These factors have been linked to other disorders such as schizophrenia and bipolar disorder. If we're going to think about future directions in research, we should investigate these further. There are also some environmental factors that are less supported by evidence, such as allergy to gluten – we don't want to forget those, but we should keep it balanced in terms of how we investigate different risk factors.