April is National Autism Awareness Month. One of the biggest goals of autism research is to determine its cause, and one of the best ways to achieve that is to rule out the things that do not cause it. So, let's acknowledge this month by listing all the things that do not cause autism.
Vaccines. A substantial proportion of people refuse to accept the reality that vaccines do not cause autism. This was never controversial in the scientific community. After the fraud Andrew Wakefield published his sham "study" linking vaccines to autism, the New England Journal of Medicine published the results of an absolutely enormous study (including more than 537,000 children) that thoroughly refuted Wakefield. Since then, the evidence has grown even stronger, concluding once and for all that vaccines are not a cause of autism.
Child Car Safety Seats. At least one lunatic believes that strapping a child into a car safety seat increases the risk of autism. How so? Chemicals. To prove his case, he shows that as the usage of car safety seats has increased over the years, so have cases of autism. QED. There's only one problem: A lot of things have increased over the years, such as sales of organic food. Possibly organic carrots cause autism, too?
Parenting style. Some people make great parents, while others stink at it. But parenting style itself won't cause autism, as was once elaborated by the "Refrigerator Mom" hypothesis. Instead, parenting style can ameliorate or exacerbate the symptoms of autism.
So, What Does Cause Autism?
For absolute certain, genetics plays a role in autism. As many as 1,000 genes may be involved. According to a paper in the Annual Review of Public Health, the genetic contribution to autism ranges from 50% to 95%. The tricky thing, however, is that each gene likely plays only a minimal role. A small tweak here, another small tweak there. We will never discover "the autism gene," which makes research incredibly difficult.
Adding another layer of complexity is the role of environmental factors and gene-environment interactions. It is entirely plausible that some environmental exposure X, if in the context of a developing fetus with genetic factor Y, creates an interaction in which autism is more likely to develop.
What sort of environmental exposures are associated with autism? Just like cancer, the research is all over the place, which is a bad sign. If epidemiologists can link just about anything to autism, then we aren't really any closer to finding the biggest causes. All of the following, with varying levels of evidentiary support and taken from the aforementioned paper in Annual Review, have been blamed for autism:
Endocrine disruptors (which, themselves, have been blamed implausibly for everything from cancer to obesity)
While factors like parental age make sense, it is difficult to believe that some of these other factors plays a substantial role in the development of autism. Both organic and conventional pesticides, for instance, are used at safe levels in the U.S. Similarly, the U.S. has some of the cleanest air in the world.
Therefore, if these factors play any role in autism, it is likely to be at the margins, especially compared to the earlier figure that autism appears to be 50% to 95% genetic. Be skeptical of claims to the contrary.