Sometimes a bad idea just won't die. Despite the overwhelming scientific evidence that there is no link between autism and vaccines, and the complete lack of evidence to the contrary, many US parents are still worried to the point that they aren't getting their children inoculated. It was even discussed in this week's Republican party debate. Why can't we shake this widely-disproven theory?
Part of the problem might be the fact that we actually don't know all that much about what does cause autism spectrum disorder (ASD). If we did, would we finally be able to put the vaccine issue to bed?
"I do think that because not very much is known about the causes, people feel sort of free to guess wildly," Andrea Roberts, a Harvard public health researcher who studies the causes of autism, told me over the phone. "Even though there's very strong evidence that vaccines are not related to autism, I think if we had more of a smoking gun it would draw people's attention to something else."
Without explanation for why a child starts to develop signs of autism, it's understandable that rational, educated people would try to find their own answers. Children get a lot of shots early in their life, around the same time when signs of autism usually start to emerge, so it's not completely crazy to assume there's a link there, even if it's incorrect.
But this assumption is dangerous from a public health standpoint. The good news is that the majority of children starting kindergarten in this country are up to date on their vaccinations, according to the Centers for Disease Control and Prevention. Though the numbers vary by state, most were above 90 percent vaccination rates for the 2014-2015 school year.
That said, the percentage needed for herd immunity (where enough people are vaccinated that the disease won't spread to those who can't get vaccinated, like babies) for many diseases is as high as 95 percent. And the dip in vaccination rates, particularly in certain pockets of the country, was enough to contribute to a measles outbreak at the end of last year, and an increase in cases of whooping cough.
Roberts said there has been a lot of research on the causes of autism, and it's not like we don't know anything about the condition. Most of the research shows causes occur in the womb and are a combination of genetics and, possibly, the health of the mother. More than 100 gene mutations have been identified as being linked to autism and studies have found a connection between the metabolic health of the mother and children who later develop autism spectrum disorder.
The trouble is, autism spectrum disorder is just that: a spectrum. It affects individuals differently and is most likely caused by a number of different combining factors, rather than one specific gene. A genetic mutation identified might only account for 1 percent of all autism diagnoses, if that, Roberts said. And besides, many autism advocates would argue that ASD isn't a disease that can be cured or prevented, or that doing so would be worth the trade-off.
"Autism itself is not a single thing. It's a set of behaviors and traits," Roberts said. "It's possible autism is actually a whole set of disorders, different kinds of autism that might have different causes."
There's a reason the symbol for autism spectrum disorder is a puzzle piece. As one autism group explained, "the puzzle piece symbol reflects the mystery and complexity of Autism Spectrum Disorder (ASD). Also, since every puzzle piece is different in some way, a puzzle piece accurately represents the diversity of the individuals affected."
So would a better understanding of autism be the key to finally shaking loose the idea that vaccines are to blame?
"That would probably kill it for good," said Michael Shermer, a research who has published multiple books on the science of human beliefs.
Shermer explained there were a number of factors that keep this idea alive. One is this need for an explanation: in the absence of a satisfying answer from medicine, people look elsewhere to fill that need, he said. Another is the power of anecdotal evidence: there are plenty of parents who insist their child developed autism as a result of vaccines. Even Donald Trump had a similar anecdote during this week's debate.
There's also a need to feel like we have control over what happens to us and our children, Shermer said.
"We naturally want to do something about it," Shermer told me over the phone. "That's why with things like Alzheimer's disease, a doctor might tell you to do sudoku puzzles or exercise. The data showing that those things make any difference is very thin, but people still ask, 'can't I do something?'"
Shermer said if we were to have a concrete set of explanations for what causes autism—and in particular anything we might be able to do to prevent it—that would finally end the popular belief that it's linked to vaccines. But it wouldn't be the end of pervasive bad ideas.
"People would probably link vaccines to something else," Shermer said. "These things are very fad-ish. They come and go, these kinds of panics. I think it would just morph into something else."