It’s hard to believe that mere decades ago, some studies purportedly “showed” that ice cream caused polio. One doctor concluded that it must be the sugar. There was even an anti-polio diet that called for withholding the frozen treat from children, along with sodas.
How could anyone reach such a conclusion?
Scentists had noticed that polio rates closely mirrored the sales of ice cream, rising when sales were more robust and falling when they went down, according to sources including Smithsonian magazine and the textbook "Introductory Statistics and Analytics: A Resampling Perspective."
It’s common for people to assume that when two things occur together, one is causing the other. There’s even a Latin phrase for this logical fallacy: post hoc ergo propter hoc, meaning that because one thing followed another, the first thing must have caused the second. In truth, though, the coincidence is called correlation, and it doesn’t prove anything about causation. That’s a truth statistics teachers pound into their students repeatedly: “Correlation does not imply causation."
In the case of polio, there was what’s called a third, “confounding factor”: the weather. Ice cream sales grow when weather is warm. That was also when children were outdoors more, playing in often unsanitary conditions.
This is a story worth remembering right now, at the confluence of Autism Awareness Month and National Infant Immunization Week, to review why the debunked association between autism and vaccination persists in some people’s minds and keeps too many parents from vaccinating their babies. The public sees, or thinks it sees, a correlation between vaccination and autism rates. But that doesn’t mean the first caused the second.
Correlation is a useful way to look for possible causes in science. But when scientists note that two things appear linked, they try to eliminate the confounding factors before reaching any conclusions that there is a possibly meaningful connection between the two. And even then, they’ll conclude that the two are associated, not that one caused the other; it takes further study, usually with randomized groups, to determine cause.
The polio story makes it easy to understand why, when Andrew Wakefield published his fraudulent study in 1998 in the medical journal Lancet—it was later retracted—saying that the MMR vaccine might be linked to autism-spectrum disorders, parents were receptive to the idea that one caused the other and some have clung to that belief despite dozens of well-conducted studies by highly regarded researchers that ruled out any association between the autism spectrum disorders and vaccines or thimerosal, which used to be employed as a preservative in children’s vaccines, though it’s found only in some flu vaccines now. (This site alone provides information on more than three dozen such studies.)
It might seem strange that parents would continue to believe in one thoroughly debunked study in the face of so much evidence otherwise. But Wakefield’s claims, no matter how wild, fit with what they had noticed: Autism rates had been rising during a time when the number of vaccines children received was also rising. Some parents of children with autism also said they had noticed the symptoms soon after their children had been vaccinated.
As it happens, though, autism symptoms often first appear at about the age when children receive their recommended vaccinations, according to the publication LiveScience. They appear in children who have been vaccinated, and those who haven’t. And a 2008 study at Columbia University found that not only was there no link between the two, but that autism symptoms often appeared before vaccinations had been given.
What about those higher autism rates, though?
To start with, many things changed during the time that autism rates rose steeply, starting in the early 1990s. Exposure to many chemicals, for example. For that matter, consumption of organic food rose during the same time, but the public is less aware of that coincidence.
The more likely cause for the increase, according to a March 2 article in the publication Spectrum, is that during the same time, the diagnostic criteria for autism spectrum disorders were expanded, which meant that children who previously hadn’t been diagnosed were now included. And in 1991, children with autism became eligible for special educational services, Spectrum reported. That might have encouraged families whose children previously had been called “intellectually disabled” to have their children re-evaluated for autism. It’s worth noting that during the time that autism rates rose, the rates of “intellectual disability” dropped.
This might not account for all of the rise in autism cases, the article said. Biological factors also must be considered.
"For example, having older parents, particularly an older father, may boost the risk of autism,” the article said. “Children born prematurely also are at increased risk of autism, and more premature infants survive now than ever before.”
In fact, there are many avenues for scientists to examine in the vital work of understanding what causes autism. If and when they succeed, we might be able to get to the root of the cause or causes of autism spectrum disorders. The continued efforts to link vaccines and autism are an unfortunate distraction from the serious work being done in this field.
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