Source: Wikimedia Commons

Vaccination for diseases like Measles and Whooping Cough was one of the most significant public health victories of the second half of the 20th Century. When I was in elementary school in the 1970s, these diseases had been relegated to the past.

But two factors caused a change in some parents’ willingness to vaccinate their kids. The first was the observation of a particular form of regressive in which children started to develop normally and then showed signs of autism. Because vaccines are something given to children around that age, some parents began to question whether there was a link between vaccines and autism. Second, a single study claimed to demonstrate a link between vaccination and autism.

That study has since been found to have been wrong. Furthermore, a lot of research done after this initial study failed to find any link between autism and vaccination.

Yet, many parents have continued to avoid vaccinating their children, and earlier this year some of the childhood diseases that had disappeared from our schools began to return.

If there is no actual evidence that vaccines cause autism (and plenty of evidence that they prevent children from getting sick), why do parents put whole communities at risk by failing to vaccinate their children?

There is a lot of psychological evidence that helps explain the persistence of these false beliefs. An initial set of studies on this topic was done by Hollyn Johnson and Colleen Seifert and reported in a 1994 paper in the Journal of Experimental Psychology: General. They had participants read about a fictitious scene. People were originally given a fact that biased the investigation and later were told that fact was false. This false information affected people’s beliefs later. They called this phenomenon the continued influence effect. In their studies, the only way to minimize the influence of false information was to give them an explanation for why they heard the false fact in the first place.

But, if we return to the example of vaccination, there has been a lot of publicity that the study that purported to show a link between vaccines and autism was a fraud. Yet, it still influences people’s beliefs and actions.

A paper by Ullrich Ecker, Stephan Lewandowsky, Candy Cheung, and Murray Mayberry added to this body of research in a way that helps to understand the vaccination case better. Their work is described in a 2015 paper in the Journal of Memory and Language.

They had participants read about a bus accident in a series of slides. Over the course of the description, two different causes of the accident were suggested. One was that the bus blew a tire before it crashed after running over construction debris. The second was that a car cut the bus off and it swerved and crashed. Some participants saw the tire explanation first, others saw the swerving explanation first.

Later in the series of slides, either the first or second explanation was retracted. Participants were simply told that this explanation was not in fact a cause of the accident. After a delay, participants were asked several questions about facts related to the accident (to ensure that they were paying ), questions about which the cause had been retracted, and several questions that required people to make inferences about the accident based on what they had read.

These inference questions are of particular importance. Once you have some sense of why an accident happened, you can make predictions about what might have prevented the accident, for example. The researchers were interested in how often people made inferences based on the cause that was retracted, because that would demonstrate the continued influence of something known to be false.

Participants did pretty well recalling facts about the accident and almost everyone correctly remembered which explanation had been retracted.

When the first cause was retracted, participants based almost all of their inferences on the second cause. That is, they correctly focused on the explanation that was still known to be true.

When the second cause was retracted, though, about half of people’s inferences were still based on that cause. That is, even though people remembered that the second cause was not true, it still affected what people thought about the accident.

Returning to the vaccination case, this may help us to understand why many people are still leery of vaccinating their children. Yes, we know that there doesn’t appear to be strong evidence that vaccines cause autism. Yet, we don’t have another explanation for what does cause autism. In the absence of an alternative explanation for why children develop autism, the debunked cause still has an affect on people’s thoughts and actions.

This research suggests that when research does uncover other factors that lead to Autism, that will help people to recognize the value of vaccinations and help stop the return of childhood diseases.

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