Back in March a study came out, though it was only recently hyped up in the media, which explored an interesting intersection between psychology, vaccination, and the communication of risk. This study by Cornelia Betsch and Katharina Sachse (2012) covered two experiments in which the researchers looked at how the wording of vaccine risk messages affected participants’ perception of the safety of vaccines.

The idea of these experiments was to explore what the researchers call the negativity bias. This bias is a tendency for negative messages to influence our perceptions of risk more than positive ones. This creates a problem for the communication of negating messages—messages that downplay a risk or offer contradictory evidence against a risk. So, relating to information about vaccines, how strongly should we word our pro-vaccine arguments?

As the science behind vaccine safety is sound, there are a few ways to make an argument for it (and to argue against anti-vaccine pseudoscience). The study outlines two different tactics: strong risk negation and weak risk negation. The study offers up the following example of both:


The claim that newborns tolerate vaccinations less well than older infants may be negated by the statement “It is absolutely impossible that newborns tolerate vaccinations less well than older infants,” which may be perceived as a strong and confident negation. A weaker risk negation may be expressed by the following statement: “It is extremely rare that newborns tolerate vaccinations less well than older infants.”


Participants in the experiments were asked to imagine that they were parents unsure about vaccination and its side effects, and were then presented with a number of common anti-vaccine arguments and their corresponding strong or weak negations (rebuttals). After this, the participants were asked how likely they would be to vaccinate their imaginary child against a fictitious illness. We could imagine this to be exploring how people would respond to the science-based community’s debunking of anti-vaccine myths.

Herein lies the issue: based on the previous literature on risk communication and the negativity bias, the study hypothesized that a strong risk negation would actually create a higher perception of risk than a weaker negation. This is counter-intuitive, as a strong negation leaves no “residual risk” (i.e., no possibility of harm), which should seemingly assure people. This would be akin to saying “It is absolutely impossible that vaccines will harm your baby.” But because of the negativity bias, even a weak negation (which implies a small risk) affects risk perception more than a strong negation (which implies no risk). That is to say, paradoxically, the study in fact found that a stronger negation of risk increased the perceived probability that a vaccine-adverse event (VAE) would affect their (imagined) child.

The study is not clear on why exactly this is the case. Perhaps a complete negation of risk is simply not “real” enough: claiming that a medical intervention is 100% safe is undoubtedly suspicious. As the negativity bias has us giving more weight to negative information, the weak negation simultaneously offers a negative aspect (“some rare side effects may occur”) while explaining vaccination. It may be that these aspects, when taken together, offer a more “real” message that has an emotional response we can focus on. Delinking this in the strong negation case could be the downfall of effective communication. It could also be that the strong negation, which makes no mention of risk, allows a cautious mind to run wild, inserting hyped-up dangers where none exist.

Of course, like any study, this one has its limitations. The sample size was relatively small (n=166), the participants weren’t actually parents, and there may have been confounding variables like effects from the availability heuristic (simply seeing the risk written down may have increased the perceived probability that it happens). But the study certainly offers an interesting and counter-intuitive finding.

There are many more parts to this study, such as exploring other moderating variables like personal involvement, prior attitudes about vaccination, credibility of the message source, and views of alternative vs. conventional medicine. However, the finding about the negativity bias in communication about risk is enough to inform some strategy for those of us who wish to, rightly, extol the safety and necessity of vaccination (and negate the damage done by anti-vaccine proponents).

Adopting a More Rational Stance

The finding of the study discussed above presents a problem: We know vaccines are safe and save lives, but we can’t discuss it in absolutes. As a pro-vaccine proponent myself, I know that I have been guilty of making statements that sound too similar to the “strong negation” case explained above and have seen many skeptics to the same. The unfounded and fear-based arguments of the anti-vaccine movement frequently get me so incensed that I forget the conditional and tentative nature of the science I am trying to promote. Vaccines are not 100% safe. We can only say that adverse reactions (which do happen) are amazingly rare. [Indeed, even if an order of magnitude more children were in some way harmed by vaccines, the cost-benefit analysis would still point to continued vaccination] Though the vast majority of VAE’s are minor, like soreness or redness at the injection site for example, they still occur.

It is odd then that in this case a cognitive bias sides with the stance of science on an issue. According to risk communication experiments like these, adopting a more conditional and scientifically accurate stance on VAE risk is better than flat-out denying any risk (though the percentages are so small that we might be warranted to do so). If a pro-vaccine message owns up to possible side effects, instead of dismissing any with a strong negation, and therefore decreases the perception of risk in comparison, that is the way to go. It may seem like an information campaign no-no to include possibly scary side effects in the message, but our cognition is quirky. In this case, an honest representation of risk happens to work well with those quirks.

This should logically extend to other anti-vaccine tropes like the imagined link between vaccines and autism. Another study would have to be done, but if the negativity bias holds true across cases, it would be better for pro-vaccine advocates to speak in the conditional voice of science. Saying “all of the evidence that we have so far shows no plausible link between vaccines and autism,” would then be better for people’s understanding of risk than saying “it is absolutely impossible that vaccines cause autism” (even though this is nearly the case).

What then does this mean for pro-vaccine discourse? I suppose that it means speaking more like medical professionals and less like pro-vaccine cheerleaders. I am convinced that getting your child vaccinated is one of the most medically important things that you can do for her, but that does not mean I can leap beyond scientific boundaries and into a place of inscrutable certainty. If we start making claims that go beyond what the science says, we become as those who claim thimerosal is a persistent and evil poison. Luckily, we already have the science on our side. Every bit of evidence that we have shows that vaccines are safe, effective, incur a modicum of side effects, do not cause autism, and save millions of lives. We just have to communicate this in a way that acknowledges the real data. Unfortunately, I don’t think this is because people value real data or know the science behind these issues, but rather it is because, in comparison to statements that leave no wiggle room, weak negations leave room for judgment and suffer less from possible backfire effects.

As does the psychology of “debunking,” the study of risk communication extends to everything from vaccines and autism to cell phones and brain cancer to “alternative medicine.” Studies like the one discussed above are by no means definitive, but they do give us a window into how to proceed as properly science-based individuals. Apparently, when it comes to communicating effectively about vaccination, and debunking the harmful tropes of the anti-vaccine movement, we need to stick to the science, admit the risks when they are present, and stay away from absolutes. Though it is incredibly hard not to adamantly shout back at those who are basically welcoming preventable childhood diseases with open arms, nothing in science is absolute, and we should represent that in the way we promote vaccine safety. A quirky psychology may just be our ticket to scientific literacy on the issue.


Journal Source:

Betsch, Cornelia, and Katharina Sachse. “Debunking Vaccination Myths: Strong Risk Negations Can Increase Perceived Vaccination Risks.” Health Psychology, March 2012.

Kyle Hill is the JREF research fellow specializing in communication research and human information processing. He writes daily at the Science-Based Life blog and you can follow him on Twitter here.