Vaccines are probably the single most cost effective public health measure ever designed by humanity, with a tremendously positive benefit-to-risk ratio. It is actual preventive medicine, that can really “boost” the immune system by priming it against a specific pathogen. A vaccine is also our best hope against pandemics like the current COVID-19 illness that is moving around the globe. Many experts hope and fear that we will only be able to truly end this pandemic when we can establish herd immunity through a vaccine. Until then, we can only slow it down through other measures.

For these reasons and more it is therefore surprising that so many people fear vaccines and even deny their efficacy. So much so that some psychologists have been trying to figure out what makes antivaxxers tick. Of course, the answer is – it’s complicated. They are a diverse group with diverse motivations, and the answer to any question of “nature vs nurture” is – both. But we can chip away at the major factors that seem to influence antivaxxer attitudes. A recent study adds to this growing picture, which I will discuss and then review some of the other studies that have shed light on this phenomenon.

The study is entitled “Vaccine skepticism reflects basic cognitive differences in mortality-related event frequency estimation,” by LaCour and Davis. They report two experiments. In the first they assessed the degree of vaccine denial in 158 subjects and then asked them a series of questions regarding the frequency of negative events, such as the frequency of emphysema-related deaths in the US. What they found is that higher levels of vaccine denial correlated with overestimations of unrelated negative events. This suggests that part of what is driving some antivaxxers may be a general tendency to overestimate risk and harm.

In their second experiment with 109 subjects they repeated the first experiment but also included neutral and positive events in the survey. They found that antivaccine attitude correlated with overestimating negative events, but not neutral or positive events. So it does not appear to be a general tendency to overestimate rare events, only negative ones. Lead author LaCour asks about these results:

“Do some people encode scary stories – for instance, hearing about a child that has a seizure after getting vaccinated – more strongly than others and then consequently remember these anecdotes more easily?” he asked. “Do they instead have certain attitudes and search their memory harder for evidence to support this belief? Is it a bit of both? How can you counteract these processes?

These are all common established psychological biases, perhaps just working a litter harder in antivaxxers. Confirmation bias is the tendency to seek out, remember, and accept information that supports what you already believe (or in a related bias – want to believe), while ignoring or explaining away disconfirming information. There is also the availability heuristic, which is the tendency to confuse how easy it is to think of an example of a phenomenon with how common or likely that phenomenon is. So if we can easily call to mind an anecdote of someone being harmed by a vaccine side effect, we then feel that the side effect is common.

People are also famously bad at statistics. We tend to be risk averse and overestimate risk. We also conflate how dramatic a risk is with the probability of harm – so people worry more about shark attacks and coconuts falling on their heads, and not the risk of driving to work every day. In other words – our worry is generally not proportional to statistical probability. This leads to bad decision-making, like refusing the huge benefits of vaccines out of worry for the tiny risk of harm.

But we also know that these cognitive biases are not all that is going on. Antivaxxers now live in their own information ecosystem, an ecosystem that partially creates antivaxxers in the first place. The question is – how predisposed do individuals need to be to fall into the antivaxxer bubble, or is it mostly just about exposure? How much selection based upon cognitive style is going on?

Another phenomenon we are seeing in the anti-vaxxer movement is a super Dunning Kruger (DK) effect. In the normal DK effect, decreasing knowledge does correlate with decreasing self-assessment of knowledge, but the gap between the two increases as knowledge decreases. So the less you know, the more you overestimate your knowledge. However, with two topics that have been studied there is a super DK effect going on where the people who know the least actually think they know the most. These two topics are vaccines and GMOs.

One study stated:

We also found strong evidence of Dunning-Kruger effects in our sample. Sixty-two percent of those who performed worst on our autism knowledge test believe that they know as much or more than both doctors and scientists about the causes of autism, compared to only 15 percent of those scoring best on the knowledge test. Likewise, 71 percent of those who strongly endorse misinformation about the link between vaccines and autism feel that they know as much or more than medical doctors about the causes of autism, compared to only 28 percent of those who most strongly reject that misinformation.

Dunning nicely summarized the phenomenon himself:

An ignorant mind is precisely not a spotless, empty vessel, but one that’s filled with the clutter of irrelevant or misleading life experiences, theories, facts, intuitions, strategies, algorithms, heuristics, metaphors, and hunches that regrettably have the look and feel of useful and accurate knowledge.

The problem is not ignorance, but the illusion of knowledge. I would add to Dunning’s list of factors – a narrative evolved out of an insular information ecosystem facilitated by social media.

There are other studies that do show a correlation between certain cognitive styles and anti-vaccine beliefs. Chief among them is a tendency to believe in conspiracies. This makes sense, because at the heart of the anti-vaccine narrative is a giant conspiracy among governments, pharmaceutical companies, and medical institutions to hide the true risks of vaccines from the public. That same study found:

Anti-vaccination attitudes were also associated with intolerance of those who limit their freedom, disgust toward blood and needles and an individualistic worldview, according to the study.

It is likely true that people with these traits preexisting are attracted to the anti-vaccine movement, but that movement also reinforces these traits. These factors also appear to be cross-cultural, with an Australian study from 2019 finding:

Compared to accepters, rejecters and fence sitters exhibited a heightened moral preference for liberty (belief in the rights of the individual) and harm (concern about the wellbeing of others). Compared to acceptors and fence sitters, rejecters exhibited a heightened moral preference for purity (an abhorrence for impurity of body), and a diminished moral preference for authority (deference to those in positions of power).

What all this tells us is that anti-vaccine attitudes are not simply a knowledge deficit problem, and we are not going to fix it with PSAs about vaccines and infectious disease. I have written previously about what might work, and what definitely won’t work, so I won’t repeat it here. But the bottom line is that anti-vaccine attitudes are complex and multifarious, and that no one strategy seems to work well or at all. In the short term, antivaxxers simply need to be exposed and marginalized. Their attitudes are literally dangerous, even before there was a worldwide pandemic.


Posted by Steven Novella

Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.