"Vaccines cause cancer." "Vaccination is a plot to control the population." "The science around vaccines isn't settled yet."

These are different arguments that one may encounter as reasons not to get vaccinated, especially on the Internet. All can be thoroughly debunked, but belief in the misinformation persists and influences people's decisions, with costs to individual and societal health.

What motivates people to believe flawed arguments, when there is extensive scientific evidence about the benefits and safety of vaccination? Psychologists believe that these motivations can stem from factors that are difficult to counter using statistics, facts, and logic alone. These factors are referred to as “roots” of individuals’ negative attitude towards vaccines.

Attitude roots can be emotional; for example, fears about negative health consequences. Roots can also be ideological, for example an individual with a politically libertarian worldview may have concerns about vaccines being made mandatory. Roots could be psychological tendencies, for example, to believe in conspiracies such as the ideation that vaccines are a plot to control the population. 

In an article just published in Nature Human Behaviour by our team, we reviewed decades of academic literature - including 152 papers and over 2,000 arguments - and narrowed down a set of eleven attitude roots relevant to understanding the motivations to reject vaccination. Using natural language processing methods, we were also able to predict the presence of these roots in a set of 582 false claims about vaccines made on social media. As an example, these three word clouds illustrate the content of anti-vaccination arguments that were classified to three attitude roots.

Importantly, we need to understand these attitude roots for two reasons. First, so we can speak to a person in a way that does not cause them to react badly to a pro-vaccination message. Second, so that our corrections of misinformation can be tailored to fit their psychological motivations.

We examined these attitude roots, and how individuals express them, in a second article that just appeared in Scientific Reports. We set out to identify them with two different methods. First, we used a taxonomy of 11 attitude roots to generate a set of prototypical anti-vaccination arguments for each root. We then looked at which of these anti-vaccination arguments individuals preferentially endorsed. Second, we also presented individuals questionnaires measuring psychological traits that represented each of the 11 attitude roots.

This proved to be a challenge because the manifestations of attitude roots can overlap. After all, a secret plot to control the population can involve sterilisation and it would also remove people’s freedoms. People can also hold multiple, reinforcing motivations for their attitudes and behaviours: for example, fear can be heightened by tendencies for conspiratorial thinking. In our research with 1,250 UK participants, we attempted to model clusters of anti-vaccination argument endorsement, but found that individuals who opposed vaccinations tended to endorse any and all anti-vaccination arguments we showed them.

When we then looked at how attitude roots might manifest as psychological traits, we found that individuals with higher levels of 11 of the psychological traits also tended to endorse all the anti-vaccination arguments more. In short, the attitude roots that these traits represented were indeed related to the strength of anti-vaccination argument belief.

We were also able to model different clusters of the psychological traits, showing that anti-vaccination attitudes (and on the flip side, pro-vaccination attitudes) had distinct and consistent trait profiles. 

Among the anti-vaccination profiles (on the left of the diagram), we found one characterised by strong anti-scientific beliefs and an alternative conception of evidence-seeking—we called this profile (coloured in red) “alternative epistemology”, as it reflects a belief system that rejects or downplays the importance of scientific knowledge and evidence. A second profile (coloured blue) was socially conservative, with high levels of religiosity, moral rigidity, and advocacy for traditions, indicating a need for decisions to conform to an approved order or hierarchy. A final distinctive profile (coloured yellow) was centred around strong ideological beliefs in free markets, suggesting objections to vaccination based on an aversion to state regulation or control. 

So what can we learn from these results? We observed that the manifestations of attitude roots are complex and highly intertwined, which makes it difficult to pin down an individual’s exact attitude root from survey data and address it via mass communication about vaccines. Nonetheless, understanding how the attitude roots cluster into psychological profiles can help us to tailor communication strategies towards different groups. 

For example, with people who generally reject the scientific understanding of facts and evidence, we can seek to find a different common ground. If an individual with this profile holds strong beliefs that alternative medicine works for them and is preferred over vaccines, it would be counterproductive to argue that there is insufficient evidence for alternative medicine and overwhelming evidence for vaccination. Instead, we could start from the position that one can indeed reap benefits from some alternative types of therapies, which potentially opens the path to a discussion how vaccines can be “in addition to” rather than “instead of” the preferred treatment.

Alternatively, with people who hold strong social or political ideologies, we can develop and deliver messages that align vaccination with their worldviews and belief systems. For example, vaccination can be “an individual choice to gain protective benefits for oneself”—such messaging is in line with the premises of free markets, which are important to advocates for individual freedoms without regulation. Explaining the collective benefits of immunisation, though typically effective as a mass communication strategy, would likely be incompatible for this group. 

Ultimately, understanding the complex motivations underlying people's beliefs and attitudes about vaccination provides the foundation for a productive conversation with them. Our research team has built on this foundation to develop further tools to support such conversations about vaccination, for example how to affirm individuals and tailor refutations of misinformation to be aligned with their attitude roots.