Several contagious diseases were nearly eradicated through childhood vaccination, but some parents have decided in recent years not to fully vaccinate their children, raising new public health concerns. The question of whether and how beliefs about vaccination are linked to political ideology has been hotly debated. This study investigates the effects of ideology on perceptions of harms and benefits related to vaccination as well as judgments of others’ attitudes. A total of 367 U.S. adults (131 men, 236 women; M age = 34.92 years, range = 18–72) completed an online survey through Mechanical Turk. Results revealed that liberals were significantly more likely to endorse pro-vaccination statements and to regard them as “facts” (rather than “beliefs”), in comparison with moderates and conservatives. Whereas conservatives overestimated the proportion of like-minded others who agreed with them, liberals underestimated the proportion of others who agreed with them. That is, conservatives exhibited the “truly false consensus effect,” whereas liberals exhibited an “illusion of uniqueness” with respect to beliefs about vaccination. Conservative and moderate parents in this sample were less likely than liberals to report having fully vaccinated their children prior to the age of two. A clear limitation of this study is that the sample is not representative of the U.S. population. Nevertheless, a recognition of ideological sources of potential variability in health-related beliefs and perceptions is a prerequisite for the design of effective forms of public communication.

Introduction

Not so long ago, contagious diseases such as measles, diphtheria, and whooping cough posed serious public health threats, especially in highly populated urban environments. Thanks to scientific breakthroughs and effective campaigns to promote childhood vaccination, these diseases were nearly eradicated in the Western hemisphere by the late 1990s [1]. Since that time, however, a plurality of citizens in the United States has decided that childhood vaccination is either harmful or unnecessary [2]. As a result, some parents have stopped vaccinating their children, and the diseases have returned, raising new public health concerns [3]. In some European countries, vaccination rates have also dropped in recent years, leading to dangerous outbreaks of measles and other communicable diseases [4].

In the U.S. today, 15% of children are under-immunized because their parents hold skeptical attitudes about the safety and utility of vaccinations [2]. Surveys suggest that 20–25% of parents believe that children receive too many vaccinations prior to the age of two, and 29–33% of parents believe—in contrast to the scientific consensus—that having multiple vaccines at a young age can weaken a child’s immune system [2, 5]. Parents who oppose vaccinations often doubt that vaccines protect against serious diseases, believe that their child’s immune system can protect itself against diseases without vaccines, and claim that the side effects of vaccination may be more harmful than the disease itself [2, 5].

By contrast, parents who support vaccinations are more likely to perceive childhood susceptibility to serious, vaccine-preventable diseases and to anticipate feeling regret if their children were to become infected with these diseases [5]. Whereas parents who are skeptical about the effectiveness and safety of vaccinations are more influenced by their peers than by medical experts, parents who support vaccinations are more likely to trust their pediatricians’ recommendations [5, 6]. Not surprisingly, there are also demographic factors such as socio-economic status, race, education, and size of household that are correlated with decisions about whether or not to vaccinate one’s children [2, 5, 7]. In any case, a significant gap has emerged between experts and the general public when it comes to the issue: whereas 82% of scientists believed in 2009 that childhood vaccination should be mandatory in the U.S., only 69% of survey respondents from the broader population agreed [8].

This brief discussion highlights the fact that parents are sensitive to the perceived harms and benefits of vaccinating their children and that there is considerable variability in these perceptions [9, 10]. The decision about whether to vaccinate one’s children appears to be based less on an attempt to understand the scientific consensus than on personalized estimates of risks and benefits. Because this is often the case when it comes to health-related decisions, research in health psychology frequently focuses on communicative attempts to modify individuals’ beliefs about health-related practices [9, 11]. Before designing effective interventions, however, it is necessary to understand whether there is systematic variability in perceptions of risks and benefits in different social groups [12, 13].

The current study investigates the possible role of ideological differences in accounting for variability in beliefs about childhood vaccination. Unfortunately, we were unable to examine this possibility in the context of a nationally representative survey of the general population. Instead, we made use of a convenience sample of Mechanical Turk workers. As a result, it is not possible to generalize our findings to the U.S. population as a whole. Nevertheless, this study provides some of the first systematic evidence that there are meaningful connections between political attitudes, on one hand, and scientific beliefs and perceptions of social norms concerning childhood vaccination, on the other.

The Role of Political Ideology It would appear that reluctance to vaccinate one’s children is connected to a number of specific beliefs that have circulated in popular culture [14]. In addition to various types of social pressures [15], receptivity to skeptical beliefs may be linked to underlying motivational orientations, such as distrust of scientific or governmental authorities as well as corporate actors, such as pharmaceutical companies and health insurance agencies. Taken in conjunction, these considerations suggest that certain motivated belief systems, such as religious [4] and political ideologies [16], may exaggerate—or, in other cases, attenuate—skepticism about the importance of vaccinating children. Political ideology could affect beliefs about vaccination in multiple, perhaps conflicting ways. Because there are prominent skeptics on the cultural left (e.g., Robert F. Kennedy Jr., Bill Maher, and Jenny McCarthy), it has been suggested that the anti-vaccination movement in the U.S. is largely a liberal development—both in terms of the movement’s origins and the people who should be most susceptible to messages that are critical of the “medical-industrial complex.” The assumption, therefore, is that liberals would be especially likely to perceive vaccines as harmful or unnecessary [17]. Along these lines, an editorial in the National Review stated that, “The countercultural Left is and has always been the core of the anti-vaccination movement. From NPR to PBS to the broadcast networks, the message of all-natural food and holistic, homeopathic medicine has been broadcast for decades, while traditional scientific medicine has been brought under suspicion as something being put over on us by the government and corporations” [18]. Generally (but not precisely) consistent with this logic, the states that have granted the most exemptions from vaccinating kindergarteners are somewhat more likely to be liberal-leaning states (such as Oregon, Illinois, Vermont, Michigan, and Maine) than conservative-leaning states—although some of the latter (such as Idaho, Alaska, Arizona, and Utah) also have fairly high rates of exemptions [19, 20]. It should be noted however, that it is impossible to draw unambiguous conclusions about individual behavior on the bases of aggregate (e.g., state-level) data because of “Simpson’s paradox,” which demonstrates that trends at the individual level may be very different from aggregate-level trends; e.g., [20]. For instance, it is conceivable that statewide trends in vaccination are driven by conservatives in “blue” states or liberals in “red” states. On the other hand, there are reasons to suppose that resistance to vaccination would be stronger on the political right. On many issues, such as evolution, climate change, stem cell research, and AIDS prevention, conservatives are less likely to trust scientific recommendations than are liberals; e.g., [21, 22]. There are probably several reasons for this, including dis-identification with scientists and educational elites [23], identification with religious authorities [21], and cognitive-motivational styles that favor intuitive, heuristic-driven ways of thinking over more systematic, deliberative modes of processing information [24]. Furthermore, libertarian opposition to governmental mandates may lead economic conservatives to be more supportive of parental decisions to refrain from vaccinating their children. For Mooney [25], Republican Senator Rand Paul’s declaration that vaccinations cause “profound mental disorders” was enough to puncture the myth “that denying vaccine safety, or avoiding vaccinations, is a liberal, granola thing.” Finally, a public opinion poll of 1,000 U.S. adults administered by YouGov and the Huffington Post revealed that—despite much ideological bickering among politicians and pundits—there are no clear stereotypes linking one group of ideological adherents more strongly than others to skepticism about vaccination [26]. When respondents were asked, “Which group do you think is MORE likely to have doubts about childhood vaccinations?” the results were equivocal at best. Twenty-six percent cited “Liberals” and 26% cited “Conservatives,” with 27% saying they were “Unsure” and 21% saying “Neither.” When these results were broken down according to respondents’ own ideological orientation, they revealed that conservatives assumed that liberals were more dubious about vaccination than conservatives, whereas liberals assumed that conservatives were more dubious than liberals [26]. What is the actual nature of the relationship, if any, between political ideology and beliefs about vaccination? Here, too, the evidence is ambiguous. A 2009 Pew Survey suggested that 71% of Democrats and 71% of Republicans felt that childhood vaccinations should be required [8]. Another poll conducted the same year by USA Today/Gallup focused on whether people had been influenced by actress Jenny McCarthy’s claims that her son had developed autism because of vaccinations. Results suggested that liberals were slightly more aware of McCarthy’s statements than were conservatives, but they were also slightly less likely than conservatives to report having been persuaded by those comments. A 2015 poll found that 61% of Democrats and 62% of Republicans believed that “The science supporting the safety of childhood vaccination is indisputable” [26], but other studies suggest that Democrats are more accepting than Republicans of scientific recommendations concerning childhood vaccination as well as many other health-related issues; e.g., [21]. A survey conducted by Kahan [27] suggested that conservative Republicans perceived the risks of vaccination to be greater and the benefits to be lesser, in comparison with liberal Democrats. Few, if any, studies of public opinion have delved more deeply into the specific contents of beliefs about vaccination and whether there are consistent ideological differences in these contents.

The Fact/Belief Distinction, False Consensus, and the Illusion of Uniqueness In addition to the obvious question of whether people agree or disagree with various statements about vaccination, it is useful to assess perceptions about whether the statements ought to be categorized as “facts” or “beliefs.” There is a long history of discussion in philosophy [28, 29] and psychology [30, 31] about how to characterize propositional statements as facts or beliefs. In these discussions, it is generally assumed that a fact is a concept from classical logic that has a set of defining features. At the same time, in practice people tend to treat facts and beliefs as “fuzzy concepts,” insofar as the features they use to distinguish facts from beliefs vary from one domain to another [32]. People tend to categorize information as factual when they believe that nearly everyone else agrees with it; cf. [33]. Therefore, exploring the ways in which people categorize specific statements about vaccination as facts versus beliefs enables us to go beyond the mere assessment of personal agreement and to incorporate perceptions of social norms. Furthermore, we are able to investigate directly individuals’ perceptions of how widespread (or consensually shared) certain beliefs about vaccination are both within and between ideological groups. Past research conducted in non-health-related domains suggests that conservatives are more likely than liberals to assume that likeminded others (and people in general) share their own beliefs [34]. Liberals and conservatives also seem to differ when it comes to the accuracy of their consensus estimates. Conservatives may be prone to overestimate the extent to which likeminded others share their beliefs (i.e., judging their beliefs to be more common than they actually are), thereby exhibiting what has been termed the “truly false consensus effect” [35, 36]. By contrast, liberals may be prone to underestimate the extent to which likeminded others share their beliefs (i.e., judging their beliefs to more distinctive than they actually are), thereby exhibiting “an illusion of uniqueness” [37]. Ideological differences in perceptions of consensus are potentially consequential when it comes to health-related decisions, insofar as human behavior is strongly affected by social norms, that is, perceptions about whether other people (especially likeminded others) are engaging in a given behavior [38]. For example, research in health psychology has demonstrated that the perceived prevalence of harmful behaviors—such as smoking tobacco, binge drinking, and illicit drug usage—affects the likelihood that a person will engage in that behavior, and that people frequently use these perceptions to justify their behavior [39–42]. Thus, the perceived prevalence of one’s own attitudes about vaccination and the extent to which those perceptions are accurate may have broad-based implications for personal and public health outcomes.