Research on the fear of disease provides insights on human traditions, norms and even prejudices

Mark Schaller, PhD, is professor of psychology at the University of British Columbia. His research focuses on human motivational systems and their implications for social cognition and social interaction. He also explores questions about the evolutionary bases of psychological phenomena and the consequences of these phenomena for culture and cultural change. Schaller recently answered questions from the American Psychological Association about his research into the “behavioral immune system” and how it protects people from disease.

How did you become interested in this topic?

During an earlier part of my career, I conducted some research looking at the effect of perceived threats on people’s prejudices. That is when I started thinking about how the spread of disease could serve as a perceived threat and have implications for prejudices against certain types of people, such as foreigners and people who just look or act in a way that violates expectations. I eventually broadened my approach and looked at implications for additional aspects of human cognition and social behavior.

What is the behavioral immune system and how does it protect us?

It’s a set of psychological mechanisms that provide a defense against infectious diseases and does so in a way that complements immunological defenses. Immunological defenses—which is what most people think of when they think of an “immune system”—fight off infections after they’ve already occurred. The behavioral immune systemopens in new window helps to prevent infections from occurring in the first place.

The way it works is that we have these psychological mechanisms that help us detect disease-related threats and then act in a way to minimize the likelihood of being infected by those diseases. A lot of these responses are reactive. I might smell something stinky, like feces, and I avoid it, or I see someone with a drippy nose and I avoid them. This kind of reactive response is often accompanied by a powerful emotional reaction, disgust, which helps to motivate avoidance of things—including people—who might pose some risk of pathogen infection.

The behavioral immune system also produces proactive behaviors that limit the likelihood that people even encounter these kinds of infectious threats in the first place. For example, people develop rituals and norms to limit the transmission of diseases. There are norms that govern forms of hygiene, how we prepare food, etc. Conforming with these norms can limit exposure to infections, and there often are negative consequences for people who ignore these rituals or violate these norms. Research showsopens in new window that this is particularly true in cultures where diseases are more prevalent. These cultures are more likely to value traditions and conformity and can be less forgiving of mavericks who violate local norms.

Research shows us that fear of pathogens and disease can shape how we view other people, including those who are different from us. Can you tell us more about these findings?

Here’s a really useful analogy developed by the evolutionary psychologist Randy Nesse: Our brains are designed like smoke detectorsopens in new window. A smoke detector can make one of two kinds of errors. It can fail to detect when my house is on fire, or it can make the opposite error and go off even when my house isn’t on fire, but I’m just burning a piece of toast or something. That first kind of error is catastrophic, whereas the second kind of error is merely irritating. Smoke detectors are calibrated to avoid the catastrophic error by being hypersensitive to even the merest whiff of smoke, which means they make lots and lots of the second kind of error. Our brains work similarly when it comes to detecting threats. If I fail to detect a serious infectious risk, that could be super-costly to me, because I might get sick and maybe even die. Our brains are designed to minimize that kind of error. Consequently, they make lots and lots of the other kind of error, leading us to respond negatively to people who might bear some superficial resemblance to a real infection threat but, in reality, may pose no risk at all. And, importantly, our brains are even more hypersensitive to this sort of thing when, for whatever reason, we feel especially vulnerable to infection.

For instance, research shows us that when people feel vulnerable to infections, they tend to become more xenophobic. One studyopens in new window, for example, looked at pregnant women’s reactions to other people during the first trimester of pregnancy, when they have suppressed immunological responses to infection. The researchers found that during this period, participants had stronger disgust reactions and expressed more strongly xenophobic responses than women in later stages of pregnancy. 

Another example comes from some experimentsopens in new window that my students and I did at the University of British Columbia. Participants in two different experimental conditions were shown different slide shows. In a control group, participants were shown images of accidents and mishaps—things that might injure or kill you but have nothing to do with infectious diseases. In the other condition, participants were shown images that made very salient the threat posed by infectious diseases. Participants then completed a task that assessed their interest in attracting immigrants to Canada from a variety of countries—including immigrants from subjectively familiar countries, such as Poland and Taiwan, as well as immigrants from more subjectively “foreign” countries, such as Mongolia or Peru. Interestingly, even compared to a control group that made other kinds of threats salient, participants who were temporarily worried about the threat of disease indicated a stronger preference for immigrants from familiar places, to the exclusion of those from subjectively foreign places. Other studiesopens in new window also show this link between disease threat and xenophobic attitudes. 

Additional research shows that when people are more worried about infectious diseases, they also respond more negatively to people whose physical appearances seem superficially anomalous in some way, such as people who are obeseopens in new window or who have visible disfigurementsopens in new window or physical disabilitiesopens in new window.

What impact can the behavioral immune system have on people’s romantic endeavors?

Attractiveness can serve as a superficial signal that somebody is not sick. In fact, research suggestsopens in new window that when people are more worried about infections, they tend to express a stronger preference for symmetrical faces, which generally are perceived to be more attractive. Analogously, other research shows that in parts of the world where pathogenic diseases are more prevalent, people tend to place higher priority on physical attractiveness in mate selection and marriage choices. 

Another set of studiesopens in new window shows that activation of the behavioral immune system leads people to be more cautious in their dating behavior. It may even influence safe sex practices. This was demonstrated in an experimentopens in new window in which undergraduates completed a questionnaire measuring their attitudes toward condom use. Depending on an experimental manipulation, they did this in a room that either smelled normal or—because the researchers had recently sprayed the walls with a novelty spray called “Liquid Ass”—smelled like feces and so, you know, triggered the behavioral immune system’s disgust response. The results showed that, compared to the control condition, students in the stinky room reported a greater intention to use condoms.

Is there any interaction between the behavioral immune system and the biological immune system?

Yes. That pregnancy study I mentioned earlier offers one example, showing that when immunological defenses are suppressed, the behavioral immune system may become more sensitive as a sort of compensatory response. Reciprocally, activation of the behavioral immune system may have immunological consequences. One example can be seen in an experimentopens in new window that I did with colleagues who are experts on psychoneuroimmunology. In one experimental condition, participants looked at photos of people who looked sick. In another condition—a control group—participants instead looked at photos of threatening people brandishing guns, most of which were aimed directly at participants. We drew blood from participants before and after they looked at the photos. That blood was then exposed to an infectious agent that stimulates white blood cells to produce a pro-inflammatory immunological response. Results showed that, even compared to the stress-inducing control condition, when people had been looking at photos of sick people, their white blood cells produced an even stronger immunological response. Other studies have produced similar findings, suggesting that there is some deep connection between the behavioral immune system and immunological systems that provide different, but complementary, defenses against infectious diseases.

A real-life pandemic is obviously different from the kind of artificial manipulations that folks like me have used in our laboratory experiments, and only actual empirical studies can tell us whether a pandemic might produce analogous psychological consequences. It certainly could. A few years ago, my students and I conducted an analysisopens in new window of political polling data immediately preceding and after an Ebola outbreak in the U.S. in 2014, which was front-page news for a little while. We found that, in both the U.S. and Canada, this Ebola scare was associated with an increased trend in intentions to vote for conservative candidates. I wouldn’t want to draw strong conclusions from just that one study, but its results are consistent with other studies linking the behavioral immune systemopens in new window to more conservative social and political attitudes. And, according to some new studies, the current pandemic may have had similar effects. One studyopens in new window was conducted during the early stages of the COVID-19 pandemic in the U.S., and showed evidence of greater xenophobia among people who were especially worried about becoming infected. Another study, in Poland, links the COVID-19 pandemic to increased authoritarianism and rejection of people who deviate from traditional sexual norms. I reckon we’re going to be seeing a lot more studies of this sort published in the near future, which will give us a much better idea of what kinds of psychological consequences are, and aren’t, produced by the COVID-19 pandemic.

What are some gaps in this area of research and what types of studies need to be conducted to fill in these gaps?

We need to know more about the psychological effects of actual real-life disease outbreaks. It will also be important to get a better understanding of the extent to which the behavioral immune system—which evolved under conditions that are pretty darn different from the way that most people live these days—may be either beneficial or counterproductive to health outcomesopens in new window in the modern world. We also need more research on the underlying physiology and neurochemistry of the behavioral immune system, as well as research on its underlying genetic basis. There is already a some researchopens in new window on these topics, but it’s going to take a lot more work in the future to connect all those dots.