Kim Mills: Although we are currently in the midst of a global pandemic, we should not forget that we are fortunate to live in a time where we have vaccines and other public health measures we can take to avoid illnesses such as polio, smallpox, cholera, even bubonic plague. And we hope that soon, COVID-19 will be vanquished by vaccines. But for our ancestors, infectious diseases were a constant danger, so much so that the threat of infection has actually helped shape human evolution.
Disgust, wariness of strangers, cultural norms around food and cleanliness–all of these behaviors may have evolved at least in part to keep us safe from infectious disease. Researchers call this suite of protective mechanisms the behavioral immune system. But are the defenses that humans evolved thousands of years ago to ward off infection suitable to protect us from the diseases we face today? Do they do more harm than good in some cases? And what is the role of disgust in keeping us safe and healthy?
Welcome to Speaking of Psychology, the flagship podcast at the American Psychological Association that examines the links between psychological science and everyday life. I'm Kim Mills. Our guest today is Dr. Mark Schaller, a professor of psychology at the University of British Columbia. Dr. Schaller coined the term "behavioral immune system," and he studies how the perceived threat of disease has shaped human psychology. He's also interested more broadly in human social cognition, stereotyping, evolutionary psychology and cultural psychology.
Thank you for joining us today, Dr. Schaller.
Mark Schaller, PhD: Hey, thank you for having me. I'm delighted to be here.
Mills: Let's start with a broad question. What exactly is the behavioral immune system and how and why did it evolve?
Schaller: The behavioral immune system I think can be thought of as a suite of psychological responses that evolved to help defend against the threat of infectious diseases, and it's comprised of psychological mechanisms that help us detect potentially infectious things in the immediate environment, to respond to those things in a way that might help us avoid them and therefore to help keep us from getting sick.
The second part of your question, why do we even have this set of mechanisms? When I think of this, I think it's useful to think about the benefits as well as the costs of what we might think of as the real immune system. Our immunological defenses against infections kick in when those things, bacteria, viruses, whatever, when they get inside us. And it's awesome that we have these immunological defenses that can mount a defense against those things, but it's hugely costly. And to actually mount an immunological defense against bacteria and so forth, it consumes an enormous amount of caloric resources. It can be debilitating. It prevents us from doing all kinds of other things.
In a sense, I think you can think of what we call our immune system as kind of like medical insurance. It's great to have, but it really stinks when we have to use it. It'd be better not to have to use it at all. And that's where the behavioral immune system comes into play. It's kind of a form of preventative medicine. These mechanisms help prevent us from having to actually mount that costly immunological defense and so serves as kind of a complementary kind of defense against the threat of infectious diseases.
Mills: You found that one of the biggest implications of this system is how it affects our interactions with other people, particularly people who are different from us in some way. What is the relationship between the behavioral immune system and prejudice or xenophobia?
Schaller: Generally speaking, when people are more concerned about the threat of disease, if they feel more vulnerable to infection, they tend to respond more harshly to people who look different, to people who act different, to strangers, to people who they perceive as being from strange or different places. And there's a couple of different reasons why that might be. One is this idea that historically, people who are strange or from strange different places might've been more likely to bring with them more exotic parasites, things that are more likely to make us sick, and that might be possible.
But I actually think a less obvious answer is probably a more compelling answer to the question, which has to do with the fact that a lot of the things that people historically have done in order to avoid infection had to do with doing things a certain way, prepare food a certain way. We have these norms of hygiene, and we have norms that govern what are appropriate ways of engaging in intimate interpersonal contact with other folks. We have these rituals and norms. And prior to contemporary technological advances in medicine and pharmacology and public health, these kind of rituals and norms were most of what people had to help them figure out how to avoid coming into contact with infectious diseases, which means that anybody who is likely to violate those norms, to do things differently, to prepare food differently, to violate norms of sanitation and hygiene, they posed a threat not just to themselves, but to anybody in the community.
People who seem likely to act different, people who are from foreign lands, it may kind of activate the sense of these people don't do the things that we're likely to do. And so getting back to your question, the idea is that this kind of xenophobic response to people who are different maybe have historically served the function of trying to avoid and keep away people who would violate local rituals and social norms.
Mills: Some of these things sound like they could be traditions, things that would be passed down from generation to generation, but was there something going way, way back that maybe the humans who prepared food in a certain way just tended to survive and therefore it got passed along in that way, almost like an instinctive thing?
Schaller: I wouldn't say an instinctive thing, but more of a superstitious thing. Food preparation is one of my favorite examples of this, because biologists who have studied spices have found that the use of culinary spices is a natural form of antibiotic. It's not just hot peppers, but just about every kind of herb and spice that people use to flavor food actually has some antibacterial properties. And so what's likely to have happened is that people just through trial and error figured out that, "Hey, if we prepare food in this kind of way, if we put hot spices on our meat, then people are less likely to get sick." And nobody really knew why, right? I mean, this was way back when before people even had a concept of germ theory, but you can figure out, "All right, people get sick if we prepare food this way and they don't if we prepare food this other way."
And so it's like, "Well, let's do that. Let's do the thing that keeps us from getting sick." They might've had a whole mythology around that, but the idea is that they would for sure tend to teach their children, they would pass that on, and so it becomes kind of a cultural norm. Without any necessary reason as to why it works, there would just be this sense, this superstitious sense of this is what works, and so folks who don't do that or seem likely not to do those things, people who seem likely to violate these norms were viewed as threats.
Mills: You've conducted some very interesting studies that suggest that feeling threatened by infectious disease can make people more conformist, more likely to follow rules. And you found this plays out both at the individual level, and at the societal level. Can you talk about that research?
Schaller: Yeah. And I mean, that follows directly upon what we were just talking about. Generally speaking, this tendency to respond adversely to things that seem like they might pose some sort of threat of infection, whether it's something that actually seems immediately infectious or some person who seems like they might pose a threat of violating norms, we tend to be hypersensitive to these perceived threats under circumstances in which we either are more vulnerable to infection or for whatever reason just kind of feel more vulnerable to infection.
And so in psychology experiments, experiments that I've done, experiments that other people have done, we've used manipulations that temporarily just make people feel a little more freaked out by disease. And we find that when people do feel more freaked out by disease, they tend to be to endorse more conformist attitudes. They want other people to conform more and they also are more likely to conform themselves. They are themselves less likely to violate norms. They're more likely to go along with majority opinion and so forth.
That's kind of what's happening at an individual, at a psychological level of analysis, but you do see this analogy at a much broader worldwide cultural level of analysis too. In some countries, simply because of meteorological variables and other sorts of things, some countries tend to just have more infectious diseases going around. And it turns out that in countries that have historically had more infectious diseases, people in general are more conformist. They're more likely to encourage obedience in children. They're more likely to conform to majority opinion themselves. They're more likely to do whatever they can to encourage not just themselves, but folks around them to uphold existing cultural traditions.
Mills: You talked about meteorology playing a role here. Are you saying, for example, in the tropics, we always talk about all of these tropical diseases. That's one of the places where weather it would be playing this role, whereas if you live in the Arctic where it's cold and the germs can't reproduce the way that they can, right, as they can in warmer weather?
Schaller: I'm not a parasitologist, so that’s not my area of expertise, but that is my understanding that–
Mills: That's kind of what we're talking about.
Schaller: More of these harmful bugs, parasites of one sort or another, just more of them can thrive in hotter, wetter places.
Mills: Do you think that some of this behavioral immune system behavior, particularly around xenophobia and prejudice, is playing out now during the COVID pandemic, are people becoming more conformist or less so? I mean, the news would tell us that people don't want to get vaccinated and yet as you see the numbers creep up and more vaccine becomes available, people are getting vaccinated. What's at play here?
Schaller: Well, that's a great question. And it's a complicated question because people have particular groups that they seem especially more willing to conform to. Let me go back to the broader question that you asked about, whether these kinds of phenomena in general seem to be playing out in the pandemic. And it is an empirical question. I'm a scientist, of course, so it's like, I want to see the data. And there have been a lot of people who have been collecting data of one sort or another bearing on these kinds of questions over the last year.
And some of those results are now appearing in academic journals and there is evidence that some of these things that we've seen in artificial psychology experiments are playing out in the real world. For instance, early on in the pandemic, political scientists collected data that was looking at whether the pandemic, the onset of the pandemic was affecting Americans' attitudes toward folks from China specifically, toward Asian Americans, toward immigrants more broadly, and they found evidence that it was, that these more xenophobic responses were being heightened by the onset of the pandemic.
There is some work that I saw recently that some psychologists, an international team of psychologists collected data in Poland that showed that the pandemic was associated with increased endorsement of very traditional attitudes, including more negative attitudes to people who violated traditional sexual norms and things like that. In various different places, we are seeing evidence that some of these phenomena that we just talked about are being exaggerated because of the fact that at a broad population-wide, worldwide level, the pandemic is making the threat of disease just extra salient for people in general and people aren't just in general feeling more vulnerable to the threat of infection, and it's having these social consequences.
Mills: There's also research that suggests that the behavioral immune system may influence our choice of mates and our romantic and sexual behavior. Can you talk about that research too? I mean, that just sounds fascinating. How does that happen?
Schaller: Well, it's a great question. And in fact, some of the first work that people did in this general line of work looked at its effects on people's mate preferences and attraction and so forth. One line of work looks at physical attractiveness and the extent to which people prioritize physical attractiveness in a potential mate, and work shows that in places, for instance, where diseases have historically been more prevalent, people place a higher priority of physical attractiveness.
Why might that be? And the answer, there's a couple of answers possible. One is that generally speaking, physical attractiveness serves as a quick and dirty proxy for health. That is, when people are sick, it affects their appearance in a variety of ways that in general, people find less attractive. Even among folks who are healthy, there's a tendency to just view physical attractiveness as a proxy for health. And in addition to that, people have argued that it can also perhaps serve as a proxy for the idea that this person is going to producing healthy children. For a variety of reasons, the argument is that when diseases are more prevalent or when people are more freaked out by the threat of disease, they place a higher priority on physical attractiveness.
There's other ways in which the behavioral immune system can affect people's inclinations, their romantic inclinations. Intimate sexual behavior and just intimate social contact in general poses an interesting dilemma for folks, because on the one hand, we get a lot of benefits from it, but on the other hand, it makes us more vulnerable to the kinds of diseases that people might spread amongst themselves. There's this kind of constant trade-off, and the idea is that people will resolve that trade-off somewhat differently depending upon whether they are or are not feeling worried about the threat of infectious disease.
When people do feel more vulnerable to infectious disease, the idea is that they'll just pull back a little bit from these romantic inclinations and there's research that shows that people are just less likely to pursue romantic opportunities when they're feeling more worried about disease. And it's not just in the realm of romance either, for that matter. There's work that shows that people just in general are less socially gregarious when they're worried about infectious diseases, because after all, it's not just intimate sexual contact that poses a risk, but close interpersonal contact in general does. And so for the same reasons, close interpersonal contact has this trade-off. There's lots of benefits from hanging out with friends, from meeting new people and so forth, but it also carries this risk of disease transmission. Consequently, when people are more worried about the threat of disease, they tend to just pull back from social contact.
Mills: Let's talk for a minute about the role of disgust and how that fits into our behavioral immune system. I mean, it obviously plays an important role for us as living beings. But what role is that? What is the purpose of disgust?
Schaller: A lot of people have studied disgust for a lot of years and have suggested a variety of roles that disgust plays, and the idea that this emotional experience that we call disgust has evolved. For instance, one expert on disgust has suggested that really a long time ago, evolutionarily speaking, disgust or some early version of disgust, distaste, served a function of just keeping potentially contaminated food stuffs out of our mouth since it's through our mouth that a lot of potential infections can happen.
And over time, particularly as our ancestors evolved into more and more of a social species, disgust evolved and took on a broader role that it didn't just serve as a way of alerting us to the threat of contaminated foodstuffs, but as kind of a first alert system for the threat of anything that might be contaminating or infectious. And so what disgust does in folks nowadays that, it really, like many emotions, it serves as kind of an alarm system. It essentially in a really powerful, immediate way is alerting me to, "Here's a potential threat of infection and it's leading me to respond in an avoidant way."
Mills: You've talked about evolution a little bit as we've been chatting here, and I'm wondering over the long-term, do you think this pandemic is going to change people's behavior in any lasting way?
Schaller: I think it would be probably silly of me to speculate upon in terms of whether this will actually have evolutionary consequences, but rather I think where one is likely to see it is in terms of not genetic evolution, but cultural evolution, that the pandemic is likely to have some lasting effects on what kinds of norms govern how people interact with each other. And I'm sure I'll be wrong about some of this because I've learned from sad experience that it's really hard to predict the future, but I wouldn't be surprised if the tendency for people to be concerned about being in densely populated, close physical spaces, people will be a little bit more resistant for some chunk of time in the future to go into these kind of crowded places.
Mills: The era of mosh pits may be over.
Schaller: Yeah. Well, this is why I think it's hard to speculate. People have short memories, right?
Mills: We'll still do stupid things.
Schaller: I may be less likely personally to dive into a mosh pit two years from now, but my kids might still be moshers in the future. Well, I think it's really an interesting question because it gets back to this idea that a lot of the things that help serve as buffers against the spread of disease are also taking away things that people value, like close interaction. And so I think that because people do have such a powerful need for social contact and so forth, that probably will recover.
What I do expect, or at least I'm very interested in seeing this, is the extent to which there will be more of a persistent norm of wearing masks. In many parts of the world, people well before the COVID pandemic tended to wear cloth masks over their faces as a way of helping to prevent the spread of just seasonal flu and colds and things like that. Yeah. And so now that's–
Mills: That may persist.
Schaller: We can't say it's a norm with everyone everywhere, but it has become much more of a norm worldwide. And I'm sure that when the pandemic is over, there will be a lot of folks who will be happy to throw their masks away and want to never touch them again. But I also bet that there will be a lot of folks who will hang onto their masks and will break them out pretty regularly during cold and flu season. And it's likely to have a lot of benefits for public health worldwide.
Mills: Yeah. I mean, it has, right? The seasonal flu numbers have been way down this year, so it's absolutely working.
Schaller: That's right.
Mills: Well, last question. What do you think are the most important questions about the behavioral immune system that remain to be answered? What else are you working on?
Schaller: I can't say I'm working on this. I'll speak more broadly on the kinds of things that folks in the broader research community are working on and what folks are likely to be working on. I think one really interesting question that goes back to how we opened and talking about that–not only is there a behavioral immune system that helps us from even getting sick in the first place, but there's also the real immune system, these immunological defenses that fight off infections when they do get inside us, and a really interesting question is the extent to which these two different kinds of immune systems actually talk to each other, how they interact.
And there is various hints of work that do suggest that they are related and they're not just fully independent. I'll give you two examples. One example comes from some work that folks have done that shows that when people actually are actually immunologically suppressed, that their immune system just isn't working as well, those folks compared to other folks tend to show more xenophobic responses to foreigners and things like that, the idea being that when people actually are immunocompromised, they compensate by ramping up the behavioral immune system.
And here's another example that some colleagues and I did a study on this and some other folks who've done similar studies that shows that when people perceive things that pose a threat of infection in their environment, or when let's say we look at disgusting pictures that it actually tends to ramp up our actual immunological defenses. People who do psychoneuroimmunology have known for years, that any kind of stressful event can ramp up our immunological defenses, but even more so than encountering other stressful things, for instance, even more so than looking at pictures that scare the hell out of me. If I look at pictures of people who themselves look sick, that tends to lead my white blood cells to mount a stronger immunological defense against infections.
Mills: It's a different part of the body that's reacting. It's not like a cortisol level. It's white blood cells, so it really is immune–
Schaller: It's not just a stress response. It actually is an immunological response. I mean, these are some really interesting hints of how these different kinds of mechanisms, these immunological mechanisms, these psychological mechanisms are in some way deeply connected. And some folks have suggested, hey, it probably makes sense to talk about this as all part of the body's integrated means of defending against infectious diseases and within an evolutionary framework, it makes sense that these things would all be deeply connected in some way, so a really interesting line of work, and I think it'll take years of work ahead is to sort out how the behavioral immune system interacts with the real immune system. That's just one example of something I think is going to be really important and interesting.
Mills: Well, thank you for joining us today, Dr. Schaller. This has been really interesting. I think you've given us a lot to think about as we make our way through the pandemic and flu season and the whole winter. I appreciate you spending time with us today.
Schaller: It's been a pleasure.
Mills: You can find previous episodes of Speaking of Psychology on our website at speakingofpsychology.org or wherever you get your podcasts. If you have comments or ideas for future podcasts, you can email us at speakingofpsychology@apa.org. That's Speaking of Psychology, all one word, at apa.org. Speaking of Psychology is produced by Lea Winerman. Our sound editor is Chris Condayan.
Thank you for listening. For the American Psychological Association, I'm Kim Mills.
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.
