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Episode 14 features psychology professor Debra Lieberman.

The Covid Tonic, Ep. 14 Transcript

By Marian L. Tupy @HumanProgress

The following is transcribed from Episode 14 of The Covid Tonic. The full video interview between Marian L. Tupy and Debra Lieberman can be found here.

Marian Tupy: Debra Lieberman, thank you very much for joining me for this episode of The Covid Tonic. I was very pleased to come across your paper on pathogens and disgust and human evolution because I think that it is a very topical subject since, unfortunately, we are still in the middle of the pandemic. So, I thought that we could spend half an hour to 45 minutes discussing your very interesting paper on pathogens and disgust. 

Debra Lieberman: Thank you for the invitation. I’m glad to be here. 

MT: Let’s start, so to speak, at the beginning. Can you explain the selection pressures of pathogens on human evolution?

DL: So, when you say the beginning, it depends on exactly what you mean by the beginning. Pathogens have been around from the get-go, and they have exerted strong pressures on all living organisms, and they have led to the evolution of some very interesting features and behaviors. 

Going all the way back, it’s thought that pathogens are the reason why sex evolved. So, instead of asexual reproduction … The idea that you get to scramble genomes and produce offspring of variable genomes [inaudible 01:45] thwart the effect of pathogens. That’s one of the benefits that’s thought to have outweighed the costs of reducing the probability of genes being able to successfully replicate 100% as occurs in asexual reproduction. So, pathogens are responsible for the evolution of sex. 

Depending on your read of the literature and how you view the jump from prokaryotes bacteria to eukaryotic cells, they’re also potentially responsible for the evolution of male and female … Once you have sexual reproduction and so forth and pathogens existing in the environment, it sets up other types of interesting pressures, ones that require organisms to now avoid becoming contaminated and infested with other types of disease-causing organisms, or organisms that could take over their bodies and use it for their own reproductive purposes. So, we have evolved along with other animals and likely other other kingdoms’ mechanisms for avoiding these negative effects of pathogens. This includes detecting their likely presence and avoiding them in various ways, cleaning and ensuring that we’re not carrying around unnecessary past parasites and other microorganisms, but also it has affected the way we select sexual partners—ensuring that we are selecting partners that are healthy and are going to increase the probability that we produce offspring that are capable of surviving.

MT: If I remember correctly from your paper, the key is that pathogens evolve at a much faster pace than we do. In other words, you have many more generations of evolution of pathogens inside the human body looking to penetrate and destroy those defenses. 

Can you just explain how the recombination of the genes helps with breaking that cycle of pathogens’ attempts to destroy human life. 

DL: So, when you are born—let’s just start with the birth of one organism. We can even take Dolly the sheep. I mean, if you remember, they tried to clone a sheep. But, whatever. Take any individual. Take a human, and they, by virtue of being human, have inherited a whole bunch of pathogens that are in, on, and around you, and so forth from day one, right. So, within the womb as you’re kind of exiting into the world, you now are exposed to this environment that is inundated with pathogens. Obviously, we can’t see them, but they’re all around us. 

Those pathogens now have a whole generation in which to adapt to the internal biochemistry of the host, so as you were saying, bacteria and so forth undergo replication far faster than we do. So, in any one given human generation, you have gazillions of generations of pathogens enabling them to better adapt to the internal biochemistry … That means being able to secure resources so that they can replicate themselves and learning, quote unquote, how to avoid elements of the immune system so they’re not destroyed. 

So, those pathogens that are more successful at getting resources and evading the immune system are going to exist in this host throughout the life of that host. Now, you can imagine at age—I don’t know—20, you now have pathogens that have honed their skills in order to take advantage and exploit the host far better than they were able to at day one. 

But now, at year 20, let’s just say, for instance, there was asexual reproduction and that host—a finger, a rib—dropped to the floor and started to sprout a whole ‘nother individual. You’d end up with another individual who has inherited these pathogens that are already really finely-tuned to that biochemistry. Give them a whole ‘nother 20 years to adapt, and they’re just going to wreak havoc. Do that again in the third generation, you’re just going to create a situation that gives an extreme advantage to these pathogens that are capable now of taking over the planet that is the host. 

But instead, if at year 20, let’s just say, or 35, you end up taking half your genome and another half of another person’s genome and recombining them, you’re basically pushing the pathogens back to square one. So now, pathogens that were really well-adapted to the internal biochemistry … Things look ever so slightly different from their standpoint at the level of DNA/RNA proteins. Their ability to actually exploit the host now are thwarted again as that next generation starts.

So now, you have an individual that’s not going to feel the full effects of their ability to exploit the host because they have different kinds of genes, but this actually presupposes that the person that you’ve recombined your genomes with has different genes. Sexual reproduction entails a lot of polymorphisms, and so the benefits of sex are due to the fact that there are multiple versions of genes that are different from the perspective of a pathogen’s point of view, but not necessarily in terms of functionality. So, they’re still performing their function, but at the DNA/amino acid protein level, they look ever so slightly different, which provides an advantage to the individual from not feeling the full effects. 

But, it also means … I’m sure you’re going to ask me about this in a little bit … This is also why we don’t tend to select close genetic relatives as sexual partners, specifically because mating with a close genetic relative is more like cloning. The idea is you’re basically recombining your genes with someone who shares very similar kinds of genes … That increases the chances that pathogens are going to be better able to exploit that resulting offspring, so you do find that children of close genetic relatives do tend to suffer a lot of defects. 

It’s originally thought that these are just recessive mutations that can be co-inherited from brother and sister, for instance, and that is true, but the I think far more important selection pressure would have been from pathogens by virtue of creating a more similar environment that’s hospitable to those pathogens who are familiar with the environments from the last generation. 

MT: That is what you, in your paper, call a ‘physiological defense’ against pathogens. But of course, you are a well-known psychologist, and your paper is actually one that is focusing on psychological defenses. So, let’s turn to those and talk a little bit about the psychological defenses against pathogens. 

DL: In addition to an immune system, which would be part of the physiological set of adaptations that are there to combat pathogens, we also have what’s been called a behavioral immune system. I call the behavioral immune system ‘disgust.’ I think there’s some debate in the literature as to whether or not they are synonymous, but I call them the same thing. 

So, part of our psychology is there to help us identify likely sources of pathogens and to strategically navigate them. Pathogens can get in in various ways, so not only can they travel—as I was mentioning just a moment ago—vertically down the generations from mom to dad to offspring, but rather they can also travel horizontally. How do you protect against this? Pathogens can get in and land anywhere. A couple of the main portals are your mouth, right, so you can ingest pathogens. Certainly, a component of our food psychology is focused on detecting those pieces of matter, those substances that could potentially cause harm. 

Pathogens and rotten foods and so forth are exactly one type of substance that would have decreased one’s reproductive success, chances of surviving, and reproducing. So, that’s not the only thing that would have jeopardized it, but, in any case, pathogens are one source of problems, of costs that we had to deal with. But certainly, touching things and coming into contact with things in the environment would have also been one way for pathogens to get in … So, putting one’s hands into feces. Yes, you have to touch feces in order to eat feces, theoretically speaking … The idea that you don’t want to touch feces, at least humans don’t typically. 

MT: Typically. 

DL: Typically, yes. I’ve heard stories. But steering clear of things like feces and vomit and blood, guts, and gore and preventing contact with that would have ensured greater protection against transmission of any pathogens. 

Those are two ways pathogens can get in. Then, the third way would be sex. 

MT: Before we get onto each of these, which is to say consumption of food, touching things, and, finally, looking for mates … Obviously, physiological defenses against pathogens are heritable. What about psychological traits, what you call the behavioral defense mechanism? Is that also heritable? Does it come from learning or is it something that we are born with? 

DL: Inherited, yes. So, these are pieces of mental software, right? We have to come equipped with components of these mental adaptations and structures that guide learning in very specific ways. Otherwise, how would this learning ever occur?

So, I would say it’s not necessarily a matter of learning versus innate or inheritance, rather, I kind of sidestep that particular dichotomy to basically say: it naturally has to be both. Genes are just genes. Genes are just things in a Petri dish. They build structures against the backdrop of very sophisticated environments, and all the while taking input from those environments in order to construct their systems for humans in terms of their particular systems that are part of disgust, so food psychology, contact psychology, and sexual psychology. These are incredibly complex systems … Pieces of mental software each requiring calibration starting from single cell all the way up to the fully sexually mature human. 

But the idea that naturally, there’s going to have to be some software there, some algorithms that are present, that are going to guide what information is required in order to solve each of these problems. It guides what counts as food. It probably has some particular strategies and rules that are built-in to the extent that they were more foolproof over evolutionary history. The presence of maggots and worms and things that are moving and wiggling was a really good sign that the meat you’re about to eat is probably old and therefore contaminated. There’s likely lots of these types of features that are quote unquote ‘baked-in,’ so to speak. 

But the idea that observing what other individuals do, particularly trusted individuals, is going to be an important feature of these evolved systems. 

MT: Sure, and not just with pathogens, but with mom telling you, “Don’t put a fork in the electric socket,” and things like that. 

DL: My mom actually did that when she was little. 

MT: But the reason why I’m so interested in what we come into the world with, already baked-in, is things like … You take a bunch of monkeys who are raised in captivity, and suddenly, you introduce a snake into their cage, and they go bonkers without ever having seen a snake before. Or, a newborn baby lizard that knows that it has to run away from a charging predator … Things like that. 

How much of that fear and other psychological traits are we coming with already baked-in? But, it seems to me that you seem to think it’s a sizable proportion. 

DL: Well, I don’t even want to call it proportion. It’s just that every component of this system requires … I mean, there’s programming underlying this. So, unless you actually have someone in your brain writing this programming right—which doesn’t seem to be happening— that programming has to be there in order to deploy in a targeted way.

Better put, those organisms that had programming that deployed in particular ways tended to do better than others that either didn’t have such programming or programming that led to less fit types of behaviors. So, it would be a bad strategy to have everything baked-in with little ability for these systems to consider environmental context, for instance—to be insensitive to motion, to color, to smell. 

I would say that we have the ability to flexibly respond to components of our environment by virtue of having sophisticated underlying software that supports that flexibility, knowing what to do in different contingent-based manners. So, I would say flexibility comes at the price of increased psychological adaptations, and so humans are incredibly flexible with their behavior, which suggests that we have a very rich underlying psychology that’s guiding what we eat, what we touch, and who we mate with. So, yeah, I guess that’s what I would say. 

MT: Turning into those three aspects—what we touch, what we eat, and whom we mate with. In your paper, when it comes to this section on food consumption, you discuss mechanical, chemical, and biological cues to pathogen presence. Could you tell us about that?

DL: Sure. As a disgust researcher, I have been narrowly focused on pathogens, but when it came time to think about consumption, it became very clear that there’s a far larger picture. Just think about a physical system that’s trying to figure out: what do I consume that’s going to increase my ability for respiration, homeostasis maintenance, and all these different biological [inaudible 18:57:00]. Instead of just thinking about how we have a pathogen avoidance that leads to disgust, I think that pathogen avoidance and the disgust that it leads to is part of our entire consumption system. 

So, you see any particular item, and you can evaluate it in terms of its expected value of consumption. What is the fitness value of consuming this particular item? You can imagine that there’s software performing a calculation of … Given the particular features that I can detect, I can estimate what the likely benefit is of eating this particular item in my current condition, in the current context. So, we have systems that are evaluating objects for sugar content, salt content, and all sorts of things … Proteins. But that’s on the positive side. 

The negative side is, what kinds of things could have hurt us? Things such as pathogens and disease-causing organisms, but also particular chemicals could potentially be very dangerous, like poisons. Then, you have other mechanical features like thorns and barbs and so forth. So, the idea that we have evolved different types of systems in order to detect the presence of these things … But, by and large, I would say the detection of these positive attributes and harmful attributes have to be integrated in some way to generate a singular decision— do I eat this? 

So, by hypothesis, it’s thought that these all get integrated by a system that’s assessing the value of consumption, and depending on one’s condition—how healthy one is, how hungry one is, how punished one can be for consuming something—there’s likely a system that estimates my expected fitness value for consuming this item. When that’s high, you perceive something as really yummy and scrumptious … Ooh sugar and fats and yumminess, right? But, that’s only when I’m really hungry, because after I’ve just eaten five tubs of ice cream, the idea of sugar and fat … It’s like, oh my god, I’m going to barf, get that away from me … Because the expected value of consuming this right now to me is very low. That generates not an appetitive response but more of an aversive response … Not as aversive as if I saw worms popping out of it, but an aversive response nonetheless. 

When you evaluate something to have a very low expected value of fitness either because it is mechanically, chemically, or, in terms of pathogens … Whether it’s a toxin, whether it just looks like it would cause internal damage, whether it exhibits cues that suggest that there are pathogens there, whether visual, olfactory or so forth … That produces a very low estimate of expected value of consumption, causing the “ew, gross” response. One of the challenges that me and my co-authors have come across with respect to this model is … Another leading figure in disgust research had said, “oh well, you don’t experience disgust when eating rocks … to a rock,” or something. 

I think to myself, I don’t know. I think if someone … Not a rock as such. I don’t see a rock and go, ‘ew,’ but the idea that someone picks up that rock and then brings it close to me and then prevents me from moving away and then tries to get it in my mouth—yeah, that’s gonna be disgusting. Yeah, gross, get it away, stop … In the very same way if someone was bringing spoiled milk to my mouth.

I would argue that our initial reaction to objects out there might not be disgust because they might not necessarily show as strong cues to being harmful or not harmful. But the idea that … Get them closer and closer and closer to one’s mouth, and that’s going to increase the negativity of all those items. I’m holding up a little dongle thing here. This little adapter thing—it’s not disgusting, right? But then, you say, would you eat that as food? I laugh. I go: “Haha, no, no, no, no.” But then, it was like, “No, eat it, eat it, eat it, eat it.” That’d be really gross. In any case—

MT: Even as you were pushing it toward the camera and suddenly it was in my face, I was thinking, “No, I don’t [want to] eat this.” 

So, the feeling of disgust will be a result of those calculations that you were talking about. I was thinking about ice cream. Ice cream is something that is just absolutely delicious, so obviously, I’m not experiencing disgust when I’m about to eat my first pint of ice cream. But after I’ve eaten two or three and I see another pint of ice cream, suddenly I developed this feeling of disgust because the set of calculations or the set of facts that go into that consumption calculations have changed. Would that be correct?

DL: Exactly. I think we talk about this in terms of the buffet effect. You go to a buffet and it’s like, oh my gosh, look at all this food, and you run wild and you eat as much … And then you go for a second plate and then possibly a third. But certainly, by the fourth plate it’s like, oh my God, I’m gonna barf. So, the question is … That’s interesting language … People like, “I’m feeling sick.” 

The idea that people are starting to use the language of disgust suggests that there is low value of consumption for the next morsel. 

MT: You also mention prior experiences and developmental state. Could you discuss those … How do those affect food consumption and pathogens?

DL:  Sure. So, a famous effect—the Sauce-Bearnaise effect—that Martin Seligman has talked about in terms of past experience eating a particular food item … If it was bad, it’s something that will very much influence and update your calculation of expected fitness value of consuming again. So, once you’re sick with something, or if you’ve experienced foodborne illness … First off, what’s very interesting about foodborne illness is as you are hovering over the toilet, what’s going through your mind? It’s not like, “When are my taxes due,” or “Did I leave the front door unlocked,” or “Is he gonna call me again?” Those aren’t the things. 

It is, “What did I eat?” Typically, it’s not, “It must have been the bread,” or “It must have been the cracker.” It’s, “Oh, it was the fish, it was the meat.” 

I find it very interesting that our … So, you talk about the kinds of software and circuitry that’s required. No one taught you to query that particular aspect of your day, four to six hours ago. That’s the idea. It’s not, “Oh, how hard or soft was the chair I was sitting on?” No, it’s a very specific part of your behavior that happened a very particular time period ago. That circuitry has to be triggered when there is an indication that one has ingested a pathogen. But in any case, what was I just talking about?

MT: The developmental stage. 

DL: Oh, yeah. So, past experience. 

Everyone has a past experience with something … Most people it’s tequila or something, although that tends not to stop people from having tequila again but much later, certainly, not the next day. Then with developmental periods … That’s very interesting because certain types of foods pose different threats at different developmental stages. So, during early development, what you find is that children’s neurological systems are still developing and secondary compounds from plants and toxins can very much interfere with and affect the development of all systems. 

So, children are famously very against eating their vegetables, much to the dismay of parents, but they’ve got good reasons for this. That’s because vegetables harbor a lot of potential plant toxins, like cyanide, that can very much influence [inaudible 28:20:00] and the development of lots of different types of systems. So, children are resistant to eating plated vegetables. This is one example of how a developmental stage can very much gear preferences. 

After systems have developed, children start to experiment with different types of vegetables, but certainly only after the window in which these systems are developed and calibrated and online. 

MT: Are there any other indications from nature that make us go, “Oh, that’s gross, I’m not gonna touch that, I’m not gonna consume that” … For example, smell or things like that. 

DL: Exactly. It ends up that the sulfur that bacteria can produce, we can detect, so there’s some interesting smells that are associated with bacterial presence … Also, the tactile cues of how mushy things are and whether or not the actual body of a fruit or plant has been compromised. 

Maggots are interesting—sort of—in the sense that when you think about maggots, they’re just soon-to-be flies. Why is that bad? I’ve actually had this experience when I was in grad school, and it was a powerful one which was, I hadn’t taken my garbage out and apparently had made some steak or something that ended up in the garbage, and I went to go change it, and I saw these maggots, and that was it. I was done. But the question is… It was incredibly powerful. Maggots serve as a timestamp for how long animate matter has … Meat has been sitting around. The presence of maggots and the size and the number of them is a good indication of how long a particular animal has been exposed to the elements and deprived of its own immune system to ward off pathogens. 

So, I find maggots to be an interesting timestamp of sorts, but then you also have—like you mentioned—smells … Smells and just visual cues that would indicate that something is rotten. 

MT: Mushiness … That would be something like instead of a hard pair you have a mushy one. That’s disgusting, and you’re not going to eat that. Would that be an example … The softer something is, the more likely it is that you are not going to eat it? Or am I not getting it right?

DL: I think you have it right, but it’s all relative. So, the idea … I’m not going to eat a pear that’s as hard as a rock, but I’m also not going to eat a pear that when I pick it up, it basically falls apart in my hand. But, then again, it depends on how hungry I am. If I haven’t eaten for a long time, if it’s a pear, is a pear, is a pear, and I will take my chances depending on how hungry I am. So, I think that, again—

MT: Of course, there are plenty of examples of prisoners in the Soviet labor camps or in Auschwitz or people who’ve been stranded on a ship somewhere who will eat infected meat. They will flick off the maggots, but they will consume it because they are making that calculation, and I never thought about it in that way, but that makes a lot of sense.

DL: Yeah, you find people who have been stranded and the mountain climbers … The idea that suddenly now, that person over there turns into a big old chicken leg. So, I think that calculations change our appetitiveness, or the appetitiveness of different kinds of substances. Or, another way of saying it, it decreases how aversive we find them. 

But, if I can add in one thing that I think is important that I don’t know is mentioned to the extent it should be mentioned in that chapter, which is … So, you mentioned your nutritional state, your health, your development and experience and so forth, but one of the things that’s also going to factor into whether or not you find this piece of morsel appetitive is going to be, what are the costs? Not only the chemical, mechanical, pathogenic costs of consuming it, but the social costs. The idea is if I’m Jewish and keeping kosher, milk and meat cannot be eaten together and you can’t eat pork. So, the idea that I find pork disgusting—

MT: Not my Jewish friends. 

DL: No, no, no— nor I, what are you talking about? 

But yeah, that’s because I don’t have a group of people who can condemn me and exact some type of cost and punishment. I do believe that one interesting facet of human nature that doesn’t seem to be present in other animals with disgust circuitry—because I do believe that other animals have this software— is that in addition to taking into account all these other features, we will forego eating something if it means we will be socially condemned. So, this integrator has to in some way take into account the social costs. You find that a lot of cultures and societies have certain taboos for certain foods, so it’s very interesting … The idea that we can use that in order to identify groups and out groups and so forth, but it affects our food psychology to be sure.

MT: Let’s now turn to the third part of your paper, which has to do with sex and mating. So humans, you argue, evaluate mates based on mate value, kingship, gender and own mates’ value. Can you explain these in context?

DL: How much time do you have? So mating psychology. There’s a lot that goes into that. The way that I’ve broken a very complex system down is really just to talk about how humans assess the mate value of another individual. You identify an individual, you categorize them according to their sex based on the features that they have—so we have procedures for determining whether people are male or female—and I think that once that assignment has occurred, then you apply a certain set of evaluations that go into estimating that individual’s mate value. It’s also possible … Well, anyways, it doesn’t matter. 

The idea is that for men … What types of features would have influenced their mate value and their ability to secure resources, protect, and invest in women and their offspring are certainly characteristics that have been well-studied by evolutionary scientists. In terms of women—basically, fertility [and] health … So, cues to youth and health seem to be two major features that would have influenced a female’s mate value. 

Basically, somehow, we are calculating the mate value of other individuals. This is one part of the equation that goes into whether or not we assess someone as a suitable sexual partner for us. Now, it ends up that we can assess the mate value of everyone. As a woman, I can assess the mate value of every other woman, and I can assess the mate value of every other man. I can assess the mate value of kin and non-kin. Fathers can assess the mate value of their daughters for the purpose of setting a bride price potentially. It’s an objective measure, basically. But just because we can assess the mate value of all other individuals, a high mate value doesn’t necessarily translate into a high likelihood that person is a proper sexual partner for me. Close genetic relatives … I don’t have a brother, which is why I can talk about this so easily … My non-existent brother is really hot, but yet, he’s not a good sexual partner for me because he’s closely related to me. 

So, it ends up that an individual’s mate value has to somewhere get combined with their genetic relatedness, or estimates of their genetic relatedness, in order to produce … Just like there is an expected value of consumption, there’s an expected sexual value. What is the expected fitness value of selecting this person as a sexual partner for me? 

A high mate value combined with a high genetic relatedness translates into a low sexual value. But remember, just as for food, context matters. So, when pickings are slim … It’s not going to necessarily affect female mating psychology, but certainly potentially males more than females by virtue of the fact that there’s less cost associated with any reproductive venture on the part of males as compared to females … But context very much matters. 

You mentioned one context, which is a person’s own mate value—the idea that one’s own mate value should very much set how attractive … And your expected sexual value of another individual. That’s been shown in a variety of really interesting experiments.  Normally, a colleague at Singapore Management University did a study a while back where he gave men a wad of cash to hold. Suddenly now, this influx of resources … And it was a lot of money he asked these men just to hold. Now, their criteria in terms of how attractive a woman was required to be to go out with them jumped. This effect didn’t necessarily happen for women who were given a wad of cash; it didn’t increase the attractiveness of the man that they wanted to date.

It’s just very interesting that perceived mate value—which can, it seems, be pushed around rather easily—is something that should affect one’s strategies for selecting a mate.

MT: So, close genetic kinship, such as brother and sister, would induce a feeling of disgust preventing them from mating under normal circumstances. Then, you could take it to, for example, gender … That a heterosexual male would not normally consider having sex with another male under normal circumstances. That idea would create a feeling of disgust unless it is, for example, in an extreme situation such as, for example, in a prison environment where these calculations may get flipped. Do I understand correctly how you think these behavioral patterns operate?

DL: Yeah, I think that’s right. So, context matters. Before I get to the jailhouse scenario … The idea that people are evaluating kinship … Kinship isn’t something that’s necessarily written on the forehead. It’s not like I come into the world and there’s an R zero that you’re seeing on my forehead. You have to infer that I am not a close genetic relative. How do you do that?

Part of my research has looked at how we figure out who our close genetic relatives are in order to have these natural aversions develop. It looks like for siblings, seeing the female who birthed and breastfed you birthing and breastfeeding another is a really great cue that that’s my sibling. So, whether you’re three, four, five, six, seven, eight, nine, ten years old, you see your mother pregnant and then give birth to and start breastfeeding an individual, that’s a sibling. But if you’re the younger sibling, and you aren’t around to see your older sibling birthed and breastfed and cared for as an infant, it seems as if the best cue that you have to go on is also a type of parental investment … The idea that you’re tracking who else seems to be getting cared for on average more by the same individuals who are caring for me. 

My research points to some interesting possibilities, which is that it looks as if it takes—if you are the younger sibling and you have an older sibling out there—14 to 15 years of tracking who they’re receiving investment from in order to get to the same certainty that they’re a sibling that comes if you’re the older sibling and seeing that younger sibling cared for and nursed as a baby. It takes 14 or 15 years to piece it all together, which is, well … First it’s two things. It seems to be right before sexual maturity, which gives me goosebumps every time I think about that because that’s pretty cool. The other thing that seems to be evident by that is why wouldn’t that be higher faster? Why does it take so long? 

It suggests, perhaps, our social ecology as hunter-gatherers, which is that children were roaming around, and a lot of alloparenting could have been occurring … So, the idea you have to sift between events of my parents caring for individuals reliably during particular periods of time that might be few and far between. If children are running around and getting food from here, from there, from everywhere, and sleeping over here, there everywhere, but when they’re sick they come home to mom, or when they’re hurt or they’re scared, they come home to mom … These clarifying moments might be very important for detecting who is a possible older sibling in the environment. When there’s a problem, who comes for aid, and who is my mom willing to help out? Who’s my father willing to help out under those circumstances?

The idea that it takes so long to figure out who your older siblings are is a question that I’ve had now because it … So, in any case, you see who your relatives are in a variety of ways by tracking parental investment. But, the important point that I was trying to make is that if these cues are absent—even if someone is a genetic relative and these cues are absent— your mind isn’t going to accurately classify them as a genetic relative. 

Fast forward to modern environments where you have siblings who were raised apart and now they meet during adulthood, there are no cues present that they’re in fact genetically related. In fact, they might find each other even more sexually attractive than the average bear for very interesting reasons. Whatever the case may be, the absence of these kinship cues prevents the development of this natural aversion. So, it’s not genetic relatedness per se, but it’s the perception of the cues that would have correlated with genetic relatedness in the past. 

It works the other way too. No cues with genetic relatives, no incest aversion, or no sexual aversion, but the opposite of cues present toward non-genetic relatives will lead to sexual aversions. That’s what has occurred in, for instance, on the Israeli Kibbutz, where our children were raised together and they developed sexual … I mean, they were raised by the same caregivers. So, the mind is like, “Ah, this is my sibling,” and it ends up they find each other less sexually attractive during adulthood. 

MT: You noted in your paper that you exclude morality as a proper domain of disgust. Can you explain the interplay between morality and disgust? 

DL: Right, well, the backstory is, when I started thinking about disgust, it came in three flavors: pathogens, sexual, and moral. 

It’s really been just having to think about the systems and the actual circuitry and information software You know the inputs, the integrators, the outputs … That really started to indicate to me that first, pathogen disgust is part of the whole system. It’s really just eating, touching, and sex as the different domains. The moral component was always a perplexing one for me, with co-authors and researchers as well. 

With Josh Tybur and other colleagues, we’ve developed a three domain disgust scale, where we measure individual differences in pathogen disgust … So blood, guts, and gore, sexual disgust, but also moral infractions as well. So, it’s part of that scale, and there it will remain, but I do think that it’s not a proper domain of disgust. That’s because it seems to me that disgust, by virtue of generating an expected value of consumption, expected value of contact, and an expected sexual value … Those three variables in the head and estimations that we create … Those are what feed into the system for social value. So, those actually can affect how much I value another individual. Do you come in contact with feces? Do you eat things that I find disgusting? You are someone that I prefer to not associate with. We have less overlap in the things that we enjoy. Are you attracted by things that I am not attracted by? 

The idea that we diverge in ways that suggest that we would not share overlap, and therefore you would not be as valued a cooperative partner … We are constantly evaluating other people in terms of their social value, I think that disgust—either by what you eat, by what you touch, by what you have sex with—are things that filter into this social value system and cause you to have lower social value for some people and higher social value for others. 

I believe that this social value system is the morality system and what generates our moral sentiments. I think disgust feeds into this system, and my disgust at something can produce a reaction that something is immoral. 

MT: Thank you. This has been absolutely fascinating.

DL: You’re welcome. 

Marian L. Tupy is a senior fellow in the Cato Institute’s Center for Global Liberty and Prosperity and editor of HumanProgress.org.


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