Paul Rozin is known for many things — he is an eminent psychologist who taught at the University of Pennsylvania for 52 years, and he has gathered honors and fellowships and published hundreds of influential papers and served on editorial boards and as chairman of the university’s department of psychology — but he is best known for his work on the topic of disgust. In the early 1980s, Rozin noticed that there was surprisingly little data available on this universal aspect of life. Odd, he thought, that of the six so-called basic emotions — anger, surprise, fear, enjoyment, sadness, disgust — the last had hardly been studied.

Once you are attuned to disgust, it is everywhere.

Disgust shapes our behavior, our technology, our relationships. It is the reason we wear deodorant, use the bathroom in private and wield forks instead of eating with our bare hands. I floss my teeth as an adult because a dentist once told me as a teenager that “Brushing your teeth without flossing is like taking a shower without removing your shoes.” (Do they teach that line in dentistry school, or did he come up with it on his own? Either way, 14 words accomplished what a decade of parental nagging hadn’t.) Unpeel most etiquette guidelines, and you’ll find a web of disgust-avoidance techniques. Rules governing the emotion have existed in every culture at every time in history. And although the “input” of disgust — that is, what exactly is considered disgusting — varies from place to place, its “output” is narrow, with a characteristic facial expression (called the “gape face”) that includes a lowered jaw and often an extended tongue; sometimes it’s a wrinkled nose and a retraction of the upper lip (Jerry does it about once per episode of “Seinfeld”). The gape face is often accompanied by nausea and a desire to run away or otherwise gain distance from the offensive thing, as well as the urge to clean oneself.

The more you read about the history of the emotion, the more convinced you might be that disgust is the energy powering a whole host of seemingly unrelated phenomena, from our never-ending culture wars to the existence of kosher laws to 4chan to mermaids. Disgust is a bodily experience that creeps into every corner of our social lives, a piece of evolutionary hardware designed to protect our stomachs that expanded into a system for protecting our souls.

Darwin was the first modern observer to drop a pebble into the scummy pond of disgust studies. In “The Expression of the Emotions in Man and Animals,” he describes a personal encounter that took place in Tierra del Fuego, where Darwin was dining on a portion of cold preserved meat at a campsite. As he ate, a “naked savage” came over and poked Darwin’s meat with a finger, showing “utter disgust at its softness.” Darwin, in turn, was disgusted at having his snack fingered by a stranger. Darwin inferred that the other man was repelled by the unusual texture of the meat, but he was less confident about the origins of his own response. The hands of the “savage,” after all, did not appear to be dirty. What was it about the poking that rendered Darwin’s food inedible? Was it the man’s nakedness? His foreignness? And why, Darwin wondered — moving on to a remembered scenario — was the sight of soup smeared in a man’s beard disgusting, even though there was “of course nothing disgusting in the soup itself”?

The most important disgust accounts following Darwin come from a pair of Hungarian men born two years apart, Aurel Kolnai (born in 1900) and Andras Angyal (1902). I haven’t found any evidence that they knew each other, but it seems improbable that Angyal, whose disgust paper came out in 1941, didn’t draw from his countryman’s paper, which appeared in 1929. Strangely enough, the Angyal paper contains no reference to Kolnai. One possibility is that Angyal failed to cite his sources. A second possibility is that he was truly unaware of the earlier paper, in which case you have to wonder whether there was something so abnormally disgusting about Central Europe of the early 20th century that two strangers born there were driven to lengthy investigations of a subject no one else took seriously.

A third possibility is that Angyal started reading Kolnai’s paper and gave up midway through in frustration. While brilliant, Kolnai’s writing has the density of osmium. His paper is rife with scare quotes and clauses layered in baklava-like profusion. Nonetheless, Kolnai was the first to arrive at a number of insights that are now commonly accepted in the field. He pointed to the paradox that disgusting things often hold a “curious enticement” — think of the Q-tip you inspect after withdrawing it from a waxy ear canal, or the existence of reality-TV shows about plastic surgery, or “Fear Factor.” He identified the senses of smell, taste, sight and touch as the primary sites of entry and pointed out that hearing isn’t a strong vector for disgust. “One would search in vain for any even approximately equivalent parallel in the aural sphere to something like a putrid smell, the feel of a flabby body or of a belly ripped open.”

For Kolnai, the exemplary disgust object was the decomposing corpse, which illustrated to him that disgust originated not in the fact of decay but the process of it. Think of the difference between a corpse and a skeleton. Although both present evidence that death has occurred, a corpse is disgusting where a skeleton is, at worst, highly spooky. (Hamlet wouldn’t pick up a jester’s rotting head and talk to it.) Kolnai argued that the difference had to do with the dynamic nature of a decomposing corpse: the fact that it changed color and form, produced a shifting array of odors and in other ways suggested the presence of life within death.

Angyal argued that disgust wasn’t strictly sensory. We might experience colors and sounds and tastes and odors as unpleasant, but they could never be disgusting on their own. As an illustration, he related a story about walking through a field and passing a shack from which a pungent smell, which he took for that of a decaying animal, pierced his nostrils. His first reaction was intense disgust. In the next moment, he discovered that he had made a mistake, and the smell was actually glue. “The feeling of disgust immediately disappeared, and the odor now seemed quite agreeable,” he wrote, “probably because of some rather pleasant associations with carpentry.” Of course, glue back then probably did come from dead animals, but the affront had been neutralized by nothing more than Angyal’s shifting mental associations.

Disgust, Angyal contended, wasn’t merely smelling a bad smell; it was a visceral fear of being soiled by the smell. The closer the contact, the stronger the reaction. Angyal’s study is even more delightful when viewed in the context of its preface, which explains that the material is based on observations and conversations “not collected in any formal manner,” and that the method, “if it may be called such,” lacked objectivity and control. Reading the paper 80 years later, as a replication crisis in the sciences continues to unfold, Angyal’s humility takes on a refreshing flavor. I’m just a guy noticing some stuff, he seems to say. Let’s see where this leads.

I first met Rozin at a Vietnamese restaurant on the Upper West Side in midsummer. He arrived in a bucket hat the color of Tang and a navy shirt with pinstripes. After ordering, we sat at a blond wood table and ate rice crepes piled with diverse vegetable elements. Rozin had ordered a green-papaya salad to share, and while spearing papaya he noted that “this, right now, is a form of social bonding — eating from the same bowl.” (He and a team did a study on it.) A fun thing about hanging out with a research psychologist is that he can usefully annotate all sorts of immediate lived phenomena, and in the case of Rozin, he may even have hypothesized the explanations himself. Our crepes, to take an example, were the width of basketballs — enough to feed six, easily — and yet we each polished off the jumbo portion. “Unit bias” is the heuristic that Rozin and his co-authors coined to describe the effect back in 2006. The idea is that humans tend to assume a provided unit of some entity is the proper and optimal amount to consume. This is why movie popcorn and king-size candy bars are treacherous, and possibly one reason French people — with their traditionally small portions — remain thin.

Rozin, who is now 85, was born in the Flatbush neighborhood of Brooklyn to Jewish parents who, though they hadn’t attended college themselves, were cultured and artistic and pleased to discover that their son was a brainiac. He tested into a public school for gifted children, left high school early and received a full scholarship to the University of Chicago, where he matriculated just after his 16th birthday. Upon graduating, he took a joint Ph.D. at Harvard in biology and psychology, completed a postdoc at the Harvard School of Public Health and in 1963 joined the faculty of the University of Pennsylvania, where his initial experiments centered on behavior in rats and goldfish. As he quickly worked his way up from assistant professor to associate professor to full professor, Rozin decided that he was tired of animal studies and wanted to focus on bigger game.

Around 1970, he turned his attention to the acquisition of reading. In Philadelphia — as in many American cities — there was a problem with kids’ learning to read. Eager to discover why, Rozin parked himself in elementary-school classes and observed something strange: A large number of children were unable to read by second grade, but those same children were always fluent in spoken English. They could name thousands of objects, and they could point to Rozin and ask, “Why is this strange man lurking in my classroom?” Compared with the vast dictionary of words filed neatly in their brains, mastering an alphabet of 26 letters would seem to be a piece of cake. Instead, it was a crisis. With a collaborator, Rozin devised an experimental curriculum that moved children through degrees of linguistic abstraction by teaching them Chinese logographs followed by a Japanese syllabary, and only then applying the same logic to English. Rozin says the system worked like a dream, but the school’s response was tepid.

“The bureaucracy, the politics — I was overwhelmed,” he said. Nothing about the process of pitching and marketing and lobbying appealed to him. He calculated that it would take years to sell administrators on the curriculum and train teachers to deliver it. Instead, he and a colleague wrote several papers with the findings and walked away. “It’s the right way to teach reading,” he said nearly 50 years later, with a shrug. “As far as I know, nothing happened with it.” At the time, he wondered if maybe some other researchers would run with the idea. But Rozin was done. His mind was elsewhere, percolating on the subject he would become best known for.

Rozin’s interest in disgust, he said, started with meat. Although he is now pescatarian “with some exceptions” (bacon), he was still a full-spectrum omnivore when he started puzzling over meat. Despite being one of the world’s favorite food categories — both nutritionally complete and widely considered tasty — meat is also the most tabooed food across many cultures. Rozin wasn’t interested in the health implications of meat or in its economic or environmental significance. That stuff had been studied. What he zeroed in on was a kind of affective negativity around meat. When people disliked it, they really disliked it. A rotten cut of beef evoked an entirely different reaction than a rotten apple. Why? Or rather, what? What was the difference between accidentally biting into a moldy Granny Smith and a moldy steak? A bad apple might be icky and distasteful, but befouled meat caused a related, but totally distinct, sensation cluster of contamination, queasiness and defilement.

It was the Angyal paper that really got Rozin’s neurons firing, and on its foundation he began to construct the theory that would go on to inform — and this is no exaggeration — every subsequent attempt at defining and understanding disgust over the following decades. In Rozin’s view, the emotion was all about food. It began with the fact that humans have immense dietary flexibility. Unlike koalas, who eat almost nothing but eucalyptus leaves, humans must gaze at a vast range of eating options and figure out what to put in our mouths. (The phrase “omnivore’s dilemma” is one of Rozin’s many coinages. Michael Pollan later borrowed it.) Disgust, he argued, evolved as one of the great determinants of what to eat: If a person had zero sense of disgust, she would probably eat something gross and die. On the other hand, if a person was too easily disgusted, she would probably fail to consume enough calories and would also die. It was best to be somewhere in the middle, approaching food with a healthful blend of neophobia (fear of the new) and neophilia (love of the new). It was Rozin’s contention that all forms of disgust grew from our revulsion at the prospect of ingesting substances that we shouldn’t, like worms or feces.

The focus on food makes intuitive sense. After all, we register disgust in the form of nausea or vomiting — nausea being the body’s cue to stop eating and vomiting our way of hitting the “undo” button on whatever we just ate. But if disgust were solely a biological phenomenon, it would look the same across all cultures, and it does not. Nor does it explain why we experience disgust when confronted with topics like bestiality or incest, or the smell of a stinky armpit, or the idea of being submerged in a pit of cockroaches. None of these have anything to do with food. Rozin’s next project was to figure out what linked all of these disgust elicitors. What could they possibly have in common that caused a unified response?

In 1986, Rozin and two colleagues published a landmark paper called “Operation of the Laws of Sympathetic Magic in Disgust and Other Domains,” which argued that the emotion was a more complicated phenomenon than Darwin or the Hungarians or even Rozin himself had ventured. The paper was based on a series of simple but illuminating experiments. In one, a participant was invited to sit at a table in a tidy lab room. The experimenter, seated next to the participant, unwrapped brand-new disposable cups and placed them in front of the subject. The experimenter then opened a new carton of juice and poured a bit into the two cups. The participant was asked to sip from each cup. So far, so good. Next, the experimenter produced a tray with a sterilized dead cockroach in a plastic cup. “Now I’m going to take this sterilized, dead cockroach, it’s perfectly safe, and drop it in this juice glass,” the experimenter told the participant. The roach was dropped into one cup of juice, stirred with a forceps and then removed. As a control, the experimenter did the same with a piece of plastic, dipping it into the other cup. Now the participants were asked which cup they’d rather sip from. The results were overwhelming (and, frankly, predictable): Almost nobody wanted the “roached” juice. A brief moment of contact with an offensive — but not technically harmful — object had ruined it.

In another experiment, participants were asked to eat a square of chocolate fudge presented on a paper plate. Soon after, two additional pieces of the same fudge were produced: one in the form of “a disc or muffin” and the other shaped like a “surprisingly realistic piece of dog feces.” The subjects were asked to take a bite of their preferred piece. Again, nearly no one wanted the aversive stimuli, which is how psychologists refer to “nasty stuff.” (When asked about the outliers who opted for the nasty stuff, Rozin waved a hand and said, “There’s always a macho person.”)

These results might seem obvious, but the experiments were designed rather craftily to elicit a disgust response rather than any of the other typical food-rejection responses, which include distaste (rejecting something because it looks or smells bad, like broccoli if you’re a broccoli hater) or danger (rejecting something because it might harm you, like a poisonous mushroom or a nonsterilized cockroach) or inappropriateness (rejecting something because it is not considered food, like tree bark or sand). Disgust was unlike the other three responses in one peculiar fashion: It could be motivated primarily by ideational factors — by what a person knew, or thought she knew, about the object at hand.