The dinner party is popping. You're enjoying the wine, music and sparkling conversation—when suddenly the soiree is invaded by an unexpected guest. Your host has just unveiled a show-stopping block of blue cheese, which is now pumping out an almost tangible odor thanks to the bacterial hordes going to town on the crumbling hunk.

The question is: Are you thinking “ooh, time to eat” or “ew, smelly feet”?

Neuroscientists, it turns out, are fascinated by this pungent scenario. They want to know why we react they way we do to stinky cheeses—with revulsion or desire—because uncovering the roots of this love/hate relationship could reveal the neural basis of disgust. Today these pioneers of the revolting are using brain-scanning to take a detailed look at what these polarizing foods actually do to our brains.

Last year, for instance, researchers at the Université de Lyon used fMRI imaging to explore the brains of both cheese lovers and haters while they were viewing and inhaling dairy. Pumping the scents of blue cheese, cheddar, goat cheese, Gruyere, Parmesan and tomme into volunteers’ noses revealed that the brain's reward center displayed aversion behavior activity among cheese haters, reports lead author Jean-Pierre Royet. Further, inactivity in a region that typically fires up when hungry people see food led Royet to suggest that those disgusted by cheese may no longer view it as food at all.

The work recently won an Ig Nobel, the parody Nobel Prize-inspired awards intended to celebrate science that first makes you laugh but then makes you think (or in this case, stink). But while scanning people’s brains as they experience an olfactory onslaught may be entertaining, it could also be illuminating.

Royet’s study included a 332-person survey that sought to quantify the extent of stinky cheese aversion. Even in cheese-loving France, he found, 11.5 percent of respondents were disgusted by stinky cheese—more than triple the rate among other foods like fish or meats. “It was quite unexpected,” he says, “but it is probably the same thing in other countries in Europe, and in the USA too.”

For the purposes of the survey, those who rated their liking for cheese between 0 and 3 on a 10-point desirability scale were considered “disgusted.” More than half of them actually rated it at rock bottom, from 0 to 1. The survey also sought to understand what exactly it was about cheese that turned so many stomachs. Six out of 10 respondents simply claimed to be disgusted by the odor and taste; another 18 percent cited a cheese intolerance or allergy.

But those results still didn’t answer the fundamental question of what it is about strong-smelling cheese that makes it revolting to so many—and by extension, what makes some foods more disgusting than others. To answer those head-scratchers, you first have to understand what disgust really is. And for that, you should turn to Paul Rozin, a well-known psychologist at the University of Pennsylvania who has been trying to answer that question since the 1980s.

In 1872, Charles Darwin took an early stab at defining disgust, writing that the the term “refers to something revolting, primarily in relation to the sense of taste, as actually perceived or vividly imagined.” Rozin, known to some as "Dr. Disgust," has refined that definition further. A more compelling way to think of disgust, he says, is by what's called the contamination response.

“Take something that you really don't like, [for instance] a bitter vegetable, and just touch it to a food that you do like,” he says. “It won't necessarily make that food inedible. You can still eat it. But touch a cockroach to it, and it will have that effect. That's because it is disgusting.”

He continues: “Or think of someone who hates the taste of cilantro and is being fed cilantro through a stomach tube. Would they really be disgusted by the idea? Probably not. But they'd be disgusted by the idea of being fed cockroaches through the same tube.” (Cockroaches are a common theme in many of Rozin’s explanations.)

In his work, Rozin has found that some foods are definitely more likely to produce disgust—and animal products top the list. One explanation may be because we realize that animal-derived foods are more likely to hold harmful pathogens, he says, though it’s debatable whether such knowledge would be innate, learned or both. “Animal products have the property that they decay rapidly, unlike plant products,” Rozin says. “So they can become a source of infection and putrefaction.”

Yet stinky cheese, while itself an animal product, presents a particularly interesting case. For one thing, the pungent smell that makes it so offensive to some isn't matched by the cheese's actual taste. That's why some smelly cheese-eaters proclaim that they “just have to get it past my nose,” Rozin notes. “It has the odor of decay that elicits disgust, but it really doesn't elicit that contamination response.”

This may seem at first paradoxical, as the senses of smell and taste are so intimately entangled. In fact, much of what we refer to as taste is actually dominated by smell, which relies on organs in your nose picking up airborne chemicals. And this may be particularly true in the case of moldy, stinky fromage, notes Johan Lundström, a neuroscientist and psychologist with the Monell Chemical Senses Center in Philadelphia.

“Remember, taste can only convey five sensations: salty, savory, sweet, bitter and sour,” Lundström says. “Everything else is per definition mediated by the sense of smell, even if it is coming from the mouth.”

While chemistry is no doubt at play, the sensation of disgust is also highly dependent on emotional and social context, he adds. Cheese perfectly illustrates the complexity of this response. For instance, “If you mix butyric and isovaleric acids you can either get a very strong sensation of vomit, or of Parmesan cheese, and it's completely dependent on context whether that is disgusting or pleasant,” he says. “Similarly, the smell of fecal matter on a farm is less disgusting than the odor emanating from a porta potty at a music festival.”

Nobody knows exactly why that's the case, Lundström adds. His best guess is that we're simply less disgusted by animal waste, because human waste is more dangerous since pathogens are less likely to spread between species.

In the recent brain-scanning study, Royet found that when cheese-haters smelled the object of their disgust, or even saw images of cheese, two small areas of their reward neural circuit became more active. This suggested to him that these areas were involved in aversion motivated behavior. Perhaps, “people who are disgusted by cheese have learned to avoid cheese because they have been ill following its consumption,” he muses. “As soon as these individuals smell or see cheese, specific structures in the brain can be activated to signal that this food represents a potential danger for them.”

There’s another twist as well. Royet also looked at a part of the reward circuit that typically becomes active when hungry people smell or see food. In cheese haters who were exposed to cheese and had to decide whether it would satiate their hunger, however, this region appeared to be deactivated.

“That is, this mechanism is no longer functional. Cheese is no long recognized as food,” he says. This is one of the four main reasons Rozin suggests for why people reject foods. They find the taste unpleasant (bitter broccoli), they think it's bad for their health (fatty stuff), they consider it a nonfood (you 'could' eat paper but you won't), or they are actually disgusted by the offering.

Royet’s findings are hardly the final word on the topic, however. Lundström suggests that those who hate moldy cheeses may not have learned aversion by negative experience at all. He suggests the opposite: They just haven’t learned to like it. Young kids, including his own daughter, may refuse stinky cheese though they've never encountered it even in utero, he adds. Other brain studies have centered disgust in the insula cortex region, which, interestingly, is also involved in self-awareness.

Such mysteries go to show that disgust is a complex response that's difficult to isolate from other variable factors—including hunger vs satiety, liking vs wanting, or pleasantness vs unpleasantness. Even a factor as seemingly straightforward as intensity can muddle the picture. “People often rate stimuli that they find disgusting as more intense than those that are desirable, even when they are exactly the same,” Lundström says.

These challenges make it difficult to explore this primal human response, Lundström says. But when you’re faced with stomaching a repulsive food, it might not matter. We still might not know exactly why some foods make our stomach turn—but we definitely know disgust when we feel it.