Eating hot chili peppers allows us to court danger without risk, activating areas of the brain related to both pleasure and pain.
As winter settles in and temperatures plunge, people turn to food and drink to provide a little warmth and comfort. In recent years, an unconventional type of warmth has elbowed its way onto more menus: the bite of chili peppers, whether from the red jalapeños of Sriracha sauce, dolloped on tacos or Vietnamese noodles, or from the dried ancho or cayenne peppers that give a bracing kick to Mayan hot chocolate.
But the chili sensation isn’t just warm: It hurts! It is a form of pain and irritation. There’s no obvious biological reason why humans should tolerate it, let alone seek it out and enjoy it. For centuries, humans have eagerly consumed capsaicin—the molecule that generates the heat sensation—even though nature seems to have created it to repel us.
Like our affection for a hint of bitterness in cuisine, our love of spicy heat is the result of conditioning. The chili sensation mimics that of physical heat, which has been a constant element of flavor since the invention of the cooking fire: We have evolved to like hot food. The chili sensation also resembles that of cold, which is unpleasant to the skin but pleasurable in drinks and ice cream, probably because we have developed an association between cooling off and the slaking of thirst. But there’s more to it than that.
Paul Rozin, a professor of psychology at the University of Pennsylvania, became interested in our taste for heat in the 1970s, when he began to wonder why certain cultures favor highly spicy foods. He traveled to a village in Oaxaca, in southern Mexico, to investigate, focusing on the differences between humans and animals. The residents there ate a diet heavy in chili-spiced food. Had their pigs and dogs also picked up a taste for it?
“I asked people in the village if they knew of any animals that liked hot pepper,” Dr. Rozin said in an interview. “They thought that was hilariously funny. They said: No animals like hot pepper!” He tested that observation, giving pigs and dogs there a choice between an unspicy cheese cracker and one laced with hot sauce. They would eat both snacks, but they always chose the mild cracker first.
Next, Dr. Rozin tried to condition rats to like chilies. If he could get them to choose spicy snacks over bland ones, it would show that the presence of heat in cuisine was probably a straightforward matter of adaptation. He fed one group of rats a peppery diet from birth; another group had chili gradually added to its meals. Both groups continued to prefer nonspicy food. He spiked pepper-free food with a compound to make the rats sick, so they would later find it disgusting—but they still chose it over chili-laced food. He induced a vitamin-B deficiency in some rats, causing various heart, lung and muscular problems, then nursed them back to health with chili-flavored food: This reduced but didn’t eliminate their aversion to heat.
In the end, only rats whose capsaicin-sensing ability had been destroyed truly lost their aversion to it. Dr. Rozin came to believe that something unique to humanity, some hidden dynamic in culture or psychology, was responsible for our love of chili’s burn. For some reason apparently unrelated to survival, humans condition themselves to make an aversion gratifying.
Not long after, Dr. Rozin compared the tolerances of a group of Americans with limited heat in their diets to the Mexican villagers’ tastes. He fed each group corn snacks flavored with differing amounts of chili pepper, asking them to rank when the taste became optimal and when it became unbearable.
Predictably, the Mexicans tolerated heat better than the Americans. But for both groups, the difference between “just right” and “ouch” was razor-thin. “The hotness level they liked the most was just below the level of unbearable pain,” Dr. Rozin said. “So that led me to think that the pain itself was involved: They were pushing the limits, and that was part of the phenomenon.”
In the human brain, sensations of pleasure and aversion closely overlap. They both rely on nerves in the brainstem, indicating their ancient origins as reflexes. They both tap into the brain’s system of dopamine neurons, which shapes motivation. They activate similar higher-level cortical areas that influence perceptions and consciousness.
Anatomy also suggests that these two systems interact closely: In several brain structures, neurons responding to pain and pleasure lie close together, forming gradients from positive to negative. A lot of this cross talk takes place close to hedonic hot spots—areas that respond to endorphins released during stress, boosting pleasure.
The love of heat was nothing more than these two systems of pleasure and pain working together, Dr. Rozin concluded. Superhot tasters court danger and pain without risk, then feel relief when it ends. “People also come to like the fear and arousal produced by rides on roller coasters, parachute jumping, or horror movies,” he wrote in the journal Motivation and Emotion—as well as crying at sad movies and jumping into freezing water. “These ‘benignly masochistic’ activities, along with chili preference, seem to be uniquely human.” Eating hot peppers may literally be a form of masochism, an intentional soliciting of danger.
Dr. Rozin’s theory suggests that flavor has an unexpected emotional component: relief. A 2011 study led by Siri Leknes, a cognitive neuroscientist then at Oxford University, looked at the relationship of pleasure and relief to see if they were, in essence, the same. Dr. Leknes gave 18 volunteers two tasks while their brains were scanned: one pleasant, one unpleasant.
In the first task, they were asked to imagine a series of pleasurable experiences, including consuming their favorite meal or smelling a fresh sea breeze. In the other, they were given a visual signal that pain was coming, followed by a five-second burst of 120-degree heat from a device attached to their left arms—enough to be quite painful but not enough to cause a burn.
The scans showed that relief and pleasure were intertwined, overlapping in one area of the frontal cortex where perceptions and judgments form, and in another near the hedonic hot spots. As emotions, their intensity depended on many factors, including one’s attitude toward life. Volunteers who scored higher on a pessimism scale got a stronger surge of relief than did optimists, perhaps because they weren’t expecting the pain to end.