Stories of pepper spray have been all over the news lately. On Fox News, Megyn Kelly wondered what all the fuss about this "food product" was, and while pepper spray is no vegetable, the compound that makes pepper spray into a weapon at 2-5.3 million Scoville units, is indeed the same compound that many humans find enjoyable when consumed in dishes that include peppers much lower on the Scoville scale. Indeed, as Deborah Blum wrote, it's the "the dose makes the poison." Even if the edible variety of capsaicin-loaded foodstuffs aren't dangerous like pepper spray is, why do humans savor the pain-inducing burn at all? I explored this question in a piece last year at The Guardian's science blog (which explains all the British spelling).

Dave's Red Hot. Mother Puckers. Green Bandit. Scorned Woman. Pain is Good. Blair's Death. No, they're not rock bands. These names represent just a small selection of the brands of hot sauce available at my local supermarket.

Humans, apparently, enjoy torturing themselves. Spiciness, after all, is not a flavour, not like sweet or salty or sour. Spicy means pain. The sensation of spiciness is the result of the activation of pain receptors in the tongue. According to psychologist Paul Rozin of the University of Pennsylvania, about a third of the people around the world eat hot peppers every single day. Why? Because they "love the burn". At a symposium on gastro-psychology during this year's Association for Psychological Science convention, Rozin pointed out that humans are the only species – we know about – that specifically seek out what would otherwise be considered negative events.

Healthy, sane humans do not stab themselves in the thighs, or bathe their eyes in lemon juice. So why do we so love to assault one of the most sensitive organs in the human body, the tongue, with what amounts to chemical warfare? Chillies are unique among foods that we should otherwise not enjoy. For example, humans also have natural aversions to the bitterness of coffee or the harshness of tobacco, but those substances have some addictive qualities, which might make them desirable. Capsaicin, the compound that provides the mouth-watering punch of chillies, does not seem to have any addictive qualities whatsoever. And yet the preference for capsaicin is almost universal; nearly every culture has incorporated it into their cuisine in some way, for milllennia.

Rozin writes:

"There are records suggesting use of chilli pepper dating back to 7000BC in Mesoamerica; they were domesticated some thousands of years after this. These fiery foods made their debut in the Old World when they were brought back by Columbus and other early explorers. In spite of their initial unpalatability, they became accepted as a basic part of the diet in many parts of the world: west and east Africa, India, south-east Asia, parts of China, Indonesia, Korea, and other smaller geographic regions, such as Hungary."

Most young children, even from cultures known for their spicy recipes, are averse to capsaicin. So maybe, then, instead of actually liking the pain, we're merely desensitising ourselves: what used to be really painful is now just sort of painful.

Since capsaicin is a member of the vanilloid family of molecules, it binds to a receptor on the tongue called the vanilloid receptor subtype 1. Upon binding to the VR1 receptor, the sensation produced by the capsaicin molecule is the same sensation that heat would cause, which explains capsaicin's burn. When scientists discovered that the VR1 receptor was a member of the larger family of TRP ion channels, the VR1 receptor was renamed TRPV1. TRP receptors are known to be sensitive to changes in temperature and are likely responsible for temperature sensation. When chilli peppers are the source of the capsaicin, there isn't any actual tissue damage; but because it binds to the TRPV1 receptor, the brain is tricked into believing that the tongue truly is on fire.

In 1980, Rozin and a colleague, Deborah Schiller, reported a study in which they compared the pepper preferences of Mexicans and Americans. Mexicans generally eat chillies several times per day, while Americans only eat chillies a few times a week. If desensitisation could explain our preference for oral pain, then Mexicans should show higher tolerance for capsaicin than Americans, and Americans should more easily detect capsaicin, even in small amounts, than Mexicans.

The data only weakly supported these predictions: the differences were seen, but were not statistically significant. Another prediction made by the desensitisation hypothesis is that individual tolerance should increase with exposure, and therefore with age. Rozin and Schiller found no correlation between age and tolerance though. Experiments conducted to try to induce a preference for capsaicin in rats, using traditional reinforcement techniques, proved futile.

In the late 1970s, Frito-Lay tried to market a brand of corn chips in Mexico that had the flavour of chilli peppers, but without any capsaicin. As would be expected in a culture that actually enjoys the burn, the product was a failure. Likewise, bell peppers, which have some pepper flavour but no capsaicin, are not at all popular in Mexico.

While most scientists still do not quite have a handle on the human preference for spicy foods, the best explanation comes from a mechanism called "hedonic reversal", or "benign masochism". Something happens, in millions of humans each year, which changes a negative evaluation into a positive evaluation, like flipping a light switch.

Rozin writes: "If the oral receptors are sending the same message to the brain in the chilli liker and the chilli hater, then the chilli liker must have come to like the very same sensation that the chilli hater, the infant, and nonhuman animals find aversive. One gets to like the burn." Only humans seem to be able to derive pleasure from the negative sensation itself. Animals have been trained to endure self-harm, but only within the context of positive reinforcement.

Perhaps we seek out the painful experience of snacking on chillies while consciously maintaining awareness that there is no real danger to ourselves. After all, people seem to enjoy – and actively seek out – many other sensations that are otherwise undesirable but are ostensibly safe: the sensation of falling provided by rollercoasters or skydiving, the feelings of fear and anxiety while watching horror movies, the physical pain experienced upon jumping into icy water, or even the feelings of sadness that come while watching a tear-jerker. Perhaps it is this cognitive mismatch itself that provides the thrill: like strapping into a rollercoaster or popping Hostel into your DVD player over and over again, the burn of capsaicin only seems to be threatening.

Want a thrill? Go out and buy yourself a bottle of One F#$%kin' Drop At A Time Hot Sauce. It comes with an eye-dropper for portion control. Benign masochism, indeed.

Photo: Flickr/dbeck03