While living in China from 2003 to 2005, I often served as the designated translator for fellow expatriates. Whenever we ate out, this involved asking our server which menu items contained MSG. Invariably I was told that almost everything is made with weijing (“flavor essence”), including, on one occasion, the roast peanut appetizer my MSG-sensitive friends were snacking on as I made my inquiry.

After observing that no one reacted to the peanuts, I was inspired to conduct a simple (and admittedly unethical) experiment. One evening, instead of translating honestly, I told my companions at a large banquet that the kitchen had promised to avoid using MSG. Everyone thanked me and happily ate their meal, dish after poisoned dish.

An hour later? Two hours later? The next day? Nothing.

I repeated this experiment on multiple occasions, always with the same result. And yet foreigners living in China routinely complained of reactions to their food that included headaches, chest pain, and shortness of breath. Was there something about my presence that conferred temporary resistance to MSG? Or could it be that MSG sensitivity was only in their heads?

In April 1968, the New England Journal of Medicine published a letter by Robert Ho Man Kwok that described a strange set of symptoms: “Numbness at the back of the neck, gradually radiating to both arms and the back, general weakness and palpitation.” Stranger still was the fact that Dr. Kwok, himself a Chinese immigrant, typically noted the onset of these symptoms 20 minutes after eating at restaurants serving “Northern Chinese food.”

An editor at NEJM titled Kwok’s letter “Chinese-restaurant syndrome,” and thus began a minor epidemic. For countless sufferers, a mystery had been finally solved. “NO MSG” signs sprang up across the United States, and, eventually, the world. Study upon study confirmed the syndrome’s existence and speculated about the science underlying it.

But after reading some of these studies, even a layperson will start to get suspicious. Take the editorial note that precedes Russell S. Asnes’ article “Chinese Restaurant Syndrome in an Infant”:

“The evidence that this infant had the Chinese Restaurant syndrome may be only circumstantial. However, the description of the symptom is accurate as attested to by the Editor’s wife who suffers from the same malady. Incidentally, she remains a devotee of Chinese cuisine.”

Science, that sworn enemy of circumstantial evidence, marched on, and slowly but surely physiological explanations of Chinese restaurant syndrome began to lose credibility. Double-blinded studies failed to turn up evidence of a clinical condition. MSG, many people noted, appears in everything from sushi to Doritos. Journalists performed experiments similar to mine, their results echoing the consensus of professional scientists: In the overwhelming majority of cases, MSG sensitivity is a psychological phenomenon.

Despite this thorough debunking, a surprisingly large number of people—generally those who lived through the epidemic—still insist they are sensitive to MSG. Google around and you’ll turn up scores of alarmist websites, which tend to combine outdated research with anecdotal, indignant rebuttals of the current scientific wisdom: “How dare you suggest my MSG sensitivity is only in my head? Why, just the other day I went out for Chinese and forgot to ask about MSG. After 45 minutes I couldn’t breathe and my heart was racing.”

Occasionally, as with vaccines and climate change denial, alarmism veers into paranoia, yielding accusations that a shadowy East Asian cabal is paying off scientists and journalists to regurgitate their propaganda. (Ajinmoto Corporation, I await your check!) For a small minority, MSG sensitivity somehow became more than a medical condition, and challenging its physiological basis poses a threat to their very identity. The harmfulness of MSG, a seemingly trivial assertion, took on the importance of a religious doctrine, a fundamental truth to be defended at any cost. But why?

In 2007, my wife’s cake shop did not offer a gluten-free option. Six years later, hardly a month goes by without a request for a gluten-free tasting. Thanks in part to celebrities such as Oprah Winfrey and Lady Gaga, nearly one-third of all consumers are now interested in gluten-free food, a multibillion dollar industry projected to exceed $10 billion by 2017. (Even children’s play sand now comes with a gluten-free guarantee!)

This is very perplexing, given that only 1 percent of the population has celiac disease and only 0.5 percent is allergic to wheat. What could possibly be causing widespread reports of nonceliac gluten intolerance, commonly blamed for a raft of symptoms including gas, bloating, diarrhea, constipation, fatigue, goose bumps, dizziness, infertility, migraines, joint inflammation, and even mood disorders?

Scientists are applying themselves to the riddle, and last February Slate’s Darshak Sanghavi reported on an Italian study that confirmed the existence of gluten intolerance (“nonceliac wheat sensitivity”) as a third, “distinct clinical condition.” In the study, one-third of patients who self-identified as gluten intolerant did in fact experience symptom relief after adopting a gluten-free diet. Case closed, right? Pass the gluten-free pasta.

Not so fast. An important implication of the study is that two-thirds of people who think they are gluten intolerant really aren’t. In light of this, the even-handed Sanghavi suggested that “patients convinced they have gluten intolerance might do well to also accept that their self-diagnosis may be wrong.” Predictably, the comment thread exploded with rebuttals: defensive anecdotes, doctrinal pronouncements about the evils of gluten, and accusations of corporate malfeasance, all of which bear a striking resemblance in tone and content to the rhetoric of anti-MSG advocates. For many, the truth of physiological gluten intolerance has now acquired a quasi-religious status.

No one likes to be told they are mistaken about the foundation of their most dearly held beliefs. It offended the faithful when Marx suggested that religions are psychological tools meant to placate the masses. Suggesting that gluten intolerance might have a psychological basis threatens a similarly foundational belief, namely that we are rational beings, competent interpreters of reality immune to mass hysteria and self-deception.

Obviously this is not the case. For one, our memories are notoriously unreliable. You may remember getting headaches from Chinese food when in fact those memories were created when you read about Chinese restaurant syndrome in the news. The same is true for memories of gluten intolerance. And don’t forget, certainty about your memories is not sufficient evidence of their truth: “Look, I know that for the last 20 years, every time I ate gluten it gave me terrible gas.” Under oath, eyewitnesses constantly forget crucial details and replace them with their own fabrications. They aren’t liars—they’re just human.

One reason for this unreliability is that memory and perception are prone to confirmation bias. Once a bias is in place, we’ll selectively remember—and notice—whatever facts help confirm it. Food historian Ian Mosby has explained the “success” of Chinese restaurant syndrome by connecting it to racialized discourse that drew on a vision of Chinese cooking as bizarre or extreme. In the case of gluten intolerance, it doesn’t take much to come up with a plausible confirmation bias. Only nine years ago, 1 in every 11 Americans was on a low-carb diet. In a country terrified of weight gain and recently obsessed with the Atkins diet, gluten makes a great villain. It’s hard not to notice the theme of weight loss on gluten-free sites. Pasta, bread, cake, cookies, pretzels—they don’t just make you fat, they make you sick! (Added bonus: Diets motivated by a medical condition are far more effective—ask any diabetic.)

Confirmation bias meets physiology in the placebo effect, a well-documented phenomenon in medical treatments ranging from sham drugs to sham acupuncture (where patients respond positively to sham needles) to sham knee surgery. People’s desire to believe in a cure actually affects their symptoms. That’s why placebo-controlled, double-blinded studies are integral to medical research. Without them, we’d be in constant danger of ascribing physiological causality to treatments that are actually psychological.

Needless to say, placebo effects aren’t always beneficial. Strong belief can also render a harmless substance poisonous, which is exactly what happened with MSG. Scientists refer to this as the nocebo effect, and it means that careful studies are necessary to distinguish between poisons and poisonous beliefs.

None of this minimizes the relief felt by those who undergo sham acupuncture, or the symptoms of those who think they are gluten intolerant. Pain is pain; chronic diarrhea is chronic diarrhea. All it means is that pain relief might not be caused by the physical presence of an acupuncture needle, and diarrhea might not be caused by the physical presence of gluten. In these instances, the symptoms may be real, but their cause (and potential resolution) is all in our heads.

It is here where the trouble begins. There’s something intensely disturbing about the notion that we can make ourselves sick. Belief, not physiology, becomes the causal agent, displacing MSG or gluten as the source of blame for someone’s suffering. This can make us feel vulnerable, stupid, and weak, as though we have the choice to be better but lack the mental acuity to manage it. Not only that, it’s hard not to feel like a psychological explanation trivializes one’s condition—hence the expression “It’s only in your head.” But things that are in our heads aren’t fake or unimportant (OCD? anorexia?), and susceptibility to a nocebo effect isn’t a sign of weakness. Anyone can unknowingly invent a false memory or react to a substance that is actually benign.

Accepting a psychological explanation of gluten intolerance is especially difficult because food aversions often turn into a way of life. Like religion, avoiding gluten requires personal sacrifice. Gluten intolerance creates communities, which, like religious communities, share stories of suffering and redemption, and share meals made special by the presence of a food taboo. It’s no wonder people take offense at the suggestion that gluten intolerance could be psychological—after all, who wants to have built their way of life on a “mere” trick of the mind?

Thinking that way is a mistake. Reductionist psychological explanations of religious beliefs can be offensive because they deny fundamental religious truths: “You believe in heaven to stave off your fear of death, not because it really exists.” Legitimate psychological explanations of medical conditions, on the other hand, deny no such truths. The question of whether and to what extent MSG and gluten cause physiological reactions is scientific, not religious. When one’s explanation of a medical condition becomes an unquestionable truth, the explanation is no longer scientific.

Many basic claims of nutrition science are unintuitive and sometimes don’t stand up to repeated research. (Salt? Cholesterol? Vitamins? Alcohol? Coffee?) At this point, scientists simply don’t have a good explanation for the mechanism and prevalence of gluten intolerance—hence the need for studies about whether it exists at all. Maybe people have always been gluten intolerant and were going undiagnosed—as is true with celiac disease. Maybe our guts haven’t evolved to process gluten—as some advocates of the Paleolithic diet claim. Maybe it’s Monsanto (conspiracy theorists rejoice!). Maybe gluten intolerance isn’t really caused by gluten, and we should actually be blaming a family of proteins in wheat called amylase trypsin inhibitors. None of these explanations are “only” in our heads, which makes them feel more acceptable. But to deny the distinct possibility that gluten could be another MSG, at least for some people, is to deny what science has confirmed, again and again, about our nature as human beings.

So when some annoying friend implies that your gluten intolerance is psychological, go right ahead and be offended. But when science suggests it? Best to listen up, question your self-diagnosis, and remind yourself that nocebo effects are nothing to be ashamed of.