The Dark Side of the Placebo Effect: When Intense Belief Kills


While people of all cultures experience sleep paralysis in similar ways, the specific form and intensity it takes varies from one group to the next


They died in their sleep one by one, thousands of miles from home. Their median age was 33. All but one -- 116 of the 117 -- were healthy men. Immigrants from southeast Asia, you could count the time most had spent on American soil in just months. At the peak of the deaths in the early 1980s, the death rate from this mysterious problem among the Hmong ethnic group was equivalent to the top five natural causes of death for other American men in their age group.

Something was killing Hmong men in their sleep, and no one could figure out what it was. There was no obvious cause of death. None of them had been sick, physically. The men weren't clustered all that tightly, geographically speaking. They were united by dislocation from Laos and a shared culture, but little else. Even House would have been stumped.

Doctors gave the problem a name, the kind that reeks of defeat, a dragon label on the edge of the known medical world: Sudden Unexpected Nocturnal Death Syndrome. SUNDS. It didn't do much in terms of diagnosis or treatment, but it was easier to track the periodic conferences dedicated to understanding the problem.

Twenty-five years later, Shelley Adler's new book pieces together what happened, drawing on interviews with the Hmong population and analyzing the extant scientific literature. Sleep Paralysis: Night-mares, Nocebos, and the Mind Body Connection is a mind-bending exploration of how what you believe interacts with how your body works. Adler, a professor at the University of California, San Francisco, comes to a stunning conclusion: In a sense, the Hmong were killed by their beliefs in the spirit world, even if the mechanism of their deaths was likely an obscure genetic cardiac arrhythmia that is prevalent in southeast Asia.

* * *

By 1986, the Hmong deaths had slowed, but remained a striking epidemiological fact. Adler was a graduate student at UCLA studying traditional belief narratives at the time. She'd been researching what she called "nocturnal pressing spirit attacks," or what scientific literature called sleep paralysis. Fascinatingly, sleep paralysis is known to just about all cultures, and it is almost always associated with nocturnal evil. In Indonesia, it's called digeunton ("pressed on"). In China, it's bei gui ya ("held by a ghost"). The Hungarians know it as boszorkany-nyomas, "witches' pressure." In Newfoundland, the spirit that comes is called the Old Hag, and the experience of sleep paralysis, ag rog, "hag ridden." The Dutch name comes closest to what English speakers know. They call the presence nachtmerrie, the night-mare. The "mare" in question comes from the German mahr or Old Norse mara, which denoted a generally female supernatural being who in Adler's words, "lay on people's chests, suffocating them." The etymology of mare isn't clear, but the term is a fruit of the Indo-European language tree, likely from moros (death), mer (drive out), or mar (to pound, bruise, crush).

Results like these seem improbable, or anti-reason, or something. But Adler's book is an attack on the "Oh, come on!" form of argument.

Across cultures, night-mare visits play out in very similar ways. Victims experience the strange feeling of being "awake." While they have a realistic perception of their environment, they can't move. Worse, they feel an "overwhelming fear and dread" accompanied by chest pressure and difficulty breathing. Scientists have a pretty good grasp of how all of this happens. The paralysis, the feeling of pressure on the chest, all that is explained quite nicely within the scientific models of sleep. During sleep paralysis, a person experiences an "out of sequence" REM state. In REM sleep, we dream and our minds shut off the physical control of the body; we're supposed to be temporarily paralyzed. But we are not supposed to be conscious in REM sleep. Yet that is precisely what happens during sleep paralysis: it is a mix of brain states that are normally held separate.

And then there is the weird stuff, the Old Hag part, the night-mare. People who have an experience of sleep paralysis tend to feel some horrible, evil being is near them. "I just knew this presence was there. An ominous presence ... not only could I not see it, but I couldn't defend myself, I couldn't do anything," one victim told Adler. This feeling is consistent across cultures, even if it goes by different names and presents through the culture one knows.

I experienced sleep paralysis twice in college. I can vouch for the sheer terror that attends the experience. I saw -- no, felt -- an evil presence to my left. I can't tell you what was evil about it or how I knew it was so nasty. But I did. As the experience progressed, it came closer. It didn't feel like my life was at risk. That was, in fact, too small. It felt like the presence was after something else, probably what you'd call my soul or my being, even though intellectually I'm a straight materialist. I woke up more scared than I've ever been in my life. Overwhelming fear. Overwhelming dread. Overwhelming fear and dread. When I read about sleep paralysis, I immediately identified that presence (which remained just to the left of my visual field) as the Old Hag.

But there is a one big difference between sleep paralysis, which impacts a substantial percentage of the global population at least once, and what the Hmong immigrants experienced in the 1980s. The Old Hag was terrifying but harmless; whatever happened in the night to the Hmong killed them.

* * *

Adler studied the Hmong and their relationship to what they call tsog tsuam for years and years. That research forms the core of her book. Adler went out into the field. She collected dozens of experiences of sleep paralysis among the Hmong both from her own interviews and other researchers. One 49-year old Adler interviewed provided this typical experience:

I remember a few months after I first came here -- I was asleep. I turned out the light and everything, but I kind of think ... and then -- all of a sudden, I felt that -- I cannot move. I just feel it, but I don't see anything, but I -- then I tried to move my hand, but I cannot move my hand. I keep trying, but I cannot move myself. I know it is tsog tsuam. I am so scared. I can hardly breathe. I think, "Who will help? What if I die?"

She brought her background in exploring traditional belief systems to bear on attacks like the one above. She found that the nighttime attacks were part of a matrix of beliefs held by both animist and Christian Hmong. A powerful folklore had built up around tsog tsuam that included both causes and cures for the attacks.

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Alexis C. Madrigal

Alexis Madrigal is the deputy editor of He's the author of Powering the Dream: The History and Promise of Green Technology. More

The New York Observer has called Madrigal "for all intents and purposes, the perfect modern reporter." He co-founded Longshot magazine, a high-speed media experiment that garnered attention from The New York Times, The Wall Street Journal, and the BBC. While at, he built Wired Science into one of the most popular blogs in the world. The site was nominated for best magazine blog by the MPA and best science website in the 2009 Webby Awards. He also co-founded Haiti ReWired, a groundbreaking community dedicated to the discussion of technology, infrastructure, and the future of Haiti.

He's spoken at Stanford, CalTech, Berkeley, SXSW, E3, and the National Renewable Energy Laboratory, and his writing was anthologized in Best Technology Writing 2010 (Yale University Press).

Madrigal is a visiting scholar at the University of California at Berkeley's Office for the History of Science and Technology. Born in Mexico City, he grew up in the exurbs north of Portland, Oregon, and now lives in Oakland.

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