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In the year 1518, the city of Strasbourg was struck by a dancing plague. Yes, a dancing plague.

And it was deadly. It started with a single woman, but within a month, there were over 400 people dancing in the streets—wildly, uncontrollably, and often to their deaths. Some sources claim that, for at least some portion of the outbreak, as many as 15 people were dying a day.

And if it's not already obvious, the dancers weren't having fun. They screamed and begged for mercy. There were literally hundreds of people, miserable and in pain… because they were dancing for no reason.

Even more strange is the fact that 1518 was neither the first nor the last time this happened. There are other documented incidents from the 1200s through the 1600s from all along the Rhine River. And while it's unclear why these plagues happened, psychologists think they can explain it through a combination of two phenomena: dissociative trance disorder and social contagion.

Now, there are other possible explanations, from it being some kind of extreme religious ritual to sleep deprivation. Researchers have even suggested the dancers might've been poisoned with ergot, a mold that grows on rye and causes spasms and hallucinations. But that seems unlikely, since rye wasn't eaten everywhere that the dancing plagues happened.

And dissociative trance disorder plus social contagion would explain quite a lot. Dissociative trances could be responsible for the dancing, while the social contagion bit could explain how it “spread” like the plague. Dissociative trance disorder or DTD is currently included in the DSM-5 under “other specified dissociative disorders.” That's a group of disorders which all involve experiencing a disconnect between what's happening in your thoughts and memories and your surroundings and actions.

And DTD is essentially when people experience dissociation as a kind of trance: a temporary state of mind where the person loses their usual sense of identity, their awareness narrows, and their movements and speech become limited. Now, it's kind of hard to measure how many people experience DTD, because unless things get really bad, people might not want to talk about feeling possessed or disconnected from themselves. Plus, a 2011 survey which identified 402 cases worldwide suggested that it might be underdiagnosed in places like the U.

S. due to cultural biases. And the low number of documented cases has made it kind of hard to study what exactly goes on in people's brains during a DTD trance. But we know from people who've voluntarily entered a trance state that trances tend to involve different patterns of activity in the brain like, a shift from more activity in the analytical left hemisphere to the more experiential right one.

That may explain why the dancers felt they were compelled to act, rather than in control of their own bodies. Also of note: during trances, natural opioids are released. Those could have dulled the dancers' pain, which would help explain why they were able to keep dancing for so long even though they were literally killing themselves.

Trances would also make sense because what a person experiences during a trance is influenced by their culture and beliefs. For example, a 1996 study looked at 10 people who had contacted the official Exorcist of the Rome diocese because they felt they were possessed by demons or the devil. They were all ultimately diagnosed with having experienced at least one dissociative trance, and the researchers argued that the commonalities of the cases showed how their culture and Catholic community had provided them a lens to express their trauma.

Oh yeah, because that's another thing: while there are lots of kinds of trances, it's widely accepted that dissociative trances, like other dissociative disorders, are triggered by trauma. Which brings us back to those plagued dancers. You see, the two biggest outbreaks happened in 1374 and 1518, which researchers have called “bitterly harsh years… even by the gruelling standards of the Middle Ages.” In 1374, people living along the Rhine River got hit by a really bad flood, plus that bout of Black Death that killed half of Europe was still within living memory.

And in 1518, they got treated to bad harvests, high grain prices, and the arrival of syphilis. Huzzah! So they were pretty miserable, and dancing plagues weren't unheard of.

In fact, paintings from the time suggest that people were kind of scared of them happening again. And generally speaking, people are more susceptible to trance states if they expect them… so maybe uncontrollable dancing isn't that strange a way for their trauma to have manifested. But how did it spread from one woman to 400 people dancing in the streets?

That's where what's known as social or behavioral contagion comes in. Behavioral contagion is the tendency to repeat a behavior after seeing others perform it. Now, this is different from conformity.

That's where you see someone else do something and then experience a conflict about it, because you want to do or say what everyone else is doing to fit in, but it's not something you would normally do. Contagion happens when you're already experiencing conflict about something. Then, seeing someone do the thing you kind of already want to do becomes enough to make you do it— even if you don't think about it like that.

Say, for instance, you have a neighbor who always leaves trash scattered around their yard, and you're not really sure what to do about it. And then, one day, you see another neighbor confronting your terrible neighbor. Next thing you know, you're up in their face yelling at them, too.

Come on, Jim! Take out the trash! Studies tracking thousands of people over time have found evidence for behavioral contagion for everything from happiness and cooperative behavior to obesity and divorce.

Even memories can be spread by contagion. People who think they maybe remember something one way can become convinced of it when someone else also remembers it that way—Berenstein bears-style. There never was a Berenstein bears!

It always had an A at the end. I promise. And sadly, the spread of terrible behaviors by contagion is also well-documented, including violence to others and self-harm.

So… why not deadly dancing? Seeing that woman in her dancing trance, combined with the cultural fear of dancing plagues, could have been just enough for others to manifest their trauma in a dissociated dance, too. And at the time, it was believed that dancing it out was the best way to rid yourself a dancing curse.

So officials actually made room for the dancers in a public space where they were sure to be seen by everyone. And the rest, as they say, is history. A lot of this is still speculation, of course.

We can never really know why hundreds of people took to the streets to dance themselves to death. But! These two ideas do cobble together a pretty decent explanation.

And they can begin to help us understand why humans sometimes do wild, hard-to-believe things. Thanks for watching this episode of SciShow Psych! I hope you learned something—that's what we here at SciShow are all about.

So tell us in the comments what part of this video you found most surprising! And if you just can't get enough about the psychology of dancing, you might like our episode on what your moves say about you. You can watch that one next!

And let's be honest—you know you're curious now.