Bitmoji / Shutterstock / The Atlantic

The dress is blue; tennis balls are yellow; and much like a glass can be both half-empty and half-full (because there are TWO HALVES), the audio clip is saying both “laurel” and “yanny” at the same time.

These are the molehills I have chosen to die on, because when it comes to viral illusions, it seems, you must choose a side. Enough of these divisive illusions have piled up now to make one wonder: How different is my reality from everyone else’s?

It all began in a simpler time—February 2015—on an ordinary Thursday evening when a photo of a dress posted on Tumblr got picked up by BuzzFeed. Was the dress white and gold, or blue and black? The question pitted brother against brother, friend against friend, caused celebrities to weigh in, and basically ground the internet to a halt. The dress was actually blue, but that hardly mattered. (And most people didn’t see it that way—in BuzzFeed’s original poll, 67 percent of people voted for white and gold, compared to 33 percent for blue and black.) What mattered was that the chasm between perception and reality had opened up, and we found ourselves teetering on the edge.

The Dress seemed like a fluke, a one-off moment of miraculous serendipity. Someone happened to take a photo of this specific dress just as the light was golden in that certain way that caused people to see different colors. And for a while, the internet rested. Or, well, it just fought about other things. Then more of these illusions started to appear. In 2016 there was a jacket that appeared to be different colors to different people. In 2017 there was a shoe.

“I think The Dress made for a new format of virality and memes,” says Cates Holderness, a senior social-media strategist at BuzzFeed, and the author of the site’s original post about The Dress. “It’s one thing or the other, there’s a weird illusion in some way, and you’ve got to look or listen and decide your camp. ‘Oh, is this the new Dress?’ is something I hear every time one of these things happens.”

2018 has brought a bounty of these phenomena. This February, my colleague Marina Koren discovered a poll posted on Twitter posing a seemingly simple question: What color is a tennis ball? Her subsequent investigation revealed yet another nexus of fierce division over what our senses tell us about the world we live in. Some said green; Roger Federer said yellow, and Marina concluded: “The color of a tennis ball is, and would remain, in the eye of the beholder.”

Now, it’s the ears of the beholders that have been divided. Earlier this week, an audio recording that sounds to some like the person is saying “yanny” and sounds to others like “laurel” spurred another round of perceptual team-forming. (Even Bitmoji—the company that lets you create memes using an avatar of yourself—made special yanny and laurel stickers for you to use in text arguments with your friends.) The man in the recording was actually saying laurel, but again, that hardly matters.

Just a couple days later, the internet is at it again, having surfaced yet another bizarre auditory illusion that has quickly gone viral. This time it’s a video of what seems to be a toy, which says ... something ... in a distorted, fuzzy voice. Listeners are largely able to make themselves hear either brainstorm or green needle, simply by thinking about one of those words before listening.

Taken together, all these viral illusions illustrate that perception is an interpretive act—the inputs that go into our brains don’t always translate the same way for every individual. Most of the time, we’re all moving through the world together, assuming we see, hear, smell, touch, and taste it the same way. And it’s newly alarming every time it’s pointed out that we’re simply not.

Am I experiencing a hundred micro-yannys and laurels every day without knowing it? Joe Toscano, who studies auditory perception at Villanova University, couldn’t answer that question, perhaps because he, like everyone, is trapped in the prison of his own mind, unable to know for sure what is real outside of it. But he did say he thinks it’s more remarkable that people don’t get this confused over speech all the time.

“The kinds of ambiguities in the signal that you have in these kinds of illusions—they occur in lots of places in speech, and we don’t seem to notice them,” he said. Sure, you might mishear a word here or there in conversation, but for the most part the sonic variability of speech doesn’t keep us from understanding each other.

I had just sent Toscano the brainstorm/green needle video before we talked, and he gave me his initial impression of what might be going on there. “Expectation plays a big role in speech perception,” he said. His lab will often create micro-versions of these kind of illusions for their research—putting an ambiguous sound in front of “ark,” for example, such that it might sound like “bark” or “park.” Then, if people are primed with images of a dog, say, they’ll be more likely to interpret it as “bark.”

In this case, “the sounds that make up brainstorm, and the sounds that make up green needle, there’s actually a lot of similarities between them,” Toscano says. “So my guess is that this recording is just right that it sits right on the edge between some of those speech sounds. And so that allows you to use your expectation to change what you hear.” The yanny/laurel recording works similarly—my colleague Rachel Gutman wrote a great explanation of the recording’s linguistics.

After we got off the phone, Toscano emailed me yet another example: a YouTube video of a man just shouting the same sound over and over again, which sounds alternately like bill, bale, pail, or mayo as images of famous Bills (Clinton, etc.), a bale of hay, pails, and jars of mayo flash on screen. This is my favorite one yet because it’s a dirtbag auditory illusion—dingy, ridiculous, and a little annoying. But it sure grabs your attention.

Part of the reason these catch on is because it’s both fun and existentially harrowing to watch (or hear) the fabric of your assumed reality warp and whirl. “We would think that perception is objective,” says Jonah Berger, a marketing professor at the University of Pennsylvania, and the author of Contagious: Why Things Catch On. If you see a white-and-gold dress, there’s no reason to think your eyes are lying to you. When enough people swear up and down they see something else, “we find it impossible. We feel like people must be playing a joke on us,” he says.

So that’s partly why. But these things are also likely to catch on because they allow people to get partisan over something that doesn’t really matter. “We love being grouped up, even if our group is based on a totally specious reason,” Berger says (#TeamLaurel; #TeamYanny).

“You’re on Twitter, and you see teams have been chosen,” says David Berreby, the author of Us and Them: Understanding Your Tribal Mind. “It’s not like religion or philosophy where you have to consciously think about where you stand; this is automatic, it’s involuntary. You either hear one thing or the other. There are teams and one of them is your team.”

And these teams get pared down and simplified as the memes spread. For example, I saw some people saying they heard the yanny/laurel recording as Gary or yearly. But that didn’t fit in the fun binary. Likewise, Holderness says during The Dress’s heyday, some people were telling her they saw it as blue and bronze—“and they got shouted over.”

In some sense it’s disappointing, that even when the scientists start weighing in (as they always do) on how these illusions work, people aren’t sharing them as a way of bonding with others over how the human brain is bizarre and full of mysteries and how we can’t take for granted that even our most basic perceptions of the world are correct, but in order to stir the pot and yell about being right.

But on the other hand: “There’s so much fighting on the internet, and so many important things that people fight about on the internet,” Holderness says. “Having something lighthearted we can fight about is important. I think that’s why these types of things go viral.”

“What’s nice about this is it’s a safe disagreement,” Berger says. “This isn’t going to hurt anybody’s feelings.”

These illusions seem almost guaranteed to go viral, because they allow people to do two of their favorite things: argue and choose sides. Plus, they’re truly novel and surprising, while also fitting into a meme category that, thanks to The Dress, we now recognize as familiar.

And unlike so much other viral content, which at this point often feels like it’s been assembly-lined in a cold, unfeeling, stainless-steel meme factory, these seem like the sort of phenomena you can only stumble on by happenstance. If Toscano were to share one of the audio illusions he creates in his lab for research purposes, it likely wouldn’t have the same effect. “Why did The Dress take off when there are many other illusions in vision that don’t spread all over the internet by wildfire?” he muses. “It just seemed like some random photograph of a dress. When people see an illusion we create in a lab, we take all the fun out of it, in a way.”

But amidst all that serendipitous fun and name-calling, viral illusions poke at a very real anxiety. “It’s not really about people being split into teams,” Berreby says. “It’s not ‘us versus them,’ it’s: ‘Oh, “us” is not what I thought “us” was,’ and that’s disturbing.”