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MELISSA BLOCK, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Melissa Block.

ROBERT SIEGEL, host:

And I'm Robert Siegel.

This next story is going to make slightly more than half of you do something that you cannot control, cannot resist, and may never understand, but you'll do it anyway.

NPR's science correspondent Robert Krulwich explains.

Dr. ROBERT PROVINE (Psychology and Neuroscience, University of Maryland): Now, you're a very silly person.

(Soundbite of laughter)

Dr. PROVINE: This could really deteriorate here. Well, we won't go there. Okay.

ROBERT KRULWICH: Oh, yes, we will. This is Provine's idea. It's not mine.

Dr. PROVINE: This excites you.

(Soundbite of laughter)

KRULWICH: Robert Provine teaches psychology at the University of Maryland. He's the author of a major book on laughter. And he saw that famous Monty Python sketch when he was a kid about a joke so funny that it's lethal, literally lethal.

If you remember, at the start of the skit, six people hear a joke that you're never allowed to hear - It'd be too dangerous. They so can't stop laughing that one-by-one they died.

(Soundbite of laughter)

KRULWICH: Here's the first one. Second. The joke is so funny - here's third. It's literally a killer. And then here comes the fifth. And then, the sixth. It's like a doomsday joke. So Provine thought why not take the same idea and try it for yawns?

Dr. PROVINE: It would be really cool if we could come up with…

KRULWICH: With what?

Dr. PROVINE: …the pursuit of the doomsday yawn.

(Soundbite of laughter)

KRULWICH: Not a yawn that kills, but a yawn so powerful, it would make everybody who sees it, everybody yawn back. This would be the totally contagious yawn.

Dr. PROVINE: Basically, I thought it would be fun coming up with sort of like the ultimate yawn stimulus, the supranormal stimulus.

KRULWICH: Which would be what exactly? Because if you ask yourself…

Dr. PROVINE: What makes a person yawn when they see another yawning? It must be the gaping mouth.

KRULWICH: And therefore, there must be some perfect shape for a gaping mouth, a perfect timing, perfect sound that would make a yawn so irresistible.

Dr. PROVINE: Oh, we could develop a super-gaping mouth, a super yawn stimulus.

(Soundbite of laughter)

KRULWICH: So Robert Provine got some of his students to yawn.

(Soundbite of people yawning)

KRULWICH: First on videotape. It was, he says…

Dr. PROVINE: Five-minute series of repeated yawns.

KRULWICH: Nothing special which he then played in his classrooms and he discovered that a basic yawn medley creates a yawn response in about 55 percent of his viewers. Fifty-five percent, which didn't seem like a lot.

Dr. PROVINE: Well, in a laboratory-type situation, 55 percent of people actually yawned.

KRULWICH: That is a lot, he says, for actual yawns.

Dr. PROVINE: Virtually, everyone reported being tempted to yawn.

(Soundbite of a people yawning)

KRULWICH: Ah, and it didn't matter anyway, whether the yawning faces were turned upside down or tilted or totally sideways or whether they were black and white or whether they were in color, still 55 percent of the audience yawned. So, the goal then was to move up, if he could figure out how, to the ultimate 100 percent effective yawn, and that's when things got really interesting.

First, just because he was curious, he decided to cover, literally blocked out the gaping mouths, so you couldn't see it on the video. And oddly, without a mouth, the same 55 percent of the audience yawned anyway.

Dr. PROVINE: That was a big surprise.

KRULWICH: You mean, if you can't see the mouth, you still want to yawn?

Dr. PROVINE: Yeah.

KRULWICH: Weird. So why would people yawn at no gape at all? So Provine decided to double his bet, he designed a yawn that was only mouth.

Dr. PROVINE: Yes.

KRULWICH: Just the mouth?

Dr. PROVINE: Just the mouth.

KRULWICH: And it turned out that looking at no eyes, no head, just a big yawning hole of the mouth got fewer yawns back.

Dr. PROVINE: It was like, oh no, the whole project's going down the tubes.

KRULWICH: Because if a gaping mouth does not trigger a yawn, what does? That's when Provine recalled that scientists have observed fetuses in the first trimester yawning in the womb.

(Soundbite of people yawning)

KRULWICH: When they yawn in there, they also stretch, which brings up what is known as…

Dr. PROVINE: A yawn-stretch complex.

KRULWICH: That says yawns and stretches are profoundly related. For example, try this…

Dr. PROVINE: The next time you start to yawn, clinch your teeth together and don't open your jaw.

KRULWICH: So you feel the yawn coming on, but if you stay clinched, you won't be able to complete your yawn.

Dr. PROVINE: Just like it's stuck. It can't resolve. You've got to stretch the jaw. So there is this relationship between - you also see this pattern in dogs and cats, for example, when they're rousing, they will stretch and yawn.

KRULWICH: So maybe, thought Provine, it's the stretch, the elongation of the face, that is the trigger that produces the yawn. After all, we all do it.

Dr. PROVINE: And in fact, virtually, all animals with backbones yawn, it's not just humans.

KRULWICH: Really?

Dr. PROVINE: Snakes do it. Fish do it.

KRULWICH: Cats do it. Dogs do it. So if you try and extend the stretch in a video, you make it go longer and further and you open wider, is that the trigger that produces more yawns?

Dr. PROVINE: Nope, it's not.

KRULWICH: It didn't work. Not at all. He got no extra yawns. In fact - and this is sad - Provine discovered that showing nothing, just talking about yawn, say here on the radio…

Dr. PROVINE: Just as we are talking about yawning this very minute…

KRULWICH: Fifty-five percent of our listeners are about to yawn.

Dr. PROVINE: Which you've already done.

(Soundbite of laughter)

KRULWICH: So for all his efforts, Robert Provine has been unable to create a yawn that beats the standard 55 percent response because apparently, there is no primary trigger.

Dr. PROVINE: It's like, you know, many things in nature. There's not a cause. There's a number of them.

KRULWICH: And the precise combination that takes the yawn from me to you -stretching face muscles, tilting head, closing eyes, making the noise - he's tinkered, he's searched, but so far, he has not found the secret recipe. So while he dreamed of a doomsday yawn…

Dr. PROVINE: It didn't work out that way.

KRULWICH: …in fact, after all these, Robert Provine isn't even sure why we yawn. There are theories: it's a way to cool off or change mood or, perhaps, a yawn is a physical expression of our human ability to share, human empathy wired into a brain, although maybe not just our brains.

Dr. PROVINE: For example, if you see your dog or cat yawn, you're to likely yawn, although…

KRULWICH: What if your cat and dog were to see you yawn?

Dr. PROVINE: Yeah. Now that is a more difficult issue.

KRULWICH: Can a dog catch a yawn from a person? Provine says, I don't know.

ADRIENNE ROSENFELD(ph): Cosby(ph), look.

(Soundbite of person yawning)

KRULWICH: But the very first couple I asked, Adrienne(ph) and Win(ph), Win's an NPR producer in New York City, when they went home and yawned straight at their dog, Cosby…

WIN ROSENFELD: Okay.

ADRIENNE: He just did it.

(Soundbite of laughter)

ROSENFELD: He just did it.

ADRIENNE: He just - oh, he's doing it again. He's just yawning. I mean, he looked over at Win and then looked over at me and just started yawning and actually did it twice.

(Soundbite of person yawning)

KRULWICH: it turns out the most mysterious thing about yawning is that it's so consistently mysterious.

Robert Krulwich, NPR News.

ADRIENNE: He's just looking at me strangely.

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