[child laughing] ["Jaws" theme playing] Hey, smart people.

Joe here.

You tired of coronavirus yet?

Don't worry we're going to talk about something else this week.

We've covered a ton of different weird bodily functions on this show.

But there's one that has always really bugged me because it's just so confusing-- tickling.

It's one of the great contradictions of the universe, like jumbo shrimp, or saying, "That's your only choice."

I mean, that doesn't make any sense.

Because tickling makes us laugh.

But we hate it.

I mean, go ahead.

Try it for yourself.

Go try to tickle someone right now.

[standby music] Actually, I'm shooting this during COVID-19 physical distancing and all that.

So maybe don't do that.

Unless it's someone in your house that you're hunkered down with, or maybe this is all blown over by the time that you're watching this.

Also, ask them permission first because that's rude and maybe assault.

But what do they do?

They asked you to stop because the almost universal reaction to tickling is to say, "No, stop it, please stop it, or I swear I will [BLEEP]."

What are you for, tickling?

What is the point of you?

Why do you exist?

Well, we're going to dig down into the soft tummy-tum of science.

And we're going to try to shake out some answers to why we're ticklish.

What is it really for?

And why is tickling the unfunniest thing that still makes us laugh?

[theme music] So I know what you're wondering.

Can you tickle yourself?

And the answer is, of course, it depends.

Because there's actually two different kinds of tickling.

But the one that we're all familiar with, the one that makes you laugh and freak out, that's called gargalesis.

This is the tickling you can't do to yourself.

And that will be important later.

Now, almost everyone has been tickled before.

We do it to make kids and babies laugh.

Maybe you do it with someone you're feeling a little flirtsy with.

But it is a real head scratcher, or maybe armpit scratcher.

Actually, that's not a good image.

People have been confused by tickling for thousands of years.

I mean, Aristotle attributed our ticklishness to the softness of our skin and the fact that we're the only animals smart enough to find things funny.

Even my boy Darwin wrote about tickling.

Now he didn't exactly hit the nail on the head.

He thought that you had to be in a good mood to be tickled, which definitely isn't true.

But he did give us the one important question that we have to ask about weird behaviors like this.

What's the evolutionary purpose?

I mean, tickling wouldn't exist if it didn't have some biological purpose either in us or in one of our species ancestors.

And we're going to try to figure out what that is.

Clue number one, this.

Only some areas of our body are ticklish.

I mean, try to tickle someone's shin.

You're not going to get much of a reaction.

But get him here and...

It turns out other primates tickle each other too during a kind of rough and tumble play.

This may hold some hints to why it exists.

Our ticklish spots are really vulnerable.

And I don't mean emotionally vulnerable.

I mean, these are the spots that a predator or an attacker would try to get you if they wanted to kill you.

You've got major arteries here and here.

You've got important for staying alive stuff like organs here, here, and here.

You curl up in a little ball.

You recoil.

It's a reflex.

Gargalesis probably evolved as a way to protect ourselves.

And those ticklish spots don't really apply to the second kind of tickling, knismesis.

That's the feeling you get when you drag something really lightly across your skin.

And you can feel this almost anywhere on your body.

The reaction could be anything from goose bumps to... [shivers] And knismesis isn't normally associated with laughter.

And it's actually pretty easy to guess where it might have come from.

This tickle is like an alarm going off on your skin, in case there's like a dozen spiders crawling up your neck or something.

That's something you're going to want to notice.

And it's definitely not funny.

That's why it leads to all the twitching and shuddering and scratching to get those potentially dangerous creepy-crawlies off your skin.

That gives you something to think about next time somebody's scratching your back.

It's like a physical touch reflex.

And you can do this kind of tickling to yourself.

But this kind of tickling isn't really associated with laughter, while gargalesis is.

But why?

Why does tickling make us laugh?

I mean, why doesn't it hurt or burn or tingle?

I mean, why is trying to protect your internal organs from would-be attackers funny?

Well, that brings us to clue number two, that you can't tickle yourself.

No matter how hard I try, I can't tickle myself with gargalesis.

It just doesn't work.

Why?

Well, it's for the same reason that you can't scare or startle yourself.

You've got a part of your brain that's always predicting your next move.

And that's why I can pick something up without looking.

My brain is predicting my movements.

It's looking for feedback, making little adjustments.

The point is that my brain knows what I'm trying to do.

And if I try to tickle myself, it just overrides the whole tickling program.

So for gargalesis, the laughy tickles, only other people can tickle us.

That tells us tickling is social.

Just like facial expressions or talking, it sends a social message.

But what is that message?

Scientists have done some pretty weird experiments to try to figure this out.

It turns out that the person tickling us doesn't have to laugh for us to laugh.

When someone's wearing a mask and tickles a baby, the baby still laughs, which is a pretty weird experiment.

Maybe don't do that.

And it turns out that adults will still laugh even if they think they're being tickled by a machine, which is also a really weird experiment.

Now we've got two clues to help us figure out tickling.

Number one-- our ticklish spots are vulnerable places that we need to protect, and number two-- you can't tickle yourself.

So maybe the reason we laugh is to get other people to tickle us, yeah, even though we hate it.

Just hear me out.

Now imagine tickling made people cry.

Well, then you'd probably stop doing it immediately.

Laughter is like positive reinforcement.

It's a social message that makes the person tickling us happy and encourages them to keep doing it.

And why should they keep doing it?

Because rough and tumble play in animals is an important part of learning and developing and growing up.

And in our ancestors, it would have been important for survival.

It would have taught us lessons about fighting and hunting and protecting ourselves.

On the flip side, if tickling was only about laughter and pleasure, if it didn't give you that runaway feeling, if you didn't recoil and try to fight back, then it wouldn't teach you that all-important lesson about protecting yourself.

And it turns out that other animals laugh when they're tickled too.

Our primate cousins make a sort of ha-ha laugh-like sound during their own form of tickle play.

And it goes back even farther on our family tree.

You can even tickle rats and get them to laugh.

And people have done it for science.

When they're tickled, they make these high-frequency noises.

Why?

Well, I don't know the mind of a rat.

But these are the same high-frequency noises they make when they play with other rats or when they're given food.

It seems to say, "I'm happy," and maybe encourage other rats to keep doing it.

So tickling is probably an ancient social behavior from way back in our evolution that helps animals learn how to protect themselves.

But importantly, the laughter that goes along with tickling doesn't have anything to do with being funny.

And this is why tickling can teach us something important about laughter itself.

We tend to think of laughter as always being associated with funny things.

But it's not.

"It's snot."

[laughs] There are people who study laughter.

And they're called gelotologists.

And one thing we know from their research is more than anything else we laugh for each other.

People are more than 30 times more likely to laugh when they're with other people than if they're alone.

And we've learned that fewer than one in five laughs are actually in response to something funny.

I mean, seriously, just watch two people having a normal happy conversation sometime.

You're going to see a ton of like, "Ha, ha," stuff like that.

We laugh because we're nervous.

We laugh to just fill space in a conversation or to let the person that we're talking to know that we're still listening.

It's more like social noise or filler.

I mean, unless you're laughing at my joke, I mean, we know that's real, right?

And you've ever been so sad or mad that you laughed?

It's kind of weird.

But social discomfort and emotional pain, well, our brain treats those a lot like actual physical pain.

And we think that laughter leads to the release of beta endorphins, hormones that act like our body's natural pain killers.

The point is that laughter is one of our most complex social behaviors.

We use it to send a ton of different messages, most of which have nothing to do with something being funny.

So maybe it's not surprising that it's linked to tickling is complex too.

So here's what we know-- Tickling is basically built into our bodies.

It's like a reflex.

And the places that we're most ticklish, well, those are the places that we're most vulnerable.

And even though we laugh, tickling is not funny.

Tickling is almost a weird form of play that in our evolutionary ancestors taught them to be safe.

And laughter is the reward for our brains that makes us want to do it to each other.

Remember, we're animals too.

And that's why tickling makes us feel so good deep down.

It helps to reinforce social bonds with the people that we love.

Wait, I take it back.

I don't like social bonding.

Stop it.

Stay curious.

Guys, thank you so much for checking out our latest video.

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These are weird times, and we really could not do it without your support.

We're working on more COVID-19 stuff to keep you informed, and our regular good-old videos to keep your brain educated and excited.

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We're so thankful to have your support.

And, if not, we're just so thankful that you're here with us and that you're staying safe.

We'll see you next time.

[child laughing] [man sings "Jaws" theme] No, no, no!

[laughing] [man sings "Jaws" theme] I'm gonna tickle you.