Why are we ticklish?

Socrates, Aristotle, Galileo, and Darwin couldn't figure it out. But one scientist has built a tickle robot (named Hektor) to try and settle the mystery of ticklishness once and for all. Guest: Konstantina Kilteni, principal investigator of the Touch & Tickle Lab at the Donders Institute To watch this episode on video, check us out on ⁠⁠Netflix⁠⁠! For show transcripts, go to ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unxtranscripts⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠For more, go to ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unexplainable⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠And please email us! ⁠⁠⁠[email protected]⁠⁠⁠We read every email. Support Unexplainable (and get ad-free episodes) by becoming a Vox Member today: ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/members⁠⁠⁠⁠⁠⁠⁠⁠Thank you! Learn more about your ad choices. Visit podcastchoices.com/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:01Speaker 00

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• 1:14Noam Hassenfeld?

  For thousands of years, there's been this one particular mystery we've never been able to solve. Socrates called it a mixture of pleasure and pain. Aristotle had his own theory. Galileo, Bacon, Descartes, Darwin, they all couldn't crack one of the most enduring questions in the history of humanity. Why are we ticklish?

• 1:42Noam Hassenfeld?

  But there is one scientist who might just be the one to do it. Konstantina Kilteni is the principal investigator at the Touch and Tickle Lab in the Netherlands. She's scoured the ancient texts. She's mapped ticklishness around the world. And she's even built a robot in her lab with a single purpose: to tickle. So today on Unexplainable, what is going on with tickling?

• 2:25Noam Hassenfeld?

  All right, Konstantina Kilteni, welcome to the show.

• 2:28Speaker 02

  Hello.

• 2:28Noam Hassenfeld?

  So how did you get into studying tickling? It doesn't feel like the normal path for a research scientist.

• 2:36Speaker 02

  The straight, yes. So I started investigating how the human brain can tell the difference between a touch that is applied to our body by ourselves versus the same touch, identical touch, that is applied to our body by somebody else or by a machine. And I remember using a lot the example of we can be quite easily, some of us, tickled by other people, but it's very difficult to tickle ourselves.

• 3:05Noam Hassenfeld?

  Right.

• 3:06Speaker 02

  And then at some point, I became very interested into ticklishness itself. Like, why are we ticklish? Like, does it have a function? And when does our brain decide that this is a ticklish touch or it's not a ticklish touch? So then I started looking the literature because I was thinking, okay, somebody must have investigated that in the past. But if you look at the work that has been done, it's surprisingly little. And you could say, well, maybe it's not an interesting question. Like, why would we start that? But actually, if you look at the texts of Aristotle's or, you know, of Socrates through Plato's text,

• 3:49Speaker 02

  or, you know, you read some books of Darwin, even Galileo Galilei has talked about it.

• 3:55Noam Hassenfeld?

  All of these guys are talking about tickling?

• 3:57Speaker 02

  Yes, all of these guys are talking about tickling. And in 2026, there is a lot to understand why are we ticklish and what is the function of ticklishness.

• 4:08Noam Hassenfeld?

  So this is a mystery dating back to Socrates, like thousands of years, and we still don't understand it?

• 4:14Speaker 02

  Two millennia, exactly. Exactly, yes, yes.

• 4:17Noam Hassenfeld?

  Yeah, when you got to this question of why are we ticklish in the first place, what are some of the possible ideas here?

• 4:24Speaker 02

  There is like the social bonding theory. Tickling is probably the first touch in life that makes people laugh, right? So there have been some neuroimaging studies, so studies scanning the human brain that suggest also that different areas are involved in the generation of the tickling induced by laughter. And also, there are different acoustic properties for the laughter that is induced by tickling. So what has been proposed is that laughter induced by tickling might be a more primitive form or laughter that we have also from our ancestors, great apes, for example, that has evolved in humans

• 5:09Speaker 02

  to a laughter that is more specific to enjoyment.

• 5:14Noam Hassenfeld?

  That's so interesting that there's a different type of laughter that you can actually recognize without context. You can listen to an acoustic signal and say, this is a tickle laugh and this is like a joke laugh. That is mind blowing to me.

• 5:29Speaker 02

  Exactly, yes, yes. There is another theory that suggests that maybe it's a way for animals to train their juveniles into protecting specific areas of the body that could be vulnerable in an arm-to-arm fight.

• 5:48Noam Hassenfeld?

  Like sort of some combat training light kind of stuff?

• 5:52Speaker 02

  Exactly, exactly, exactly. But it is in a family and social context, so it serves learning how to protect the areas of the body. There is also the theory that suggests that ticklishness is a reflex behavior. As the knee-jerk reflex, you give a stimulus, you have a response, but we don't know its function yet.

• 6:14Noam Hassenfeld?

  So you're basically saying we have some theories, but we don't really understand exactly why it might have evolved, why we're ticklish in the first place?

• 6:21Speaker 02

  Exactly, yes, yes.

• 6:23Noam Hassenfeld?

  Okay, so you just published a study this year that takes on a bunch of these ticklish unknowns. Can you tell me what you were trying to do with that paper exactly? What you were studying, what you were asking?

• 6:36Speaker 02

  One of the questions that we had, and I think it's linked a little bit to what is the function of ticklishness, is why are you ticklish in certain areas of the body and not in others? And we had 150 Dutch people, 150 Chinese, and 150 people of Greek cultural origin. We gave them a figurine of the human body back and forth, and we asked them to report the areas that are most sensitive to touch, most sensitive to pain, where they experience hedonic sensations, so pleasurable sensation from touch on their body. So at the end, we had ticklishness body maps. We had touch body maps, pain body maps.

• 7:21Speaker 02

  We had a pleasure body maps. And we got very excited because we had the map of ticklishness that was quite stable across cultures and across people. And then you can go back to what has been proposed across these two millennia and try to ask questions like, is this map explained by theory A or theory B? So for example, Aristotle said that humans are tickled because of their skin delicacy.

• 7:48Noam Hassenfeld?

  Was Aristotle right? What did you find?

• 7:51Speaker 02

  He was not.

• 7:52Noam Hassenfeld?

  Oh.

• 7:53Speaker 02

  Your eyelids is one of the areas that have the thinnest skin in your body, but it's not a ticklish area, right? Your footsoles could have thick skin, but they are ticklish. So it didn't go together, this theory. Then there was, I think, a general proposal, which is like we are ticklish in areas of the body that are most sensitive to touch. And we tried to see if that map with our ticklishness map had any relationship. No. And think about it, right? Your lips or your fingertips are areas that have very good tactile sensitivity and a lot of receptors underneath.

• 8:36Speaker 02

  But these are not the areas that are the most ticklish. So popular theory, yes, but it was not supported by our data.

• 8:44Noam Hassenfeld?

  Great, okay.

• 8:45Speaker 02

  Then we had a suggestion that erogenous areas of the body are ticklish. So we had data where the participants indicate where they experienced pleasure sensation on their body when being touched by other people. It didn't work either. So and then we had Darwin.

• 9:07Noam Hassenfeld?

  Hmm.

• 9:08Speaker 02

  Who said a lot about ticklishness in his books.

• 9:10Noam Hassenfeld?

  Really? Okay.

• 9:11Speaker 02

  Yes. Yes. He was very intrigued by the phenomenon. So one of the things he said about ticklishness was that we are ticklish areas of the body that do not frequently receive touch.

• 9:24Noam Hassenfeld?

  So you're saying like, uh, the fingertips are sensitive, but maybe they're not ticklish because they're they're touching things a lot.

• 9:33Speaker 02

  Yes, exactly. So it would be that, um, people don't touch you on the armpits and that's why touch there is interpreted as ticklish. So then we tried to see if there is a link between where people experience touch in their daily life and the ticklishness map. And then we got a positive response.

• 9:53Noam Hassenfeld?

  Oh.

• 9:54Speaker 02

  So we did find that the areas of the body that we receive less frequently touched by others or by ourselves are areas that are more ticklish. But something we need to keep in mind here, like two things. First, it could be reciprocal. It could be, well, they don't touch me on the armpits because I'm ticklish, right?

• 10:16Noam Hassenfeld?

  Okay.

• 10:16Speaker 02

  And the second thing is that from all the theories we tested, this was the best relatively to the other theories, but it's not that it explains 100% of what we see in the ticklishness map.

• 10:30Noam Hassenfeld?

  How much of it do you think you would attribute to Darwin's theory?

• 10:34Speaker 02

  Yes, it was around 34%.

• 10:37Noam Hassenfeld?

  Okay. So like less, less than half.

• 10:39Speaker 02

  Yes, there is a lot of the pattern that remains unexplained.

• 10:43Noam Hassenfeld?

  Yeah, like the majority is unexplained.

• 10:45Speaker 02

  Yeah.

• 10:46Noam Hassenfeld?

  So my question then around this big thing is how is it possible that we have this thing that scientists and philosophers and thinkers have been wondering about for over 2,000 years and we don't know what it's doing? How is it possible that we know so little about this?

• 11:08Speaker 02

  Yeah, it's a very good question. And I think it just, it has been very difficult to investigate because it involves, you know, like movement, it involves touch, it has emotional components, it induces arousal in the physiological system. And if you look at the previous studies, the experimenters were basically manually tickling the participants, right? So it was like a human to human interaction. And one of the problems of previous research was that they were using so different methods and manual methods that it was a little bit difficult to replicate these results. So we thought if we want to study it, we would need to do it in a way that is very replicable. So we can reproduce it in multiple studies

• 11:52Speaker 02

  and also other researchers can use our methods. So that's why we built the tickle robot that we call Hector.

• 11:59Noam Hassenfeld?

  You've built a tickle robot?

• 12:01Speaker 02

  Yes.

• 12:02Noam Hassenfeld?

  And you said its name is Hector?

• 12:05Speaker 02

  Yes, we call it Hector.

• 12:07Noam Hassenfeld?

  What?

• 12:08Speaker 02

  We call it Hector. No, because like I'm, I'm Greek and I love Greek mythology. So we have Hector, which is the tickle robot, and we have a epolitity, which is like the MRI, like the robot compatible with brain scanning. Yes.

• 12:24Noam Hassenfeld?

  Oh, my God.

• 12:25Speaker 02

  And names from the Greek mythology.

• 12:30Noam Hassenfeld?

  Wait, and why is, why is the tickler Hector?

• 12:34Speaker 02

  I mean, Hector was a very good warrior. So we were like, yeah, you know, it has to survive all the stimulation with the participants. Yes.

• 12:43Noam Hassenfeld?

  Yeah, he passed the tickle combat.

• 12:45Speaker 02

  Exactly.

• 12:50Noam Hassenfeld?

  In a minute, the tickle robot.

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• 16:19Noam Hassenfeld?

  Um, tell me about this robot. Like, what does it look like? What does it do? How does it work?

• 16:24Speaker 02

  So participants are sitting on a chair and they have their feet on top of a wooden platform that has two holes. And then what we can do is that as if this were the foot sole, we can stimulate the foot soles of the participants with a probe. Okay. Why, why this is good for us? Let me take a little bit the experimenter's point of view. So we wanted to have a robot that we can control 100%. We want you to be able to see how strong the touch has to be for somebody to say that it's ticklish, how fast, how big the probe has to be, how long does it have to last. And the reason why we wanted to do this, it

• 17:07Speaker 02

  is because we want to understand how a touch is converted into ticklish touch. Okay? Like, what is the function? And for the participants, the participants don't see anything. They just know that they are going to receive strokes on the left or right foot. They don't know where, they don't know when, and they don't know how the strokes are going to be. So while the participants are sitting on this chair and they have their feet on the platform, we, as experimenters, we can control what the stimulus is going to be like. And at the same time, we record a lot of data. So we can record brain responses. We record

• 17:46Speaker 02

  a lot of physiological responses like heart rate, respiration, if the participant changed the way they breathe, for example, if they start laughing. We also record videos where we can analyze facial expression. So you can have some participants, you know, for example, when they receive the stroke on their foot, it might be like, ooh, you know, something like that. Or they can be like full-blown laughter or smile or, you know. So we can record these facial expressions and quantify them.

• 18:19Noam Hassenfeld?

  What do the participants think of all this? Like, are they signing up for a study knowing that they're about to get tickled by a robot?

• 18:25Speaker 02

  We don't tell them that it's about ticklish.

• 18:28Noam Hassenfeld?

  Oh, okay.

• 18:29Speaker 02

  And the reason why we don't tell them that is because we don't want to attract participants that for some reason they really want to get tickled. And something that I should tell you also, when we started building this robot, we had this very big question mark in our heads, like, what if we are investing now a lot of time? And then we find out that... People cannot be tickled by a machine, so it has to be another human tickling them. You know, like it really needs a social interaction. So this was a very big risk, but it can be super successful in some people. So, you know, sometimes you are running an experiment and you hear this person that cannot stop laughing from the ticklish stimuli that you receive on the food. You know, it's an uncontrollable laughter.

• 19:13Speaker 02

  Of course, you also have people that the responses are moderate. And you also have other people that will tell you it's not ticklish, it's not ticklish, it's not ticklish.

• 19:22Noam Hassenfeld?

  Was it like a eureka moment when you built this robot and the first person comes in and they get tickled? And you're like, ah, it works?

• 19:29Speaker 02

  Yes. To be honest, like we were building this device for almost two years. And we were like, um, this stimulus should be like that to be more ticklish. So we somehow tuned the robot based on our perceptions. And then we saw that similar, let's say, tuning parameters exist for the majority of the people who experience ticklish sensations in the lab.

• 19:56Noam Hassenfeld?

  What are you actually trying to do with this tickle robot research? And what have you found out so far?

• 20:03Speaker 02

  Yes, so we are doing a lot of experiments right now with this, with Hector, our precious Hector. So the first experiments that we wanted to do were exactly to try to find if there are common parameters in eliciting touch that feels as ticklish. Then we have another line of research that investigates whether you laugh because you enjoy being tickled, right? It's part of the response to laugh. It's part of the response to smile, maybe to scream, maybe to tickle back, maybe to retract your body. You have this paradoxical nature. And on top of that, you have some people liking and some people not liking. We have another line of research where we are investigating ticklish sensation in people with autism. We know that people with autism

• 20:48Speaker 02

  might experience the hypersensitivity to sensory stimulation. So we wanted to see if ticklishness is indeed linked with autism. This is a project we have just started, so I don't have anything to share about that.

• 21:03Noam Hassenfeld?

  That's an interesting idea.

• 21:04Speaker 02

  Yes, so we are trying to really establish the basic stuff.

• 21:09Noam Hassenfeld?

  So is this tickle robot research kind of like you're coming into a field that hasn't had a lot of experimental research done? It's been a lot of like self-reports, a lot of like armchair philosophy theorizing, and you're trying to give sort of a fundamental experimental basis to it?

• 21:28Speaker 02

  Yes, I hope so. But we need good methods. So I think we are trying to establish some basic methods for this.

• 21:36Noam Hassenfeld?

  And where do you hope this research goes? Like, why are you interested in the connection between ticklishness and autism? Or why are you asking some of these bigger questions about what's happening in the brain when someone's being tickled?

• 21:50Speaker 02

  I mean, my first question about ticklishness, which remains, I would say, the big question I have, is I don't understand why are we ticklish? And what does it serve for? And I think, as I told you before, for the different theories, we have pros and cons. So, for example, we said the social bonding theory, but actually, when you are being tickled, the first reaction you have to the stimulus is that you are trying to escape the tickler and you are trying to protect your body. It's not that you expose your body because, you know, you really like it. So you're like, yes, give me more, right? So I think there is a very interesting paradox there. Then I told you about this other theory that says about the vulnerability. Like, you are trying to teach your juveniles how to protect their body, but actually, the areas that are ticklish

• 22:34Speaker 02

  are not necessarily areas that are the most vulnerable. So to me, it's a big question. Why are we ticklish? And what if it has a function that we don't know about it, right? But to address what is the function of a human behavior, it's a very big question. And it's very difficult to address it with, I don't know, research of one year or two years.

• 22:56Noam Hassenfeld?

  Do you think you're going to be the one to crack it after Socrates and Aristotle and Galileo were enabled?

• 23:02Speaker 02

  The next Greek, the third? No, no, no, no. It really depends on what we're going to find, I think. I find this question very, very interesting. And it intrigues me that we don't know yet about it. But I think we're working on that.

• 23:22Noam Hassenfeld?

  All right, well, I will say that next time I'm in the Netherlands, I am more than happy to come, volunteer, be tickled for science. But until then, Konstantina, thanks so much for coming on the show. It was really, it was really great to chat.

• 23:39Speaker 02

  Thank you very much for hosting me. And you are super welcome to participate in all our experiments, since you are very ticklish as well, right?

• 23:46Noam Hassenfeld?

  Yeah, I am not exactly sure how much I will enjoy it, but I will do it for science.

• 23:56Noam Hassenfeld?

  If you want to see this episode on video, check us out on Netflix. This episode was produced by Valerie Schenkman and me, Noam Hassenfeld, who's edited by Joanna Solotaroff. Our video editor is Jacob Reynolds. Our animator is Karim Karia. Our fact checker is Melissa Hirsch. And Meredith Hodenot runs the show. Mixing and sound design from Chris Janayala, music from me, and Bird Pinkerton backed away from the subway door. Somehow, the birds had found her. They were coming for the guitar.

• 24:30Noam Hassenfeld?

  Thanks, as always, to Brian Resnick for co-creating the show with me and Bird. If you have thoughts about the show, we would love to hear from you. Please email us at [email protected]. Or if you want to support the show, join Vox, become a member. Just go to vox.com/members. And if you signed up because of us, let us know. We'd always love to hear it. Unexplainable is part of the Vox Media Podcast Network, and we'll see you right back here next time.