There are many forms of touch in the human vocabulary, like caresses, squeezes, slaps, and pinches. Yet there’s only one type of touch known to routinely elicit laughter: tickling. From the time we’re babies in our caregivers’ arms to adulthood, a few wiggly fingers on the stomach can unleash squeals and cackles. But why? Why is it that we’re thrown into fits of uncontrollable laughter by tickling?
“It’s a very unique response,” says Sandra Proelss, a PhD candidate at the Bernstein Center for Computation Neuroscience in Berlin, Germany. “There are some fascinating idiosyncrasies with it.” Proelss has studied the human tickling response in her neuroscience research. Though there are still some major mysteries and big unknowns, scientists like her have come to a handful of interesting answers and explanations for why–exactly–we’re so ticklish, and what’s going on during a tickle session.
What is tickling?
First, an important distinction. The word tickle refers to two very different sensations. The first is a light brush against the skin, like a hair falling down your back, or the feeling of a feather on your arm. The technical term for this is knismesis. It might trigger you to flick something away or scratch the affected zone, but it has more in common with itching than anything else and it probably won’t make you laugh.
In contrast, there’s gargalesis, tickling induced with a repetitive, strong pressure in a particularly sensitive area of the body. “These two things should be considered completely separate,” says Shimpei Ishiyama, a neuroscientist at the Central Institute for Mental Health in Mannheim, Germany who studies neuronal mechanisms of tickling.
[ Related: Why do some people sneeze so loudly? ]
The purpose of the knismesis is obvious: to reduce the likelihood of an unwanted critter from biting or burrowing. “You want to protect your body’s surface from parasites,” says Ishiyama, as with a cow that flicks its tail to ward off flies. Explaining gargalesis is less straightforward.
What’s the purpose of being ticklish?
We don’t know for sure why this type of ticklishness exists, but there are a few theories. One has to do with defending vulnerable body parts, says Ishiyama. An outsized, disarmingly joyful reaction might stop a would-be attacker in their tracks. Or, it’s possible that ticklishness could prime to be more protective of our soft and fragile centers. However, he’s skeptical of the idea given that our most vulnerable body parts and our most ticklish body parts don’t quite line up. For instance, most people are extremely ticklish on the soles of their feet–but that’s not exactly the bit of our bodies most in need of protecting, Ishiyama points out. “There are a lot of exceptions… I don’t think it’s really a conclusive theory,” he says.
More likely, in his estimation, is that tickling and ticklishness are a type of play and social bonding–and potentially combat practice may be an auxiliary benefit. For one, humans aren’t the only ticklish animals. Other great apes and some rodents (namely rats) demonstrate a similar sort of response to tickling stimulus. What every ticklish animal has in common is that “they are highly social mammals” that play, roughhouse, and communicate with each other.
Other points also seem to support the play theory. Ticklishness is emotion and context dependent, says Proelss. People and animals are more ticklish when they’re in a positive and playful mood. Responses also vary based on familiarity, with people less likely to react with laughter to tickling if a stranger is doing it, she adds. Anxiety can dampen ticklishness. And, except for a few exceptions linked to disorders like schizophrenia, we can’t tickle ourselves–it’s a response that exists solely in social settings.
Finally neuroscientists have tracked some of the brain regions involved in a tickle session. In a study of rats, the same somatosensory circuits involved in play behavior also light up when a subject is tickled. Other brain regions that become active with tickling include those responsible for processing touch, one associated with the fight or flight response and vocalizations, and emotional zones like the amygdala and anterior cingulate cortex.
Is tickling always fun?
Laughter is a sign of joy and pleasure, and tickling–though there’s an aspect of discomfort to it–is mostly a joyful activity. This is best demonstrated by experimental and anecdotal observations that indicate both people and rats will pursue opportunities to be tickled, even if in the moment of contact they thrash to feign escape from the claws of the tickle monster, says Ishiyama. In domestic rats, tickling can even be used as a reward system during training, notes Proelss. (In some humans, tickling can veer so far into pleasure that it becomes the basis for a sexual fetish, as documented in a 2024 study co-authored by Ishiyama.)
Strangely, humans and rats do display ambivalence and signs of fear in situations where they know a tickle is likely. But that doesn’t stop members of either species from going back for more, Ishiyama says. The mix of emotions brought on by a prospective tickle session may be similar to that of someone eagerly anticipating a haunted house tour or scary movie. “A tiny bit of fear in pleasure is what makes play more fun because otherwise it isn’t really thrilling,” he explains.
Yet there can be too much of a good thing. Tickling triggers a big response both neurological and physically. It prompts involuntary movement, changes your breathing rhythm, lights up multiple brain regions, often leaves someone gasping for air, and can quickly turn stressful. Historical accounts attest that, at points in the past, tickling was used as a form of torture because–again–outside of the positive context of play and familiarity it can become a profoundly uncomfortable sensation, especially when it’s out of your control. In instances like this, Proelss says the victim’s response would look different–a lot more like distress and less like laughter.
Even the fetishists that Ishiyama surveyed who seek tickling out for sexual gratification note that the activity is a combination of pain and pleasure, akin to BDSM. Nearly 40 percent of the study participants indicated they experience distress during consensual tickle sessions.
What don’t we know?
Even with a handful of scientists on the case, tickling remains an understudied phenomenon, says Ishiyama. “I’m an advocate of positive emotion study. My ultimate goal is to understand the brain mechanism of fun.” However most neuroscience research focuses on diseases and disorders, he says. Perhaps as a consequence, there’s still many unknowns when it comes to ticklishness.
For example, some people are very ticklish, while a select few aren’t ticklish at all. There seems to be some genetic component at play, says Proelss, but “it’s one of the big mysteries,” she adds. Then, there’s the precise neurophysiological mechanism behind tickle self-suppression, which we’re getting closer to, but haven’t cracked yet. We don’t know how many other animals experience ticklishness, notes Ishiyama–he’s just begun to undertake research on mice. Age-dependency is a continuing question–ticklishness seems to diminish with maturity, and why is unknown. Plus, the evolutionary path that’s led to tickling in multiple species is unclear.
We’ve tickled our way to some insights, but we’ll need more laughing rats, chimps, and people before the whole tangled tickle mess unravels.
This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us.