Beginning with a dictionary definition is the tactic of the cowardly writer, and yet, here we go: According to an aggregation of medical dictionaries, "tickling" denotes "a peculiar itching or tingling sensation caused by excitation of surface nerves, as of the skin by light stroking," occasionally accompanied by "involuntary laughter."
That gives us a good foundation from which to build, but still doesn't explain the central question: What the hell's up with tickling?
Sure, the question sounds like a bad comedy set, but from an anatomical, biological -- hell, even social -- standpoint, why are humans ticklish? Could it have been crucial to our evolution, and remained, like the appendix, a part of who we are even though it serves no purpose? Does it serve a function unnoticed or long forgotten?
Tickle laughter is different from other laughter
Let's start by doing what any scientist worth their salt would do: Hook some willing participants up to fMRI machines and make them listen to people getting tickled.
What researchers who did just that found is that when tickled people laugh, it's an entirely different function than "social laughter," the noises you make when you find something funny, or even "taunting laughter," which is apparently a thing that needs studying.
For one, tickling laughter sounds different -- it has a "higher acoustic complexity" -- and activates parts of the brain that process auditory information, as well as a part of the brain related to language comprehension. In short, tickling laughter has meaning. But what?
Well, the researchers describe tickling laughter as an "unequivocal and reflex-like social bonding signal," related in part to how your brain processes language and working memory. It helps you form bonds with those close enough to tickle you, and your response signals that you have a relationship with the tickler. Social laughter grew out of this basic bonding response, but it can appear in scenarios that have nothing to do with touch; we humans have evolved to laugh at movies, for example, while chimps remain totally ignorant of Hollywood.
So why is tickling occasionally unpleasant?
But all of that doesn't account for the strange paradox of tickling. As you've likely experienced at some point in your life, after a certain point, the tickling sensation can morph from one of fun to one of pain. This is where the dark side of tickling lives.
Paradoxes are exactly what researchers have found when looking into the human response to tickling -- ticklees in one study showed simultaneous expressions of both pleasure and displeasure, and other research has categorized tickling as a sensation that resembles pain or itch, but isn't unpleasant.
One of tickling's defining characteristics is that you can't tickle yourself, which is where tickling's dark side turns pitch black.
Laugh until you cry
There's a long history of nonconsensual tickling as torture and/or interrogation methods, from ancient Rome (where a detainee's feet were dipped in salt and a goat was brought in to lick it off) to the Han dynasty of China (where tickle torture was reserved for those of noble descent, as it would be excruciating but leave no mark on the victim).
In his book The Men with the Pink Triangle, Nazi concentration camp survivor Heinz Heger describes watching a fellow prisoner tickled to death. More recently, the documentary Tickled examines the world of "competitive endurance tickling" fetish videos; it seems mostly benign, before the film takes a left turn into the story behind the videos.
So we return to the original question: If tickling is one of the most powerful reactions our bodies can have, expression a range of often contradictory emotions, why is it there?
Tickling makes you YOU
In 2004, the neuroscientist Robert Provine wrote a report that tried to answer this question. On a physiological level, one of the reasons we may have developed such a strong response to tickling is that it was probably a way to protect us from parasites and other creepy-crawlies; it's why kids squirm to escape a tickler's grasp.
But clearly tickling has evolved beyond a defense mechanism. One place to look for pieces of the tickling puzzle is other species, especially chimpanzees. Provine notes that chimps produce a laugh-like sound when tickled, though it differs from human laughter because we possess the breath control to produce speech.
It's not just apes, though -- the tickling response extends to most mammals, indicating something more basic is at work. A 2003 study looked at the "laughter" of rats, that is, vocalization chirps higher than 50kHz, which are produced during bouts of playfulness.
It may be how we develop our entire sense of being.
Rats only produced the chirps by human hands after they'd learned to trust the handlers, which isn't dissimilar to what happens with young children. Anyone who's had a playdate with a toddler knows this all too well. Once a baby knows you, the old "I'm going to get you" game works like a charm. All you have to do is start creeping forward and twiddling your fingers, and the giggles start right up.
Battles like these may allow young humans to give themselves a sense of self. Remember how you can't tickle yourself? As a young child, you learn how to separate your body from others through this phenomenon. Suddenly, you become yourself, and others become others. "[S]uch a mechanism may be at the foundation of a sense of identity and a first step toward the evolution of personhood and the neurological computation of its boundaries," writes Provine. He goes even further, saying that this response may be even more powerful than sexuality: "Solo tickle is even emptier than solo sex -- you can masturbate to climax but you cannot tickle yourself."
So tickling serves a twofold purpose by identifying you as an individual, but also an individual in need of social interaction. "Laughter," Provine says, "reveals us as a social mammal, stripping away our veneer of culture and language, challenging the shaky hypothesis that we are rational creatures in full conscious control of our behavior."
By feeling a tickle, then tickling someone else and seeing their reaction, toddlers may begin to glimpse themselves as entirely new beings, a conceit that will accompany them for the rest of their lives. In fact, tickle battles occur even before toddlers recognize themselves in mirrors and begin to use first-person language like "I, me, and my."
And, to return to the dark side of tickling, what is torture if not an attempt to take away a victim's sense of self, a sense which produces the will to live? Tickle torture undermines humans' most basic sense of self and joy.
Tickling, then, may not just be some weird quirk in our brains, or even a lingering defense mechanism from our early ancestors. It may be how we develop our entire sense of being. Think about that the next time someone tries to tickle you.
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