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.