Ever wonder why people are ticklish? If so, you're in not alone—great writers and thinkers, including Aristotle, Sigmund Freud and Charles Darwin have pondered the subject. But they were not able to answer the question.  In fact, so far, no one has—but that’s not to say people aren’t studying it. While there’s not a lot that’s definitively known about tickling, there’s more research on the subject than I’d expected.

Defining tickling

Tickle researchers (yes, there are people who do this for a living!) separate tickling into two distinct categories:

  1. Gargalesis is the kind of tickling that makes you laugh and squirm (Let’s call this laughter-associated tickling.)
  2. Knismesis is the kind of tickling you experience when you run your fingernails or a feather lightly over your skin—or, as a creepier example, imagine the feeling of a spider crawling up your leg.  It’s probably not something that would make you laugh.  (We’ll call this light-touch tickling.)

Researchers also note that the laughter that comes with tickling seems to be different than the laughter that comes from watching a funny movie. For example, there’s a “warm up” effect with humorous laughter—if you hear several jokes in a row, the later ones may seem funnier than the first because the first jokes got you warmed up. The same is not true with ticklish laughter. One study found that people didn’t laugh harder when being tickled after watching a funny movie than people who had not watched the movie. One prominent tickle researcher, Christine Harris, Ph.D., compared this idea to the fact that crying from cutting an onion and crying at a funeral are very different states.

Tickle fight!

Disagreement exists in many areas of science, and the study of tickling has its share. The main source of conflict in tickling research centers around this question: Is laughter-associated ticklishness reflex, or is it a social behavior? That is, does it happen involuntarily and beyond one’s control or does it require a relationship between two members of the same species?

On the "reflex" side of this tickle fight is a study that showed that people reacted similarly when they thought they were being tickled by a machine as when they thought they were being tickled by a person. (The blindfolded subjects were actually tickled by a person both times, because the researchers wanted to make sure the tickling felt the same; but study subjects believed they’d been tickled once by a machine and once by a person.) If tickling was purely a social activity through which two people interact, a machine shouldn't be able to do the tickling.

Additional evidence supporting the reflex concept is the finding that people can be trained to laugh in response to a neutral (“unfunny”) word if that word has been repeatedly associated with tickling. This is similar to Pavlov's famous experiments with dogs, in which the dogs salivated at the sound of a bell after they were trained to associate the sound with food.
Scientists on the social behavior side of the argument say that tickling can't be a reflex because people can learn to inhibit their tickle response, and that a person's mood alters whether he or she can be tickled. With other reflexes, mood makes no difference, and you can’t control the response even if you try.

So, who's right? We may never know. The answer could be some combination of these two ideas, or there may be some other explanation that proves both the reflex and the social behavior concepts wrong.

Why are people ticklish?

This question, too, is one with no solid, science-based answer. But that hasn't kept tickle experts from speculating. Some theories about the cause of laughter-associated tickling include:

It’s a way that people bond. For example, a mom tickles her baby, the baby laughs, the mom smiles, and they share a happy moment together. Babies generally start to laugh in response to tickling at around 6 months old, just as other complex and interactive social behaviors are rapidly developing. Similarly, kids who tickle each other may bond over the experience. Among adults, tickling can be a kind of sexual foreplay.

Ticklish spots of the body, such as the abdomen, tend to be important areas to protect during combat, and learning to protect them from tickling as a child may help you protect them in more serious fights. (One study found the abdomen to be the second-most ticklish area; only the underarm was more ticklish.)

This idea has some holes in it, though—such as the fact that the head and hands are vulnerable during a fight, but they are not particularly ticklish. And why should the armpit be “protected” by ticklishness?

Increased skin sensitivity on various areas of the body develops before birth to encourage a fetus to stay in the healthiest positions in the womb. However, this theory seems to date back to the 1850s, and though it is referenced in some recent papers, there doesn’t seem to be any modern evidence to support it.

As for light-touch tickling, the current thinking is this: If you feel a bug or parasite crawling on you, it behooves you to get it off. Light-touch tickling generally produces a desire to rub the area, which would do just that.

By the way, people aren't the only animals that laugh when they are tickled. Primates seem to be ticklish, and some research suggests that rats may be, too. And many, if not most, animals react to light-touch tickling.

Why can't you tickle yourself?

Once again, there is no convincing, scientific data on this subject. But, one idea is that, like the startle reflex, laughter-associated tickling requires that you not know it’s coming. Some studies back this up by showing that people laugh more when they are blindfolded and don’t know where or when they’ll be tickled. If you try to tickle yourself, you’ll know where and when it’s going to happen and that might “cancel out” the tickle you’d ordinarily feel.

There could be advantages to not being able to tickle yourself. For example, I wouldn’t want to involuntarily start laughing during a meeting with my boss just because I’d scratched my stomach!

Is being tickled fun?

A final mystery about tickling is why we laugh even though most adults say that they don’t like to be tickled—and some people hate it. Kids, on the other hand, seem to enjoy it more than adults, as long as it doesn’t go on too long. It’s as if being tickled is pleasurable when we’re young but something we grow out of as we age. Whether that’s true or not is just one more mystery about ticklishness.

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