DEAR DOCTOR K:

I am a very ticklish person. My husband thinks it's hilarious how easy it is to make me giggle and squirm. What causes ticklishness?

DEAR READER:

Ticklishness is indeed a mysterious phenomenon, and not just for you. To this day, no one has adequately answered the question of what causes it.

When discussing ticklishness, most people mean the kind that makes you laugh and squirm. Let's call this laughter-associated ticklishness. There's another type, like what happens when you run your fingernails lightly over your skin. But I'll focus on the first, since you asked.

Although there isn't a solid, science-based answer for why we're ticklish, that hasn't kept people from researching and speculating. Some of the top theories include these:

-- Tickling bonds people. A mom tickles her baby, the baby laughs, the mom smiles, and they share a happy moment together. Similarly, kids who tickle each other may bond over the experience.

-- Ticklish spots of the body, such as the abdomen or neck, tend to be more vulnerable. Learning to protect them from tickling as a child may help you protect them from harm.

I doubt this theory, however. I think it has some holes in it. For example, the head and hands are particularly vulnerable to injury, but they are not particularly ticklish.

-- Increased skin sensitivity on certain areas of the body develops before birth to encourage a fetus to stay in the healthiest positions in the womb. This centuries-old theory doesn't seem to have any modern evidence to support it.

The biggest question is whether laughter-associated ticklishness is an uncontrollable reflex, something that is "hard-wired" inside us at birth. Many experts believe that is likely, but others think this kind of ticklishness is something we learn as babies by interacting with our parents and others. My own guess is that both theories are true: We are born with a tendency to be ticklish, and then interactions with others reinforce that tendency.

One interesting fact is that you can't tickle yourself. Once again, there is no convincing scientific data to explain this. If you try to tickle yourself, you'll know where and when it's going to happen, and that might cancel out the tickle.

One idea is that, like your startle reflex, laughter-associated tickling requires that you not know it's coming. Indeed, some studies have shown that people laugh more when they are blindfolded and don't know where or when they'll be tickled. However, I'm not sure I believe that theory, either. Even if I'm not surprised, even if I know that someone is trying to tickle me, and then she does it -- I still laugh. What about you?

Why are we ticklish? Why do we yawn? Why do we yawn when we see someone else yawn? Why do we laugh at certain things? Why are so many adults -- including big, powerful adults -- terrified by tiny, non-poisonous spiders? There are so many things that seem to be part of being human that we don't even begin to understand. The question is: Do we need to?

(Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information: www.AskDoctorK.com.)

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