Why do we laugh when we are tickled? Perhaps we enjoy it or find it funny. But if so, why do most people, especially adults, say they hate it? And why don't I laugh when I try to tickle myself?

Illustrious thinkers have pondered these and other ticklish mysteries for more than 2,000 years. Socrates suggested that the sensation is to some degree pleasant but to a greater degree painful.

Aristotle raised the question of why one cannot tickle oneself: "Is it because one also feels tickling by another person less if one knows beforehand that it is going to take place . . . ?"

Francis Bacon and Charles Darwin agreed that humorous laughter requires a "light" frame of mind. But they differed on ticklish laughter. Darwin thought that the same light state of mind was required. Bacon said no. When tickled, he noted, "men even in a grieved state of mind yet cannot sometimes forbear laughing."

The kind of tickling that brings about laughter in humans usually is a pressure repeatedly applied to certain areas such as the ribs, armpits and belly. This may be limited solely to primates, the animal group that includes humans, great apes, monkeys and lemurs.

Most experts seem to agree that chimpanzees and perhaps other apes tickle each other in the course of rough-and-tumble play, producing the equivalent of laughter. There are even stories of chimps signaling to their human caretakers a desire to be tickled.

But is our response to tickling really similar to the kind of laughter produced by comedy and jokes? Probably not. Most people find humorous laughter enjoyable but do not like being tickled.

Few adults actively seek it out. In fact, there is little doubt that prolonged tickling can be extremely unpleasant. Some accounts report that medieval warriors sometimes tortured victims to death using nothing but unrelenting tickles.

Many writers have assumed that being tickled is more pleasant for children than adults, noting that children sometimes ask for it. This should be viewed with skepticism. For one thing, each succeeding generation of children independently rediscovers "tickle torture" as a way to torment playmates.

Small children sometimes do seek out tickling. But they likewise enjoy games in which parents play at startling or menacing them ("I'm gonna get you!"). So do they really like the tickling? Or is it really the thrill-seeking? Or just the closeness to parents?

A few years ago, psychologist Nicholas Christenfeld and I began to study those questions.

We reasoned that, if ticklish laughter and the laughter elicited by humor reflect the same psychological state, there ought to be a "warm-up" effect familiar to comedy audiences, that is, people laugh harder at later jokes in a series than earlier ones. Presumably, this is why top comedians insist on being placed toward the end of a program.

We brought student subjects into my laboratory and had them watch a videotape containing highlights of several comedy films. Most readily laughed and smiled while watching. A research assistant tickled the subjects either shortly before or shortly after they watched the tape.

We didn't find a warm-up effect. When tickled, subjects who had been "warmed up" with the comedy tape laughed, smiled and squirmed just the same as subjects who had not first seen the film. Likewise, having just been tickled did not make people laugh more while watching the film.

BECOMING TICKLISH

If it's not closely related to humor, what is the origin of ticklish laughter?

Several researchers have suggested that, at least in children, it could be involved in developing a sense of humor.

The argument goes something like this: Parents of easily tickled babies may engage in more physical play because they are delighted with the child's laughter. That "positive reinforcement" makes the parents want to tickle some more; the infant's giggle becomes a reward.

Such parents will probably branch into other forms of humorous physical play and eventually to stimulating their children mentally. As a result, those kids will be encouraged to laugh at humor in words and gestures as well as tickling.

But with that idea come problems. For one thing, researchers in the 1970s studied how laughter emerged during the first year of a child's life. Ticklish laughter was first seen at around six months. If anything, that's slightly after, rather than before, the emergence of the most primitive forms of humor -- for example, laughter when the parent playfully says "I'm gonna get you!"

One recent study showed that ticklish laughter can trigger conditioning, a form of learned behavior in which a specific event or "stimulus" causes a predictable response. A few years ago, researchers tickled students while simultaneously exposing them to non-funny, "neutral" words. Later, they found that the words alone produced laughter.

But it's also possible that ticklish laughter is itself a conditioned response, that is, children may come to laugh when tickled because tickling has always occurred in playful, happy situations. Or perhaps children laugh when tickled because the tickler is laughing, which creates a sort of feedback loop.

The Social Side

How much of the tickle response is caused by the personal interaction that accompanies tickling? Darwin, writing in 1872, thought that "the mind must be in a pleasurable condition; a young child, if tickled by a strange man, would scream in fear."

Similarly, writer Arthur Koestler suggested in 1964 that laughter occurs only when the person being tickled views it as a harmless and playful mock attack. Naturally, if ticklish laughter is an interpersonal behavior, it might explain why we cannot tickle ourselves.

If that idea is correct, then people should laugh only when they believe that a person causes the sensation. A person who believes that he or she is being tickled by a machine should not laugh. Common sense seems to endorse this idea.

Christenfeld and I surveyed undergraduates and found that 50 percent thought a tickle machine could not produce laughter. Only 15 percent thought that it would be as effective as a person.

We then set about to fashion a tickle machine. But we soon realized that having a machine actually tickle people would not provide good answers -- if people tickled by a machine didn't laugh, we could never be sure whether the lack of laughter resulted from their knowledge that it was a machine or from the device's failure to mimic accurately the movements of a human hand.

A more decisive experiment required a mock tickle machine and a little deception. We created such a machine in our laboratory, complete with a robotic-looking hand, a vacuum-cleaner hose and a water-vapor generator to provide convincing sound effects. The hand did not actually move.

In the experiment, subjects were advised that they would be tickled once by the human experimenter and once by the machine. Then they were blindfolded, under the impression that they could pay closer attention to the tickling sensations.

In fact, all tickling was done by a human being secreted beneath a cloth-draped table next to the subject. The tickler carefully applied the same stimulation during both the "machine tickle" and "human tickle" parts of the experiment.

Subjects laughed just as much when they believed that a machine was tickling them as when they thought that a person was doing so. For half of the subjects, the experimenter left the room during the machine-tickle phase. Even when subjects thought that they were alone with the machine, they readily laughed and smiled in response to being tickled.

Involuntary Response?

If ticklish laughter is not the same as our response to humor and not inherently an interpersonal response, then what is it?

One possibility is that it is similar to a reflex, that is, an automatic nerve reaction. That may seem odd since you can't tickle yourself. But you can produce a perfectly respectable knee-jerk reflex by tapping your own knee with a rubber hammer.

There is, however, an ancient reflex you definitely cannot produce in yourself: startle. The violent reaction produced by a loud sound requires unpredictability, and it can be inhibited by even a very faint warning signal.

Maybe ticklish laughter, too, requires appropriate and vigorous stimulation that cannot be anticipated, just as Aristotle proposed. After all, our experimental subjects laughed more when they had their eyes closed than open.

Another possible reason that you can't tickle yourself may be similar to a feature of vision. The world doesn't appear to jump every time you move your eyes because the brain has taken into account the fact that it issued the command to move. So perhaps when the brain issues the command to tickle, it cancels the sensation of ticklishness.

Some scientists have actually studied the question, using high-tech functional magnetic-resonance imaging (fMRI) devices that show exactly what parts of the brain are most active at a particular time. They found that different parts of the brain's outer cortex were excited when people tickled themselves than were activated when others tickled them.

In sum, results from the handful of studies suggest that the inability to tickle oneself may be due to nerve impulses being stifled at a fairly basic level, although no one knows exactly how.

Getting Physical

Which of our many nerve tracts are involved? Some clues may come from the fact that different parts of the body respond differently to tickling.

More than a century ago, researchers surveyed 700 people, who reported that children were most ticklish on the soles of their feet, the underarms, the neck and under the chin. Research in my lab suggests that college students are most ticklish anywhere along the sides of the torso, beginning at the armpits and extending to the waist, and on the soles of the feet [see chart].

Work with animals and humans suggests that tickle involves nerve mechanisms that carry pain sensations. For example, when surgeons sever pain fibers in the spinal cord as a treatment for persistent pain, responsiveness to tickle -- specifically, stroking the sole of the foot with cotton wool attached to a stick -- seems to be reduced.

However, tickle-induced laughter is retained in at least some patients who have lost pain sensation as a result of similar surgery.

Tickle also may depend on nerve fibers related to touch. When blood circulation to an arm or leg is cut, sensitivity to touch and tickle is eliminated before pain sensitivity. Thus, it appears that tickle may involve signals arriving on both pain and touch fibers.

So, you might think that the most ticklish areas of the skin would be spots where our sense of touch is keenest. This doesn't seems to be the case.

For example, pressure sensitivity is greater in the palm than the sole of the foot. And when people are asked to tell whether they have been touched at one point or at two nearby points, they can gauge it more accurately on the palm than on the sole. However, the foot is a much better place to elicit ticklish laughter than is the palm.

What's in It for Us?

Perhaps the explanation for ticklish laughter lies in the currently fashionable effort to explain various aspects of human behavior in evolutionary terms.

In this view, a behavior that increased our ancestors' ability to adapt to their surroundings -- thus keeping them alive longer and, most importantly, allowing them to reproduce more often -- naturally would tend to appear more frequently in future generations. So what big advantage could tickling bring to humans?

One has been mentioned earlier: Tickling may help to strengthen the bond between parent and child. In addition, sharing smiles and laughter bind people in social groups that keep their members alive and reproducing longer.

Some writers have noted another possible evolutionary function: Ticklishness is greatest in places on the body that are most vulnerable in hand-to-hand combat. Being ticklish in such spots gives you an advantage by motivating you to protect these areas.

One difficulty with this idea is that hands and fingers are highly vulnerable to injury during fighting but are among the least ticklish spots. In addition, what protective benefit would smiling and laughing give you in a combat situation?

With reservations, I offer a third suggestion that combines elements of the two proposals just mentioned.

Consider again the basic, very odd facts of tickling. People exhibit defensive movements and generally report not enjoying the sensation. But at the same time, their facial expressions are saying, "Boy, I'm having fun!" Perhaps this disconnection between outward expression and inward feeling is in itself an advantage.

The discomfort from tickling motivates the growing child (or ape) to develop combat skills. The facial expression, on the other hand, tells playmates, "Keep doing what you're doing. I like it!"

In other words, the smiling and laughter encourage the tickler to continue. If tickling produced negative facial expressions, children would be far less likely to engage in it during play. And that would halt development of combat skills that might have survival value.

Might not this arrangement leave the ticklish person vulnerable to enemies? Indeed, tickle-torture by children suggests that it sometimes does.

But most tickling is done by parents, siblings or friends engaging in play. It is this context that, in my view, frequently fools us into thinking that the actual sensation of tickle is pleasant.

It also may have beguiled many theorists into assuming that certain conscious ideas, such as "this is a friendly source" or "this is a mock attack," must be in place for tickling to elicit laughter.

Christine R. Harris is a research scientist at the Center for Brain and Cognition at the University of California, San Diego. This article and the chart above are adapted from the July-August 1999 issue of American Scientist, the magazine of Sigma Xi, The Scientific Research Society.

CAPTION: Ticklish Spots (This chart was not available)