Ms. Harris said the finding that ticklish laughter did not indicate mirth corroborated the experiences of many people, including herself. ''My second cousins did tickle torture, and I do remember laughing, even though I found it unpleasant,'' said Ms. Harris, who describes herself as moderately ticklish.

In fact, in the researchers' latest, still unpublished study employing a so-called tickle machine, they found that far from being the jolly social interaction that many people had assumed it to be, ticklish laughter may be a simple reflex.

Researchers blindfolded 32 undergraduates and told them that a person and a machine would tickle them on the feet, each for five seconds. However, the tickle machine actually did nothing. It looked plausible, with a robotic hand attached to a hose, which was attached to a metal box, on top of which were control panels with lights and buttons and inside of which was a nebulizer, a noisy, vibrating machine used by asthmatics. But in reality, both times students were tickled by a person.

Even when students believed they were alone and being tickled by the machine, they smiled, laughed and wiggled as much as when they knew it was a person. Ticklish laughter, Ms. Harris concludes, rather than being social interaction, appears to be a reflex, much the same as the one a doctor elicits from a patient's knee with a little rubber hammer.

The findings corroborate one of the few earlier tickle studies, published in 1941 in The Journal of Genetic Psychology by Dr. Clarence Leuba at Antioch College in Yellow Springs, Ohio. Experimenting on his own two children, he tickled the infants only while wearing a mask, and never in playful situations. Nonetheless, his children laughed when tickled, suggesting that laughter was an innate reflex rather than a learned, social response. (How these children fared later in life is not known.)

But if tickling is indeed a reflex, that raises an even more perplexing question: why can't people tickle themselves? Ms. Harris suggests that it may be the same reason that people cannot startle themselves. What is missing is the element of surprise.

Despite the new study, Dr. Fridlund contends that people cannot tickle themselves because ticklish laughter is social and requires a tension that can only be created between two or more people in much the same way as with a joke.