Why are some people not ticklish?

C.B.

Peabody

A. Because tickling involves the sensitivity of our skin, we do start out with natural differences in ticklishness, much as we have differences in our hearing or eyesight.

But tickling is different because it's social; it's not like sight or smell, says Robert R. Provine, a professor of psychology and neuroscience at the University of Maryland who studies the tickling response and is the author of the forthcoming book ``Quest for Laughter.''

``The tickling story is far more interesting than anyone has anticipated,'' says Provine. ``If you think the social component is not important, try tickling a stranger.''

Although tickling, like reflexes, involves neurological mechanisms, it is far more than an involuntary response. You can give yourself a knee-jerk reflex, but you can't tickle yourself, says Provine. Tickling is a form of communication used between friends, family and lovers, and one that Provine thinks may play a key role in the evolution of our social and sexual behavior.

Chimps tickle each other during play. Children are playfully tickled by their parents and other children of both sexes. Above the age of puberty, though, tickling usually is a form of flirtation, he says.

So why are some people more sensitive than others? Provine theorizes that the differences lie largely in who is doing the tickling. You may be highly sensitive to being held down and tickled by an older brother, for instance, because it's somewhat aversive. But a lover's tickling may not annoy you at all.

Acquiring sensitivity to ticklish stimuli might have been highly useful in our evolutionary past, says Provine. It would have warned us if a poisonous scorpion was crawling along our neck. And the parts of our bodies that are highly vulnerable to injury, like our feet, chest and armpits, are among the most ticklish.