Why Do We Tickle?
Trying to unearth the evolutionary basis for peculiarities of psychology and physiology is a notoriously treacherous enterprise. But it is also irresistible and currently in vogue. What, we might wonder, could be the adaptive value of tickle?
One possible function is that tickling may help facilitate the bond between parent and offspring. A child smiles when tickled, which produces smiles in the caregiver. This reciprocal smiling and laughing produces positive social interaction. We like sharing such facial expressions with conspecifics, because it usually connotes positive motives. The one thing this doesn’t help explain is why people usually find tickle unpleasant, rather than pleasant, and why they so often resist it.
Another possible evolutionary function was bandied about by several writers at the turn of the century. Ticklishness, these writers suggested, is greatest in places on the body that are most vulnerable in arm-to-arm combat. Being ticklish in such spots confers an adaptive advantage by motivating individuals to protect these areas (Gregory 1924). Consistent with this notion is the 1984 observation by University of Iowa psychiatrist Donald W. Black that ticklish spots are also places where protective reflexes are often found. One difficulty with this hypothesis is that the hands and fingers are highly vulnerable to injury during fighting, but they are among the least ticklish spots. And although we might find them disarming, it is not clear what protective benefit the smiling and laughter might confer in a combat situation.
With some reservations, I offer a third suggestion that is basically a hybrid 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 of tickle, but they simultaneously display a facial expression suggesting “Boy, I’m having fun!” Perhaps the disconnection between outward expression and inward feeling is itself adaptive. The discomfort from tickling motivates the growing child or ape to develop combat skills in much the same way that other rough-and-tumble play does. The facial expressions, on the other hand, tell conspecifics “Keep doing what you’re doing; I like it!” In other words, the smiling and laughter encourage the tickler to continue. If tickling produced a negative facial expression, conspecifics would be far less likely to engage in it during playful bouts—thus cutting off the 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 it sometimes does. But recall, most tickling is done by parents, siblings or friends engaging in play. It is this very context that, in my view, frequently fools us into thinking that the actual sensation of tickle is pleasant and has 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 in order for tickling to elicit laughter.
It may not be possible conclusively to test adaptive stories like the one I have just offered. However, my suggestion does make some predictions. Even among apes, if my suggestion is correct, we should be able to observe that the outward expressions of tickle actively promote the interactions. A close study of the role of tickle in the interactions of nonhuman primates thus should help to decide the plausibility of this hypothesis. Of course, as Stephen Jay Gould and others have noted, it is always possible that a biological phenomenon may be not adaptive per se, but merely a side effect of a mechanism designed for another function. If this is true, the mystery of this aspect of our primate heritage may long remain just that.