"Solo tickle is even emptier than solo sex," psychologist Robert Provine once wrote. After all, "you can masturbate to climax, but you cannot tickle yourself."
Why is tickling an activity reserved for two or more people? And why exactly did we evolve to be ticklish in the first place?
Let's cut right to the chase. One of the longest-standing, leading hypotheses about tickling is that our brains are programmed to tune out anticipated stimuli, including — perhaps most tellingly — tactile perceptions that result from our own movement. In a 2006 study, published in PLOS Biology, U.K. researchers Paul Bays, Randall Flanagan and Daniel Wolpert provide evidence that we do this by predicting self-generated sensations, and subtracting the expected sensory consequences of our actions.
It's important to point out that this mechanism is fundamentally different from what the researchers call a "postdictive" process, in which your perception of stimulation is altered only after the event is determined to be self-generated. Their findings instead suggest that our bodies are constantly predicting what they're about to experience, and adjusting their perceptions of those experiences accordingly.
Psychologists and neuroscientists call this "sensory attenuation" — by blunting the perception of self-imposed stimuli, your body is better prepared to respond keenly to the unexpected, an ability that Bays and his colleagues say "may have evolved to enhance the perception of sensations with an external cause." This gets at the heart of why tickling can't be self-imposed. Knead at your ribs with your fingertips, or squeeze sporadically just above your own knee, and your body is programmed to ignore it. Your body is just too busy looking out for unexpected, external stimuli to deal with your self-prodding.
But what kind of external stimuli are we talking about? For example, does it matter whether another person is doing the ticking, or can it be an animal? What about inanimate objects? Surprise tree branches, for instance — can they tickle, too?
Questions surrounding tickling's dependence on external stimuli tie back to two major hypotheses. The first is the "interpersonal" explanation, which states that tickling is fundamentally interpersonal, and therefore requires that another person be the source of the touch. The second is the "reflex" explanation, and suggests that the only prerequisite for a ticklish response is an element of surprise.
There is a wealth of evidence that would seem to support the interpersonal explanation for tickling. "Most everyday tickle is… another social context for laughter and a form of communication," writes Provine, who argues for tickling's significance in bonding between friends and family members, in a perspective published in a 2004 issue of Current Directions in Psychological Science. "[Tickling binds] us together in a laugh-ﬁlled give-and-take that may be the basis of all social play." He continues:
Consider the social choreography of tickle. The ticklee may push away the offending hand of the tickler and escape, only to return, renew the interaction, and counterattack. For infants who cannot yet talk, being tickled, along with the as sociated laughter, is an entrée into social relationships with caregivers. Laughter signals ‘‘I like it; do it again!'' Crying and fending off the other person signals the game has gone too far.
Being ticklish could also be the body's way of learning to protect itself during hostile encounters, which are frequently interpersonal in nature. "Tickle battles," notes Provine, are "the most benign form of human conflict." His observation resonates with those made by psychiatrist Donald Black in a 1984 issue of The Journal of the American Medical Association, who points out that many of the body's most ticklish regions, like the neck and ribs, are the ones most susceptible to injury during combat.
All that being said, none of the above examples demonstrate that tickling is dependent upon interpersonal contact; who's to say that our tendency to associate tickling with social interaction isn't simply an emergent feature of a deeper, hardwired motor pattern, as postulated by the "reflex explanation" mentioned earlier? Consider, for example, the sensation you experience when you feel a spider crawling up the back of your neck. It probably won't send you into fits of laughter, but it won't enhance your social bond with creepy crawlies, either.
But the spider/neck example also draws attention to the difference between a soft, annoying tickle and a more dramatic, laughter-inducing one. The distinction between the two is rooted as far back as 1897, when psychologists G. Stanley Hall and Arthur Allin suggested in The American Journal of Psychology that the gentler sensation be referred to as "knismesis," (light tickle), and the more intense be termed "gargalesis" (heavy tickle). The former has been likened to a "moving itch," and can actually be self-induced with little more than the light brush of a fingernail; the latter, of course, cannot.
According to UC San Diego psychologist Christine Harris, most people believe that ticklish laughter associated with gargalesis requires that the stimulation be attributed to another person. So where do we land, if we're dealing solely with heavy tickles — the interpersonal explanation, which holds that tickling is fundamentally dependent upon another person, or the reflex explanation, which suggests that the only prerequisite for a laughter-inducing tickle is an element of surprise, similar to a reflex?
This is precisely the tossup that led Harris to ask a very interesting question: can a machine tickle? Her anser: yes; and if her findings are correct, they cast a serious shadow doubt on tickling's interpersonal explanation. You'll find her full research paper here, but I'll leave you with this summary of her findings:
Thirty-five subjects were tickled twice—once by the experimenter, and once, they believed, by an automated machine. The reflex view predicts that our "tickle machine" should be as effective as a person in producing laughter, whereas the interpersonal view predicts significantly attenuated responses. Supporting the reflex view, subjects smiled, laughed, and wiggled just as often in response to the machine as to the experimenter. Self-reports of ticklishness were also virtually identical in the two conditions. Ticklish laughter evidently does not require that the stimulation be attributed to another person.