"It has been observed at least since the time of Aristotle that people cannot tickle themselves, but the reason remains elusive. Two sorts of explanations have been suggested. The interpersonal explanation suggests that tickling is fundamentally interpersonal and thus requires another person as the source of the touch. The reflex explanation suggests that tickle simply requires an element of unpredictability or uncontrollability and is more like a reflex or some other stereotyped motor pattern. To test these explanations, we manipulated the perceived source of tickling. 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, as interpersonal accounts imply."
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