One of the most basic observations about tickling is that one does not laugh and smile when one tries to tickle oneself. The inability to self-tickle is compelling evidence that tickling requires some factor beyond just a particular sort of physical stimulation of the ribs, feet, or other body part. However, the precise nature of the additional factor has been the subject of some debate (Harris, 1999). On one side is a reflex view, suggesting that what is lacking in self-tickle is unpredictability or uncontrollability. On the other side of the debate is the interpersonal view, proponents of which suggest that one cannot tickle oneself because tickling is fundamentally an interpersonal experience. The notion that the tickle response is a reflex was advocated by G. Stanley Hall and other writers around the turn of the century (Hall & Allin, 1897; Sully, 1902). A writer for the Edinburgh Review (“Art,” 1912) went as far as to claim that “in the phenomenon of tickling we get laughter produced as a purely physiological reflex, so unconnected with mental appreciation” (p. 385). More recently, Black (1984) noted that laughter “is unique in that it is both a reflex and a psychosomatic event. . . . For example, as a reflex, laughter can be elicited by tickling; as a psychosomatic event, it can be elicited by a circus clown dropping his pants” (p. 2995). A similar view was suggested by Francis Bacon (1677), who observed that, when tickled, “men even in a grieved state of mind, yet cannot sometimes forbear laughing” (p. 151). Stearns (1972) and Fridlund and Loftis (1990) have also argued that tickle should be considered a reflex. If tickle is a reflex (or some other innate stereotyped behavioral response such as a fixed action pattern), why can we not elicit it in ourselves? Several possible mechanisms that are consistent with the reflex notion might be responsible for inhibiting self-tickle. For one, self-tickle may be impossible because the element of surprise is missing. On this view, tickle may be more akin to the startle reflex than to the knee jerk; one can no more startle oneself than tickle oneself. Another account has been suggested by Weiskrantz, Elliot, and Darlington (1971), who argue that to some extent the command, or efferent signal, works to prevent people from tickling themselves. Thus, what is sometimes termed exafference (stimulation uncorrelated with a motor command) may be required in order to elicit the tickle response. While several writers have referred to the tickle response as reflexive, tickle may be more complicated than a standard reflex. It may be more appropriate to view the behavior as a stereotyped motor pattern that requires a particular releasing stimulus. Although the specific mechanisms and terms vary across writers, what unites this approach to tickle is the notion that the inhibition of self-tickle reflects relatively low-level physiological mechanisms. We will return to these complexities in the Discussion section. In contrast, the interpersonal approach to tickle assumes that we cannot tickle ourselves because the stimulation must be known to come from another person, rather than simply unpredictable or exafferent. For example, KeithSpiegel (1972) wrote that “not just anyone or anything can do the tickling and elicit laughter. It must be administered by a ‘friendly’ source and done in a playful manner” (p. 18). Koestler (1964) suggested that laughter will occur only if the person being tickled views it as a harmless and playful mock attack. Shultz (1976) aptly points out an important implication of Koestler’s suggestion: “The tickling must come from another person; otherwise it could not be interpreted as an attack” (p. 32). Levine We gratefully acknowledge Meg Notman’s assistance with the creation of “Mechanical Meg” and her willingness to tickle the feet of strangers. We also thank Benjamin Liu for providing us with several references from antiquity. Address requests for reprints to C. R. Harris, Department of Psychology–0109, University of California, San Diego, La Jolla, CA 92093-0109 (e-mail: firstname.lastname@example.org). Can a machine tickle?