What is it about people’s feet, ribcages, necks and armpits that make us want to tickle them, and run the risk of being kicked in the face? Moreover, what is it about wanting to be tickled, to be suspended in a state of temporary helplessness, while laughing at the top of our lungs? Tickling is a mainstay in physically intimate relationships, especially those between children and their caregivers. Gentle or otherwise, tickling often incites inconsistent responses from the ticklee. There’s also reason to believe the smile or laugh a tickle incites might not even be authentic.

Is the mighty tickle simply a facade?

A tickle is most often an unpredicted, surprise touch, which sends signals to the hypothalamus, the part of the brain responsible for regulation of involuntary responses. Upon being tickled, the hypothalamus subsequently kicks the body of the ticklee into fight-or-flight mode and enables it to act out in the form of complex reflexes — these could be shrieks, laughter, or some kind of motor movement, such as kicking, doubling up, or flinching away.

There are some functional, albeit not comprehensively researched, theories as to why these reflex responses accompany the process of being tickled; one, given by researchers at the University of California, San Diego in a study, is “[the] tickle evolved to promote protection of areas that would be most vulnerable during arm-to-arm combat. The idea is that ticklishness in such areas motivates one to protect these areas and thereby confers an adaptive advantage (i.e. increased one’s ability to survive and reproduce). This provides a possible explanation for the pulling away and fending off movements frequently encountered during tickling.”

Researchers add, however, that this theory doesn’t explain why our arms or hands are not ticklish, which logically are the most vulnerable to getting maimed in this hypothetical combat situation. Another theory they offer is that tickling, in modern combat-free society at least, is often carried out between intimate friends and family. When accompanied with smiling or laughter, it creates positive associations within the interaction and can contribute to social bonding between the tickle participants.

Laughter while being tickled, however, needs to be taken with a grain of salt, as it may be a conditioned response, according to the UCSD study. It says that when children are tickled in the context of play, they learn to laugh because of social cues given by those who are tickling them, which happens usually in jest and in a playful manner. “This repeated pairing could lead to Pavlovian conditioning whereby laughter then becomes associated with tickling movements, even when not paired with other humorous situations,” according to the study. “Another possibility is that children laugh when tickled because of the laughter of the tickler which creates some contagious loop (e.g., a parent’s laughter causes the child to laugh which increases the parent’s laughter and so on).” Another 1940s study, however, in which a father wore a mask (so as to not betray any of the above cues) tickled his two babies, the infants still laughed, which led him to conclude that tickling may not be a conditioned response, but an inherently natural one.

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The incitement of laughter during tickling, whether attached to positive feelings or not, is independent of who is doing the tickling, the UCSD study found. Researchers built a mock tickle machine (in reality, it was a research assistant who was tickling the participants, who thought it was an automated machine) and studied participants’ reactions when a person tickled them, and when they thought a machine was tickling them; they found no difference in how the tickle was perceived.

But the UCSD researchers found that any seemingly positive reactions to being tickled, such as the “Duchenne smile” — upturned lips, lifted cheeks and crinkled eyes — are not necessarily the same as smiles elicited from pure joy or positive emotions. While people admitted to exhibiting Duchenne smiles during the process of being tickled, they also reported the absence of a pleasant feeling. Researchers noted, “ticklish smiling need have no closer association to merriment and mirth than crying when cutting onions has to sorrow and sadness.”

The nature of a tickle differs, however, depending on if it’s self-tickling, or tickling done by someone else. Tickling, according to psychologist G. Stanley Hall, can be differentiated into two types: knismesis, akin to a feather slightly brushed across the skin that incites slight discomfort or itch on the tickled surface; and gargalesis, which is a stronger, more forceful kind of tickling. The key differences between the two are that knismesis seldom invokes laughter, and it can be done to one’s own self; gargalesis only works when it’s someone else doing the tickling, and usually ends in laughter, or worse, violent reflexes. In this case, the unpredictability of tickling comes into play — it’s difficult to surprise one’s own self, and hence, knismesis often proves to be harmless.

A person’s propensity to be tickled and to exhibit strong reflex responses to it vary not only from individual to individual, but also from moment to moment, and in intensity — this is an unresearched phenomenon, but might be a result of the differing skin sensitivities in people. But looking at the small amount of inconclusive research, some of which says tickling can be quite unpleasant for the ticklee, it’s safe to conclude that we should leave people alone, and let them breathe in peace.