If tomorrow we had to move and establish personal ties in another country, we would probably experience more cultural clashes over hugs or caresses than over tickling. Different forms of social contact have been studied for a long time, but there are still unresolved questions regarding their cultural or evolutionary origin. Specifically, that tactile stimulus that simultaneously provokes laughter and an irresistible urge to flee generates curiosity and theories since the times of Aristotle, a sign that, a priori, it seems quite universal, although many certainties are still lacking. We are talking about what is technically known as gargalesis, which differs from the light tickling caused by the touch of a feather, whose corresponding technical term is knismesis.
A study published this Monday in Nature Human Behaviour asked 448 people from Greece, the Netherlands, and China about how they experience tickling to try to draw a bodily and intercultural map. The researchers wanted to shed light on some central, still unanswered questions about the physical experience itself, possible cultural differences, and the reasons behind the uneven sensitivity across the body. The first thing they found was near unanimous agreement: 98% have been tickled and almost 96% have tickled someone else at some point. Additionally, 71% consider themselves quite or very sensitive to this stimulus.
With all the information provided by the study participants, they were able to design the first detailed bodily map of tickle sensitivity. This allowed them not only to locate tickling on a kind of heat map reflecting the frequency with which people pointed to each specific body region. This precise topology of tickling is also a rich database that they were able to contrast with five theories that have historically tried to explain why we feel tickling more in certain parts of the body.
Some of these, such as the skin thickness theory, are so old that Aristotle already suggested it. However, existing data on epidermis thickness rule out that tickling is stronger where the skin is thinner, as it does not match the study’s map. The “vulnerability theory” is also discarded: where arteries have a larger diameter, near organs, a violent blow is considered more dangerous. This map also does not coincide with the tickling map from this study, so they discard that the reflex to flee when tickled is a training mechanism against stimuli in those critical survival points.
The study’s map also does not match the areas where participants indicated more pleasure, ruling out the “hedonic theory”; nor with the parts of the body where the skin has more nerve endings, which rules out the “physiological theory.” On the contrary, they found a certain negative effect of these two regarding the tickling experience, meaning the maps would be somewhat antagonistic, with pleasure or more innervated areas among the least sensitive to tickling.
The theory that best fits the map from this study is the exposure to touch theory, suggested by Darwin’s visionary. As he proposed, it seems that “the parts of the body that are touched least in everyday life tend to be the most sensitive to tickling,” in the words of Konstantina Kilteni, lead author. However, as anticipated, this factor does not explain the whole phenomenon, and the neuroscientist adds that their results “indicate that tickling arises from a combination of biological, psychological, and social factors.” The explanation of our response to tickling is, therefore, “more complex than previously thought,” she adds.
To conclude, they found that the maps describing other bodily sensations of the participants, such as pain, pleasure, or detection of our own touch, did not match much with the tickling map. This allows them to affirm that this sensory experience has its own pathways and is separate from those of other tactile stimuli, even from the futile tickling we might try to do to ourselves. “That sensation is expected and the brain reduces its intensity, but when another person touches us, there is an element of surprise, so the sensation is much more intense and prone to provoke tickling,” says Kilteni.
Universal experience
The study shows that sensitivity to tickling is extraordinarily universal. “People from different cultural backgrounds pointed to the same parts of the body as the most sensitive, described similar reactions, and shared very similar social experiences,” summarizes Kilteni. Most participants from the three cultural groups agreed on laughter as the main reaction and pointed to the neck, armpits, abdomen, and soles of the feet as the areas with the most tickling. For the neuroscientist from the Karolinska Institute (Sweden), this suggests that “tickling constitutes a fundamental feature of human biology, rather than something we learn from our culture.”
Most have tickled and been tickled mainly by family and friends. Many participants also include partners, and in interaction with them, they add one more body part: the inner thigh area. However, tickling by strangers or with a mere professional relationship was very rare. “Our study did not investigate situations with strangers, so we cannot draw conclusions about fear or lack of consent, although it is very likely that these social factors are important,” reflects Kilteni.
Almost half of the participants opposed the idea that laughter during tickling necessarily reflects enjoyment, and up to 26% labeled it as unpleasant. But if there is a stage of life when we consider it more fun, it is childhood, which is also when we receive it most from our close adults, something confirmed by the study responses. “This pattern was also identical in the three cultures studied and, interestingly, it is the same observed in our closest evolutionary relatives, chimpanzees and bonobos, suggesting it could have deep evolutionary roots,” explains the neuroscientist.
A possible explanation is that tickling is more frequent in childhood because adults enjoy it less than children, although they did not directly investigate why tickling decreases with age. “We focused on adults because they could inform us about their experiences both during childhood and adulthood,” says Kilteni. They suggest that perhaps this loss of enjoyment in adulthood coincides with the stage when sexual play emerges and, although these are only conjectures, it matches the low association participants made between tickling and sexual enjoyment.
Some of the apes with whom we share a very close kinship, such as chimpanzees and bonobos, also tickle each other. Moreover, they use their hands and fingers to stimulate the armpits, soles of the feet, abdomen, or neck and provoke laughter in their peers, especially those with whom they share a bond. Exactly like humans. And in them, age also marks the same roles and the frequency with which this physical interaction is experienced, reasons that reinforce the evolutionary origin hypothesis supported by the authors.
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