There are no cultural differences in the "body map" of ticklish spots…the areas that are touched less are more ticklish.

Charles Darwin's 150-year-old insight regarding the evolution of human emotions has once again come to light. Tickling, a familiar sensation, remains a mystery despite decades of exploration by philosophers and scientists. For over two millennia, from ancient Greece to modern times, various theories have attempted to explain why certain body parts are more ticklish than others.

However, none of these theories have been definitively proven. A recent study by researchers from Radboud University and Karolinska Institute in China, the Netherlands, and Greece has provided new insights into this age-old question. Their findings, published in the journal Nature Human Behaviour, reveal that ticklishness is a universal human experience, shared equally across cultures, and affecting the same body parts.

The common ticklish areas are the neck, armpits, waist, and soles of the feet, with the occasional addition of the inner thighs in romantic relationships. Machine learning models trained on one cultural group accurately predicted ticklish areas across different cultures with near-perfect accuracy. The study challenges long-held theories such as the thin-skinned argument by Aristotle, the protective advantage theory, and the neurophysiological theory that more densely packed sensory nerves lead to greater ticklishness.

The researchers found that only Darwin's "touch-exposure theory" adequately explains the phenomenon, but it accounts for only about half of the ticklishness. Other factors, such as the lack of frequent touching and low sensitivity in certain areas, contribute to ticklishness, accounting for an additional 57% of the effect. The researchers emphasize that ticklishness is not just a physical sensitivity but a complex psychological and physiological response based on past experiences.

It is also suggested that tickling serves social bonding and intimacy within human societies, with family and friends being the primary tickle partners. The study also found that tickling is not exclusive to humans, as chimpanzees and bonobos also engage in tickling, mimicking human laughter. Furthermore, people tend to tickle others during childhood and among peers as they grow older, mirroring similar patterns observed in chimpanzees and bonobos.

This research concludes that tickling is a unique sensory system that has been conserved through evolution to aid survival, though the study was limited to laughter-inducing tickling. Further research is needed to understand the lighter tickling sensation.