Dutch Scientists Build a Tickle Robot to Study Why Humans Are Ticklish
Aristotle, Erasmus, Galileo, and Darwin all attempted to explain why humans are ticklish, yet none found a definitive answer. The scientific term for tickling is gargalesis. This phenomenon appears across cultures and throughout history, with few variations and modest differences between men and women, suggesting its roots lie in biology. Tickling responses have also been observed in chimpanzees and bonobos.
Two researchers at Radboud University in Nijmegen, the Netherlands—Xiong Ziliang and Konstantina Kilteni—decided to take a different approach. They turned to robots and sensors to study the phenomenon.
The laboratory is located in a windowless basement of the Maria Montessori building. The environment was chosen to strip away all other stimuli that might make participants laugh, keeping variables to a minimum.
The robot uses a metal needle to simulate tickling, with adjustable parameters ranging from gentle stroking to poking. Different contact methods can be measured. Participants wear electrode caps to record brain responses. The goal is to decompose tickling from a subjective feeling into measurable physical contact and neural responses.
A separate survey of 448 participants revealed highly similar experiences. Most tickling is done by adults to children, usually within families or between siblings. Partners and friends also tickle each other, but less commonly. Most respondents reported laughing, squirming, or trying to turn away. The most ticklish body parts were consistently the sides, armpits, belly, and feet.
Researchers noted that although tickling is associated with play and laughter, many people find it uncomfortable and easily overwhelming. The Nazis even used it as a torture method. There are indications that individuals diagnosed with autism are more sensitive to tickling, and ticklish people may even live longer. Other theories link tickling to sexual stimulation, as partners often tickle each other, usually targeting rarely exposed body parts.
Darwin speculated that tickling is uncomfortable because it involves touching body parts unaccustomed to being touched. Aristotle believed it acts on thin skin. But the Nijmegen researchers say neither theory fully answers why humans are sensitive to tickling.
Xiong Ziliang points out that the back is rarely touched, yet it is fairly resistant to tickling. This counterexample shows that skin thickness or touch frequency alone cannot explain the distribution of ticklishness. Armpits and feet have thin skin, but the back is also not thick, yet it is not ticklish. The sides and belly are touched relatively often, yet they are highly sensitive.
The researchers have not yet given a final answer, but the lab's establishment means this ancient question now has repeatable, quantifiable research methods. The robot can precisely control contact force, speed, location, and frequency. Brain electrodes record corresponding neural responses, and the 448 questionnaires provide behavioral and social data. Together, these three elements move beyond the speculation of philosophers. Whether tickling serves a biological function—such as a defense mechanism, a social bonding tool, or a byproduct of the nervous system—remains undecided.