Why tickling makes us laugh even when it annoys us and other mysteries that baffle neuroscience

There are some everyday human behaviors that seem simple, yet inexplicable. One of them is what scientists call gargalesis, which the rest of us know as tickling. These are the behaviors that provoke an involuntary and uncontrollable fit of laughter, even when you don’t want to. From Aristotle to Charles Darwin, people have pondered the physical and cognitive mechanisms triggered by tickling. However, despite its triviality, science has yet to fully understand it.

It’s unknown why certain areas of the body are more sensitive to tickling than others. Or why some people enjoy being tickled while others hate it. It’s also not fully understood why people can’t tickle themselves. In short, the primary function of tickling in humans, as in other primate species, remains a mystery.

Konstantina Kilteni is a researcher at the Donders Institute for Brain Cognition and Behavior in the Netherlands, and for years she has been working on experiments that analyze how the human brain distinguishes between self-generated and external touch. And although “I hate being tickled,” she admits, she is obsessed with her study.

“There are many implications in the study of tickling that we don’t usually consider,” Kilteni explains to EL PAÍS. The scientist points out that the study of tickling can contribute both to sensorimotor neuroscience in infants and to the understanding of touch perception in people with schizophrenia. “Tickling is a useful model for studying the complex interaction between movement, sensation, and social context, with implications in many areas of science,” she adds.

In this spirit, the scientist published a review on May 21 in the journal Science Advances in which she poses five fundamental questions about tickling that neuroscience still has to unravel, and for which there is no definitive answer, although scientists are now a little closer.

Why are some areas of the body more sensitive?

The soles of the feet and armpits are often the weakest spots for tickling, as has been shown in tests on children and older adults. The most intuitive answer to this question is usually physiological. That is, we think we are more ticklish in regions with greater sensitivity to touch or pain. However, this is not the case. The soles of the feet and armpits are not the areas with the highest density of cutaneous sensory receptors, those that detect stimuli on the skin.

That’s why some alternative theories have been proposed over the years. “Specifically, it has been suggested that the areas most sensitive to tickling are the most vulnerable in hand-to-hand combat,” Kilteni points out. Therefore, tickling would be an evolutionary reflex for survival. This hypothesis, however, has been questioned because there are more vulnerable areas during combat, such as the arms, which are not particularly sensitive to tickling.

It was Darwin who proposed that tickling is related to atypical contact. “He suggested that our armpits are not usually touched, which explains why unexpected contact there is often perceived as tickling,” the researcher notes. With the soles of the feet, the theory works the other way around: since they are accustomed to constant, hard contact with the ground, subtle stimulation with the toes is what causes the tickling. Kilteni isn’t entirely convinced: “This explanation could be quite simplistic.”

Why do we laugh even when we don’t enjoy being tickled?

Socrates described the sensations produced by tickling as ambiguous: with elements of both pleasure and pain. Experiments on infants confirmed this idea, as they oscillated between positive and negative states: both seeking contact and avoiding it. This is the duality produced by tickling, which was even used as a method of torture during World War II.

Figures from an experimental study with 84 people show that the reality is pretty even. One-third of people find tickling pleasurable (some even include it in their sexual behavior), another third are indifferent, and the last third explicitly stated they don’t enjoy it.

Now, why does it always provoke laughter? “As a social behavior, laughter can communicate different emotions and have different connotations, ranging from happiness and joy to even embarrassment and aggression,” writes Kilteni.

Some studies have analyzed the different parameters and acoustic properties of tickled laughter and compared them with those of joyful laughter. And it turns out they are different kinds of laughter. The joy of tickling “could be a primitive response, a reflex rather than enjoyment,” the researcher suggests.

Why can’t we tickle ourselves?

On this point, science has no doubt: tickling cannot be self-induced. What’s not entirely clear are the reasons. Darwin proposed that the element of surprise plays a fundamental role. “You can’t tickle yourself because you know in advance when and where you will experience contact,” Kilteni summarizes.

The simple — and most widely accepted — explanation is that, to save resources, the brain can predict and suppress self-generated sensations. This is why the perception of the touches we inflict on ourselves is attenuated. The author insists that further studies are needed to fully confirm this hypothesis.

Why are some people more sensitive?

Answering this question using the rigor of the scientific method is particularly complex. The studies that have been conducted are difficult to evaluate because each participant perceives the tickling differently. “We still don’t know if this is due to physiological causes or personality traits,” Kilteni points out.

Understanding these differences is one of the main challenges for tickling researchers, as tactile experiences depend not only on receptors in the skin, but also on “a unique and complex combination of factors, ranging from genetics and physiology to more transient psychological and cognitive states.” In other words, it’s a lottery of individual factors that is very difficult to unravel. These factors include everything from skin stiffness to how neurons respond.

It’s also assumed that children are generally more sensitive to tickling than adults. This could be explained from an evolutionary perspective: greater sensitivity could help children develop laughter and, subsequently, their sense of humor in adulthood.

Although the differences between tickling in children and adults could also be explained by children’s greater thrill-seeking, rather than tickling itself. Another dead end for neuroscience.

What is the evolutionary function of tickling?

Some scientists argue that tickling played a crucial role for our ape ancestors and early humans. Others believe it’s a byproduct of other tactile perceptions and doesn’t actually have any evolutionary advantage or disadvantage; it’s simply there. “It could be a social activity, a play mechanism, or have affective and bonding aspects,” Kilteni explains.

Those who defend the first idea point out that tickling was fundamental in teaching young individuals to prepare for hand-to-hand combat and awakening the instinct for self-defense to protect vulnerable areas of the body. But they also believe in a social theory: tickling is a playful behavior that serves to foster bonds between couples, friends, and family members. Critics argue that if tickling were a social gesture, it would be paradoxical for people to instinctively recoil from a startled response to tickling, as most often happens.

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