Wiggly fingers approaching the armpits can elicit giggles from some people. For others, even a feather caressing the toes will bring about no response. Scientists are perplexed by the variability and the origin of the tickle response. One neuroscientist, David J. Linden, a professor at Johns Hopkins University School of Medicine, explains whether the childlike response is learned or innate and why it is almost impossible to tickle oneself.

The Evolution Conundrum
Some scientists have argued that being ticklish is a defensive reflex against attack, but Dr. Linden finds that explanation wanting. While some ticklish parts of the body would be vulnerable in battle, others, like the toes and feet, wouldn’t result in a mortal wound if struck, he says. He compares being ticklish to having an itch, which most experts believe evolved as a protective measure against infestation by insects or worms. “Itching and tickling are similar in that they are both sensations that demand an immediate physical response,” Dr. Linden says. “We can take that as a clue—that perhaps the tickle response is some sort of reflex gone awry.”

Still, the neuroscientist says, “We honestly don’t know why humans are ticklish.”

A Cross-Cultural Phenomenon
Dr. Linden says there is no indication being ticklish is inherited. He has seen tickling across every culture, and says the behavior is often informed by social norms, taboos and the setting in which it takes place. “If someone is really angry, you can’t tickle them,” he says.

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