It's a mystery that's challenged some of science's greatest minds, including Charles Darwin, Galileo, Francis Bacon, and Plato [PDF]. One thing is for sure: It’s not because we find it funny. In fact, many people find tickling very unpleasant. So why does it make us laugh?
There are two kinds of tickling phenomena: Gargalesis, the heavy tickling that produces laughter, especially by targeting sensitive areas like the armpits and stomach; and knismesis, which is caused by light movement and tends to elicit an itching sensation rather than laughter. You can't tickle yourself because your brain knows it's coming.
When the nerve endings in your epidermis are stimulated by a light touch, they send a signal through the nervous system to your brain. Using functional magnetic resonance imaging machines (fMRI), researchers have determined that two areas of the brain create that tickle sensation: the somatosensory cortex, the area responsible for analyzing touch, and the anterior cingulated cortex, which is involved in creating pleasurable feelings.
Another fMRI study has shown that both laughing at a joke and laughing while being tickled activate an area of the brain called the Rolandic Operculum, which controls facial movements and vocal and emotional reactions. But tickling laughter also activates the hypothalamus, the area of the brain that regulates the fight or flight response—and fires when you’re anticipating pain. This has led some scientists to believe that laughing when you’re tickled could be a natural signal of submission to an aggressor, which would reduce the duration of any attack. It also explains why we may laugh at just the threat of being tickled.
Robert Provine, neuroscientist at the University of Maryland, Baltimore County and author of the 2000 book Laughter: A Scientific Investigation, says that laughter during tickling creates bonds between babies and parents. “When people say they hate being tickled and there’s no reason for it, they forget that it’s one of the first avenues of communication between mothers and babies,” he told Slate. “You have the mother and baby engaged in this kind of primal, neurologically programmed interaction.”