You probably know that you can’t tickle yourself. And although you might be able to tickle a total stranger, your brain strongly discourages you from doing something so socially awkward.

These facts offer insight into tickling’s evolutionary purpose, says Robert R. Provine, a neuroscientist at the University of Maryland and the author of the book. Tickling, he says, is a mechanism for social bonding between close companions, helping to forge relationships between family members and friends.

Laughter in response to tickling kicks in during the first few months of life. “It’s one of the first forms of communication between babies and their caregivers,” Provine says. Parents learn to tickle a baby only as long as she laughs in response. When the baby starts fussing instead, they stop. The face-to-face activity also opens the door for other interactions.

Children enthusiastically tickle each other, which some scientists say not only inspires peer bonding but also might hone reflexes and self-defense skills. In 1984, psychiatrist Donald Black of the University of Iowa noted that many ticklish parts of the body, such as the neck and the ribs, are also the most vulnerable in combat. He inferred that children learn to protect those parts during tickle fights, a relatively safe activity.

Tickling while horsing around also may have given rise to laughter itself. “The ‘ha ha’ of human laughter almost certainly evolved from the ‘pant pant’ of rough-and-tumble human play,” says Provine, who bases that conclusion on observations of panting in apes that tickle each other, such as chimpanzees and orangutans. In adulthood, our response to tickling trails off around the age of 40. At that point, the fun stops; for reasons unknown, tickling seems to be mainly for the young.