You probably know that you can't tickle yourself. And although you might be able to tickle a total stranger, your brain also strongly discourages you from doing something so socially awkward. These facts offer insight into tickling's evolutionary purpose, says Robert Provine, a neuroscientist at the University of Maryland, Baltimore County, and the author of the book Laughter: A Scientific Investigation.
Tickling, he says, is partly a mechanism for social bonding between close companions and helps forge relationships between family members and friends. In adulthood, 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.