Tickle torture was common in ancient China. Since it can be extremely unpleasant for the victim without doing permanent damage, tickling was particularly useful for intelligence gathering. At the risk of being overly macabre, it is strange to think of hysterical laughter emanating from a torture chamber.
Which brings me to the question: why oh why does tickling make us laugh even though we dislike it? How bizarre! Furthermore, why are some places on our bodies more ticklish than others? And why can't we tickle ourselves? A number of famous scientists have turned their attention to these questions at some point in their careers, and there are several theories as to the evolutionary purpose of the tickling response. Here we go.
Charles Darwin (of all people) noticed that the effect only occurs when accompanied by a certain degree of unpredictability, which explains why we can't tickle ourselves. This idea has been confirmed by recent experiments in which MRI scans are done on people while they're being tickled. Our brains inhibit the tickling response when it can predict where we are going to be touched, as when we tickle ourselves.
Darwin supposed tickling to be a form of social and/or sexual bonding, which is why we feel more ticklish in response to tickling by people we know. Many scientists agree with this summation and consider tickling to be especially useful in solidifying the parent-child bond.
Others have noticed that places typically considered the most ticklish, such as the armpits and neck, are also the most vulnerable in hand-to-hand combat. They theorize that tickling as part of childhood play is a way of honing self-defense skills. While laughter encourages the tickler to continue, pain or unpleasantness causes the tickled person to fight back with combative and defensive moves. This may explain why, in adulthood, our fastest reflexes are invoked to protect these ticklish (and vulnerable) areas.
Galileo contemplated tickling in a philosophical context. To him, ticklishness was evidence that the self is separate from the other. "This sensation," he wrote in The Assayer, "belongs to us and not to the hand." An insight from the days before neuroscience.