The word evolved from the Middle English tikelen, perhaps frequentative of ticken, to touch lightly.
The sensation of surprise elicited by tickling protects against crawling animals and insects, such as spiders, mosquitos, scorpions or beetles, which may be why it evolved in many animals, including rats. However, the continued laughter produced by tickling cannot be explained with this evolutionary survival advantage alone.
Tickling is almost certainly a form of social interaction. One feature of tickling is that we do not laugh when we tickle ourselves, only when other people tickle us. This implies that the brain may have a different mechanism for responding to the two types of tickling. Charles Darwin theorised on the link between tickling and social relations, arguing that tickling provokes laughter through the anticipation of pleasure. If a stranger tickles a child without any preliminaries, catching the child by surprise, the likely result will be not laughter but withdrawal and displeasure. Darwin also noticed that for tickling to be effective, you must not know the precise point of stimulation in advance, and reasoned that this is why you cannot effectively tickle yourself.
Researcher Sarah-Jayne Blakemore confirmed Darwin's propositions by investigating how the brain distinguishes between sensations we create for ourselves and sensations others create for us. Blackmore used robotic arms to tickle people and found them to be as effective as real people in provoking laughter. When her subjects used a joystick to control the tickling robot, however, they could not make themselves laugh. This suggests that when a person tries to tickle him- or herself, the cerebellum sends to the somatosensory cortex precise information on the position of the tickling target and therefore what sensation to expect. Apparently some cortical mechanism then decreases or inhibits the tickling sensation.
Washoe, a chimpanzee who learned to use the American Sign Language, has been reported to frequently make the sign for "tickle me" to researchers, similar to children who enjoy being tickled.
The idiom tickled pink means "pleased or delighted".