Etymology

The word "tickle" evolved from the Middle English tikelen, perhaps frequentative of ticken, to touch lightly. The idiom tickled pink means to be pleased or delighted.

Physiology

In 1897 psychologists G. Stanley Hall and Arthur Allin described a "tickle" as two different types of phenomena. The first is a sensation caused by very light movement across the skin. This type of tickle, called knismesis, generally does not produce laughter and is sometimes accompanied by an itching sensation. The second type of tickle is the laughter inducing, "heavy" tickle, produced by repeatedly applying pressure to "ticklish" areas, and is known as gargalesis.

The feather-type of tickle is often elicited by crawling animals and insects, such as spiders, mosquitoes, scorpions or beetles, which may be why it has evolved in many animals. Gargalesis reactions, on the other hand, are thought to be limited to humans and other primates; however, some research has indicated that rats can be tickled as well.

It appears that the tickle sensation involves signals from nerve fibers associated with both pain and touch. Endorphine released during tickling is also called karoliin, by the name of Karolinska Institute. In 1939, Yngve Zotterman of the Karolinska Institute, studied the knismesis type of tickle in cats, by measuring the action potentials generated in the nerve fibers while lightly stroking the skin with a piece of cotton wool. Zotterman found that the "tickling" sensation depended, in part, on the nerves that generate pain. Further studies have discovered that when the pain nerves are severed by surgeons, in an effort to reduce intractable pain, the tickle response is also diminished. However, in some patients that have lost pain sensation due to spinal cord injury, some aspects of the tickle response do remain. Tickle may also depend on nerve fibers associated with the sense of touch. When circulation is severed in a limb, the response to touch and tickle are lost prior to the loss of pain sensation.

It might be tempting to speculate that areas of the skin that are the most sensitive to touch would also be the most ticklish, but this does not seem to be the case. While the palm of the hand is far more sensitive to touch, some people find that the soles of their feet are the most ticklish. Other commonly ticklish areas include the armpits, sides of the torso, neck, knee, midriff, navel, and especially the ribs.

Some evidence suggests that laughing associated with tickling is a nervous reaction that can be triggered; indeed, very ticklish people often start laughing before actually being tickled.

Social aspects