Tickling and Humor:

Richard D. Alexander, Ostracism and Indirect Reciprocity:

"In the case of tickling, it seems a reasonable hypothesis that vocal or other physical responses originally functioned (i.e., had as their evolved significance) to keep the tickler tickling. Two curious aspects are that (1) we cannot successfully tickle ourselves ... and (2) our tendency to be ticklish renders us vulnerable to a kind of cruelty in the form of unwanted tickling. I see this vulnerability as paralleling the vulnerability of humans to have evolved to appreciate and use humor that does not involve physical events like tickling to have this appreciation and sensitivity as well turned against them. Vulnerability to excessive tickling, or excessive responsiveness to tickling, sometimes takes the form of being 'goosey,' meaning to be intolerant of tickling - or even the threat of tickling - that a word, gesture, body movement, or simply a stare can be a form of torture and cause a susceptible individual to do extraordinary things ..."

[See also: John Cleese on Laughter, Henri Bergson on Laughter]