I suppose there's some truth to an innate sensitivity to tickling.....but I also think that the reactions to being tickled are in small part related to how it's applied, as well as to where it's applied, and how an infant sees the reactions of the person who is doing the tickling. Most times, the 'ler in such cases is one of the parents....it's quite tough NOT to tickle an infant and smile like a goof all throughout....or giggle when we see them smile. I think this is where scientific studies began to figure that ticklishness was also part and parcel based on a 'learned response.' Babies mimic facial expressions that they can see and process right from the womb (this has been proven)....babies can often be made to smile just from seeing a smiling face alone, and it doesn't necessarily HAVE to be from someone they see every day. But our bodies are rich in nerve endings (some parts more than others, obviously), and if the brain (of an infant) perceives a tickle along the bottom of the foot as a funny feeling (barring the plantar reflex), the baby could indeed smile and react to it.
I think the 'fear' factor Vae made mention of is a whole 'nother balllgame. The skin of our bodies (and the nerve endings just beneath the surface) serve as a defense system to certain stimuli. When we feel an itch on our shoulder, once we get the signal to scratch it...that's when we do...the act of scratching an itch is our defense (in a matter of speaking). When a fly lands on us....sometimes we feel it immediately....other times, we're not quite aware of it until it begins to walk around a little and finds a more sensitive area (albeit inadvertently). Once the sense of it walking on us reaches our brains...we react. Same with tickling...and we all know this.....it's part and parcel of why the feet of someone we're tickling can react the way they do....wrinkling up...flexing...wagging about. I think the 'fear' of tickling develops when one who may have been tickled too excessively at stages in their life when they were unable to 'defend' themselves from the tickling and couldn't control just how MUCH tickling they were receiving. It can be pretty unnerving (especially to young children) when you feel you have NO control over the sensations that are bombarding your body in one sitting....and with tickling....a child could indeed reach a panicky state when the tickling makes them laugh so hard and so heavy that they lose the ability to catch their breath. Children that endure tickle sessions like these entirely too often can indeed develop a dislike/fear of being tickled that can certainly last well into their adulthood.
