I somehow remember a thing about tickling being actually two mechanisms, biologically speaking. One is the light, very superficial tickling which only makes you smile, the other is the more intense kind which is able to induce full-on laughter. And they've both have very different evolutionnary hypothesis as to why they exist in modern humans.
AFAIK, the light kind can be self-inflicted. The other can't.
AFAIK, the light kind can be self-inflicted. The other can't.