Judging from the comments made here I think it's a given that especially sensitive people might be able to make themselves laugh. However no-one can generate the same level of response that a seperate tickler would be able to. Schizophrenics however, have no such problem according to the scientific study I read about. Of course the accuracy of such a study is always open for individual people to question, so at the end of the day it's just another point of view.