Why can't we tickle ourselves?

When we tickle ourselves something prevents the sensation being as strong as when someone else does it. It is suggested that the "something" may be: (i) the predictability of the stimulus; (ii) the presence of feedback from the movement of the arm doing the tickling; (iii) the presence of a corollary discharge from the voluntary movement of the tickling arm; (iv) the absence of a social/sexual context. To study these, Ss rated perceived "tickle-strength" in situations where they were tickled: (a) with their eyes closed; (b) with their eyes open; (c) with their own arm doing the tickling, but being moved by someone else; (d) by themselves. One group of Ss was divided into same sex and different sex subject-experimenter pairs. The results showed significant effects of predictability and sensorimotor feedback.