Why can t a Japanese girl say Stop it! to the molester who is touching her body? Previous studies indicate that linguistic sex differences reflect the societal hierarchy of the speakers status. Nonetheless, in Japanese language, women cannot use some phrases regardless of their social positions. Those forms are used exclusively by men. On the contrary, the examination of conversation data reveals that sex differences occur with striking infrequency. In order to elucidate this complication, I postulate two types of rules, prescriptive and proscriptive, that operate upon speakers to maintain mutual exclusiveness between two sexes. Proscriptive rules prohibit transgressions of the boundaries between the sexes. Since preferred images of a gender change over time, prescriptive rules are fluid. On the other hand, the violation of proscriptive rules brings serious penalty to speakers. This book also explores the historical process in which female students speech, which was accused as coarse speech at first, became the model speech for Japanese women.