This is, no doubt, the most common opinion, and it is frequently repeated in text-books. It is scarcely necessary, however, to point out that only the opinions of those who have given special attention to the matter can carry any weight. R. W. Shufeldt ("On a Case of Female Impotency," pp. 5-7) quotes the opinions of various cautious observers as to the difficulty of detecting masturbation in women.
This latter opinion is confirmed by Näcke so far as the insane are concerned. In a careful study of sexual perversity in a large asylum, Näcke found that, while moderate masturbation could be more easily traced among men than among women, excessive masturbation was more common among women. And, while among the men masturbation was most frequent in the lowest grades of mental development (idiocy and imbecility), and least frequent in the highest grades (general paralysis), in the women it was the reverse. (P. Näcke, "Die Sexuellen Perversitäten in der Irrenanstalt," Psychiatrische en Neurologische Bladen, No. 2, 1899.)
Mammary masturbation sometimes occurs; see, e.g., Rohleder, Die Masturbation (pp. 32-33); it is, however, rare.