Among single females who were actually masturbating, the average individual was reaching orgasm about once in every two and a half to three weeks. ... We shall find that this is more or less true of the frequencies of several other types of female activity and of the total sexual outlet of single females. This is one of the most remarkable aspects of female sexuality, and one which most sharply distinguishes it from the sexuality of the male. Hormonal factors may be involved. (Alfred Kinsey)