By the time they reach adulthood, almost all persons are aware that their bodies go through a series of changes when they become sexually aroused and proceed to masturbation or sexual intercourse. The most obvious changes for the males are erections and ejaculation, and for females increased vaginal lubrication and enlargement of the clitoris and the external fold of the vagina. Helen Singer Kaplan modified the Masters and Johnson sexual response cycle based on her work treating sexual dysfunction problems in men and women. Dr. Kaplan added a prior stage of sexual desire, while eliminating the resolution phase that she believes to be an absence of sexual response rather than part of the cycle itself. In addition, Kaplan eliminated the plateau phase as defined by Masters and Johnson because she believed that it is essentially a continuation of the excitement phase and because it is of little value in sex therapy, due to the virtual impossibility of a patient distinguishing it from the excitement phase. Therefore, her model has only three phases: sexual desire, excitement, and orgasm

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