Sexual disorders are categorized into three general groups. The groups are sexual dysfunctions, sexual deviations, and homosexuality. These are not necessary abnormal, but what is considered out of the norm by either the mental health establishment, or society, or both. The first group, sexual dysfunctions, have increased significantly over the last decade. Which, in turn, has increased the progress in the study and treatment of sexual dysfunctions. One important aspect that came out of William masters and Virginia Johnson's research is classifying the various forms of dysfunctions. Closely related is Kaplan's system which groups sexual dysfunctions according to the phase in the sexual response cycle in which they occur. There are three phases in the sexual response cycle. The first one, called the desire phase, involves sexual fantasies and interest in sexual activity. Failure to feel this in either a man or woman is called hypoactive sexual desire. The second phase is called the excitement phase. In this phase, the sexual fantasy, or encounter is usually followed by physiological changes. The absence or weakness of these responses is called erectile dysfunction (or impotence) in men, and general sexual dysfunction in women. The third phase is called the orgasm phase. This is the phase of sexual response. If the man is unable to exert control, and ejaculates very quickly, this is called premature ejaculation. If, on the other hand, ejaculation is greatly delayed, or does not happen at all, this is called retarded ejaculation. A delay or absence of orgasm in women is called orgastic dysfunction. There are two more types of sexual dysfunction that do not fit completely with this typology. The first is a female disorder called vaginisimus. This occurs when, during coitus, the walls of the vagina contract involuntarily, making insertion of the penis difficult or impossible. The second disorder is dyspareunia, or pain during intercourse.