In 1966, William Masters and Virginia Johnson published a book detailing the results of their observations of nearly 700 people who agreed to participate in their study of physiological responses during sexual behavior. Unlike Kinsey, who used personal interviews and surveys to collect data, Masters and Johnson observed people having intercourse in a variety of positions, and they observed people masturbating, manually or with the aid of a device. While this was occurring, researchers recorded measurements of physiological variables, such as blood pressure and respiration rate, as well as measurements of sexual arousal, such as vaginal lubrication and penile tumescence (swelling associated with an erection). In total, Masters and Johnson observed nearly 10,000 sexual acts as a part of their research (Hock, 2008). Based on these observations, Masters and Johnson divided the sexual response cycle into four phases that are fairly similar for most people: excitement, plateau, orgasm, and resolution (Figure 10.17). The excitement phase is the arousal phase of the sexual response cycle, and it is marked by erection of the penis or clitoris and lubrication and expansion of the vaginal canal. During plateau, the vagina swells further and blood flow to the labia minora increases, and the penis becomes fully erect and may exhibit pre-ejaculatory fluid. People experience increases in muscle tone during this time. Orgasm is marked in females by rhythmic contractions of the pelvis and uterus along with increased muscle tension. In males, pelvic contractions are accompanied by a buildup of seminal fluid near the urethra that is ultimately forced out by contractions of genital muscles, (i.e., ejaculation). Resolution is the relatively rapid return to an unaroused state accompanied by a decrease in blood pressure and muscular relaxation. The refractory period is a period of time that follows an orgasm during which an individual is incapable of experiencing another orgasm. The duration of the refractory period can vary dramatically from individual to individual with some refractory periods as short as several minutes and others as long as a day. As people age, their refractory periods tend to span longer periods of time. Figure 10.17 This graph illustrates the different phases of the sexual response cycle as described by Masters and Johnson. In addition to the insights that their research provided with regards to the sexual response cycle and the multi-orgasmic potential of females, Masters and Johnson also collected important information about reproductive anatomy. Their research demonstrated the oft-cited statistic of the average size of a flaccid and an erect penis (3 and 6 inches, respectively) as well as dispelling long-held beliefs about relationships between the size of an erect penis and its ability to provide sexual pleasure to a partner. Furthermore, they determined that the vagina is a very elastic structure that can conform to penises of various sizes (Hock, 2008). In a similar manner to Kinsey’s work and other studies, Masters and Johnson’s research has been criticized, built on, and in some cases proven wrong. The research represents an important element of the history of sexual research, and is not viewed as entirely accurate today.