Successful reproduction is the ultimate definition of evolutionary fitness. Because fertilization occurs within the reproductive tract of humans, intimate contact between male and female is necessary for spontaneous conception to occur. Therefore, from an evolutionary view, human sexual behavior should ultimately be directed toward the physiology of coitus, which results in the deposition of sperm within the female reproductive tract. Of course, this purely procreational approach to sexual behavior is too simplistic. Humans differ from most animals, whose mating is seasonal and determined by hormonal cycles, in being sexually receptive regardless of fertility potential. Human sexuality is defined not only by procreation, but also by recreation and pleasure. The nonreproductive aspect of human sexuality is quite plastic and subject to individual and cultural influences. What is pleasurable to one individual may not be so to another. Normative behavior in one culture may be unacceptable in another. What does seem to be common to all human sexual responses is that both physiologic and psychologic satisfaction are central and motivating.
Most sexual encounters pass through five stages. The first stage, sexual attraction or arousal, was not included in initial descriptions of the human sexual response cycle. The latter four stages were first defined by the pioneering work of Masters and Johnson. Using hundreds of observations made during heterosexual interactions and masturbation, they divided the human sexual response into excitement, plateau, orgasm and resolution phases (Fig. 15.1). Although the validity of some of the data gathered by Masters and Johnson has been subject to question, their model remains the single best description of the physiologic aspects of the human sexual response.
Phases of the sexual response
Sexual attraction or arousal is the most individualized stage of the human sexual response. In many respects, sexual attraction and arousal are closely tied to personality. They are also the most culturally determined. For example, incest taboos forbidding marriage and intercourse between closely related family members are almost universal among cultures over time. In contrast, attention to women’s breasts or weight in a sexual context varies tremendously among cultures. Interestingly, two variables of attractiveness do appear to be both universal and related to reproductive success: youth and health.
The nature of erotic stimuli can also be quite varied and include mental images, smells, sounds and physical events such as touching or stroking. If self-report and measurements of pelvic blood flow are used to indicate the level of arousal, men and women seem to be equally arousable. They differ dramatically, however, in the types of things that result in arousal. Novel or unpredictable situations and explicit visual stimuli, particularly body images, appeal to men more than women. Women generally prefer images with an emotional, romantic or familiar context. An individual’s physical health and mental state contribute greatly to the threshold at which they can be aroused by a given stimulus.
During the next phase of the sexual response cycle, the physiologic excitement phase, sexual interest is stimulated by these psychologic or physiologic stimuli. This aroused state intensifies during the plateau phase. If stimulation is sufficient, orgasm or climax occurs. Orgasm is typically experienced as an explosive and pleasant release of sexual tension. Finally, during the resolution phase, sexual arousal dissipates. The physiologic changes associated with arousal and orgasm return to baseline. Although both men and women progress through the same phases of sexual response, they may differ in length and intensity in any given sexual encounter. The most notable physiologic difference between males and females is the presence of a refractory period in men. This is a part of the resolution phase following orgasm. During this period of time, sexual arousal cannot be restored and orgasm cannot occur in men. In contrast, sufficient stimulation can induce orgasm in women at any point during the resolution phase.
The basic physiologic responses of the human body to sexual stimulation are twofold. The primary reaction is vascular congestion. The secondary response is generalized muscle tension or myotonia. Reflexes activated within the spinal cord are modulated by the higher central nervous system and control each response.
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