Copulatory behavior in men and women

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Throughout history there have been many descriptions of human copulatory behavior. Most of them belong to the fine arts, some to pornography and a few to the medical sciences. Descriptions satisfying some minimum requirements of objectivity and accuracy are more rare. Without discrediting the early pioneers in sex research, I will start my analysis of copulatory behavior with the extraordinary data collected by Alfred Kinsey and colleagues and published in two landmark books (Kinsey et al., 1948, 1953). Those interested in research on human sexuality prior to the Kinsey studies can find an excellent summary in a charming book (Bullough, 1994). However, there is one extremely interesting text preceding the Kinsey era that I cannot avoid mentioning. In 1926, the Dutch gynecologist, Theodoor Henrik van de Velde, published an account of human sexual behavior (van de Velde, 1965; I have had access only to a reprint of the 1926 edition. All following citations refer to the 1965 reprint), including plenty of advice as to how to increase the pleasure derived from sexual activities. Human sexual interactions were presented as a sequence of stages, beginning with prelude, continuing with love-play, followed by coitus and finally the epilogue or postlude. The progression between stages and the associated level of sexual excitation in a few cases of sexual intercourse is illustrated in Figure 2.2.

Black Coital SexStages Coital

FIGURE 2.2 van de Velde's illustration of human coital interactions. (a) The changes in sexual excitation during an ideal copulatory encounter. With ideal is understood that the man and the woman reach orgasm at the same time. Excitation is defined as the summation of sexual desire and pleasure, bodily and psychic. (b) Similar to panel (a), but here the sexual interaction occurs with an inexperienced woman without adequate coital stimulation. (c) Similar to panel (a), but now the woman is sexually experienced. The prelude was omitted, but the woman's low initial excitation was compensated for by her experience. (From van de Velde, 1965. Copyright 1926. Reprinted with permission.)

FIGURE 2.2 van de Velde's illustration of human coital interactions. (a) The changes in sexual excitation during an ideal copulatory encounter. With ideal is understood that the man and the woman reach orgasm at the same time. Excitation is defined as the summation of sexual desire and pleasure, bodily and psychic. (b) Similar to panel (a), but here the sexual interaction occurs with an inexperienced woman without adequate coital stimulation. (c) Similar to panel (a), but now the woman is sexually experienced. The prelude was omitted, but the woman's low initial excitation was compensated for by her experience. (From van de Velde, 1965. Copyright 1926. Reprinted with permission.)

The prelude is described in this way: 'As soon as the first stirrings of the impulse of approach are perceptible, the prelude to sexual union begins' (van de Velde, 1965, p. 102). Further: 'In the prelude the impulse of approach works through the three senses of sight, hearing and smell. Taste and touch come into play at closer range, and it may be said that touch, once in action, soon becomes paramount' (van de Velde, 1965, p. 104). Although this was written long before incentive motivational theory existed, van de Velde's description of the initial stage of sexual interaction is stated in exactly the same way as incentive motivation theory would have it, that is the senses register certain incentive stimuli which activate sexual approach behaviors.

The love play arises from the impulse for sexual gratification and it almost always begins, in Western cultures, with the erotic kiss, according to van de Velde. It proceeds with tactile stimulation of various parts of the body, eventually concentrating on the genitals. During the love play, sexual arousal (erection in the man and vaginal lubrication in the woman) needs to be produced. If the woman shows signs of insufficient lubrication, the man is recommended to ameliorate the situation through repeated mechanical stimulation of the vulva. 'And this may best, most appropriately, and most expeditiously be done without the intermediary offices of the fingers, but through what I prefer to term the kiss of genital stimulation or genital kiss: by gentle and soothing caresses with lips and tongue' (van de Velde, 1965, p. 118). Likewise, if the man's erection is insufficient, the woman may provide oral stimulation to the penis. These suggestions must have been far ahead of their time. In 1926, the practice of cunnilingus or fellatio, even as preparations for penile-vaginal intercourse, was not socially acceptable and even constituted a felony in many places.

When erection and vaginal lubrication have been obtained, the third stage of sexual interaction, coitus, may begin. This stage starts with the insertion of the penis into the vagina, reaches culmination in the ejaculation of semen and in the orgasm in both partners, and finally ends with penile withdrawal. The 'sexual tension' increases stepwise from the introduction of the penis until orgasm in both the man and the woman. van de Velde illustrates the degree of 'sexual tension' in a series of figures, which 40 years later were employed by Masters and Johnson (1966) in their important book on human sexuality.

The postlude or epilogue is not a stage that attracts much of van de Velde's attention. He briefly mentions that it may be entirely absent in couples who do not feel love for each other. In the ideal case, 'it should echo, and vibrate, and reawaken in the preliminaries of a new act of sexual communion' (van de Velde, 1965, p. 102).

An interesting aspect of van de Velde's book is that he does not mention the factual basis for his analysis of human sexual behavior. I assume that his clinical experience forms the basis for all his assertions, but the more detailed nature of this experience is never mentioned. This is a serious limitation, but despite this limitation we need to accept that his discussion of human sexual behavior has had considerable impact. His book is said to have sold more than 800 000 copies and, as we will see in a few pages, the notion of human sexual behavior as characterized by stages still survives. What also survives is the insistence on penile-vaginal intercourse. No other form of sexual behavior is considered by van de Velde. He makes it explicitly clear that he refers exclusively to normal intercourse between opposite sexes and this has continued to be the case in almost all subsequent studies of human sexual behavior. One noteworthy exception is found in the work of Masters and Johnson (1966), which also includes data obtained during solitary masturbation.

van de Velde described and gave advice concerning the perfect penile-vaginal intercourse. Although extremely interesting and influential, his book did not contain any data as to what humans actually do when they have sex, with whom they have sex, at what age they start and stop to have sex, and so on. All these and many other questions remained unanswered until the Kinsey group published their reports on human sexual behavior. It must immediately be mentioned, though, that they did not report data on human copulatory behavior based on direct observation. Almost all their data stem from interview responses. In these interviews, people were asked to recall and tell their sexual history. Questions like the age at the first masturbatory experience, at the first intercourse, kinds of copulatory behavior displayed and with whom, number of partners, homosexual activities and so on were asked. The veracity of the answers was checked in several ways and data from interviewees showing signs of giving false answers were discarded. However, it is well known that the tendency to provide false information is largest when sensitive questions are asked (Coxon, 1999; Jagannathan, 2001). Most people seem to consider that questions about their copulatory behavior are sensitive and the likelihood of obtaining answers based on imagined social desirability rather than on facts is not negligible. Most unfortunately, available data do not allow for any direct evaluation of the coincidence between self-reported sexual behaviors, be it in the form of written questionnaire answers, face-to-face or telephone interviews, or computer assisted procedures, and the actual behaviors displayed. In fact, some authors suggest that self-report data are essentially useless (Brody, 1995; Lewontin, 1995). I tend to agree with this proposal, but at the same time I have to recognize that extensive reviews of the vast literature suggest that some useful information might be obtained through self-reports (Catania et al., 1995; Weinhardt et al., 1998). Nonetheless, Kinsey intuitively figured out that people are prone to lie about their sexual activities when answering a questionnaire. Lying to an interviewer staring into your eyes is much more difficult, thought Kinsey, and abandoned written questionnaires very early in his research. Although interviews performed by extraordinary interviewers like Kinsey and associates may be superior to other forms of data collection, they are not perfect. The fact that people have to rely on their memory when answering most, if not all, of the questions inevitably leads to some uncertainty as to the correspondence between what actually occurred and what the person remembers occurred. Nevertheless, despite all their defects, the Kinsey data are probably the most reliable we have concerning human copulatory behavior. Later studies, based on written questionnaires or telephone interviews, provide data that probably are far less reliable than the Kinsey data. I will, in fact, avoid such data whenever possible. They are, anyway, well known from front-page headlines in newspapers and long articles in men's magazines.

The main findings reported in both Kinsey books were that human copulatory behavior is more variable than expected and less dependent on social rules than hoped. Many behavior patterns, like cunnilingus (stimulation of the vulva with the mouth and tongue), fellatio (stimulating the glans penis or the penile body with the mouth and tongue), anal intercourse or the display of copulatory behavior with individuals of one's own sex, were all far more common than ever imagined. These facts were still more remarkable considering that these behaviors were illegal in most states of the USA and severely punished if prosecuted. In the late 1940s, the penal codes of many states established maximal prison terms of around 20 years for sodomy or fellatio, for example. Some other American and many European countries had similar laws. The discovery that many otherwise exemplary citizens engaged in sexual activities 'contrary to nature' as they were regarded at the time, produced a profound social shock and a violent outcry from the guardians of morals, like Anglican bishops, cardinals of the Holy Catholic and Apostolic Church of Rome, members of the United States Senate's Government Operations Committee, Lutheran pastors and many others. They tried to combine their moral complaints with scientific criticisms, with the obvious aim of reducing the credibility of the Kinsey data. The rather unorthodox sampling procedures employed were one of the main targets of attack. It was argued that the estimates of the frequency of some copulatory behaviors were much exaggerated because of biased samples and that, in reality, the copulatory behavior of Americans was much closer to the social norms than the Kinsey data suggested. This point may have been reassuring for many conservatives, but the data showing that copulatory behavior was far more diverse than previously believed do not appear to be solely a consequence of biased sampling. In fact, a group of respected statisticians, including John Tukey and William Cochran, were asked to examine the procedures behind the data presented in the book on male copulatory behavior and they concluded that the sampling procedures employed by Kinsey probably were the best available. The statisticians found some minor errors in the data presentation, but basically the results were accepted as scientifically sound (Cochran et al., 1954).

An important innovation by the Kinsey group was to quantify the intensity of sexual activity by estimating the number of orgasms reached per unit time. This was called the total sexual outlet and usually expressed as the number of orgasms achieved per week. Earlier research had preferred to express the intensity of sexual activity as the number of coitions per unit time. The underlying supposition was that the only sexual activity in which people engage and which, therefore, is the only activity worth studying, is penile-vaginal intercourse. The Kinsey data show that this is not the case. Penile-vaginal intercourse is only one of many ways in which women and men may obtain orgasm, and this important fact was first acknowledged by Kinsey. Among the many consequences of this notion is that sexual activities must be regarded as a means to obtain orgasm and the specific sexual activity that leads to that end is uninteresting. Such a conception of sexuality is totally different from the point of view of Saint Augustine and of most other Christians, and it is also contrary to the opinion of biologists. To these, the end point of sexual activity is procreation as mentioned several times already. Here we have another source of criticism of the Kinsey approach to sexuality: its lack of moral considerations and the absence of condemnations of non-procreative sex. Those reading Swedish would likely enjoy a funny little book illustrating the fury provoked by the scientific rather than moral analysis of human sexuality characterizing Kinsey and his collaborators (Sundstrom, 1949). Similar books were certainly published in many other languages.

The Kinsey studies of copulatory behavior in the USA had clearly shown that human sexual behavior is most variable within a country. Others had taken an anthropological approach and studied copulatory behavior in different cultures

(Ford and Beach, 1951). Again, diversity seems to be the best descriptor. However, anthropological data may not always be entirely reliable, since most descriptions of sexual acts are based on hearsay and very few on actual observations. Nevertheless, it does not appear to be too risky to suggest that human copulatory behavior is far more variable than in any other primate. I even dare to suggest that no other mammal shows the amount of intraspecies variation as the human.

The Kinsey studies as well as Ford and Beach's (1951) account are basically descriptions of general motor patterns of copulatory behavior. The information contained in terms like intercourse with rear entry, anal intercourse etc. is not sufficient if we are looking for quantitative information on aspects of copulatory behavior. Questions like the latency from penile insertion to ejaculation, or from the initiation of masturbation until orgasm, or the mean duration of cunnilingus or fellatio are not easy to answer from the kind of descriptive data offered. Furthermore, on the few occasions when quantitative data on some of these aspects are presented, they are based on verbal reports of memories of copulatory activities. The degree of coincidence of these memories with reality is difficult to evaluate. Laboratory observations of these and many other copulatory behaviors could provide the required information, but such studies were unconceivable for a long time. It appears that Kinsey had started to make direct observations of copulatory behavior (Jones, 1997), but at his death in 1956 nothing had been published.

It was not until the early 1950s that scientists began to observe human copula-tory behavior in a laboratory setting. The groundbreaking work performed at Washington University by William Masters and Virginia Johnson from 1954 until the early 1960s is still unsurpassed in quantity and probably also in quality. In their classical description of human copulatory behavior, Masters and Johnson (1966) distinguished four arbitrary stages: arousal, plateau, orgasm and resolution. The notion of stages or phases was explicitly taken from van de Velde, although the phases were slightly modified and renamed. The world famous figures used by Masters and Johnson (1966) to illustrate 'the sexual response cycle' were also adjustments of van de Velde's (1965) figures. Figure 2.3 illustrates the Masters and Johnson (1966) curve. Its similarity to the curves shown in Figure 2.2 is probably not a pure consequence of coincidence. The most noteworthy modifications were that van de Velde's first phase, the prelude, was omitted and that orgasm was added as a phase of its own. van de Velde regarded orgasm as the end of the coition phase. It is of some importance to keep in mind that the four stages, perhaps with the exception of orgasm, do not correspond to qualitatively different events occurring during human sexual intercourse. Rather they reflect the chronological progress of human sexual activity, from erection and vaginal lubrication (arousal) through penile insertion and intravaginal thrusting in men and the consequent mechanical stimulation of the vulva, clitoris and vagina in women (plateau) to ejaculation and the experience of orgasm in men and rhythmic contractions of the perineal musculature and the experience of orgasm in women. The resolution phase corresponds to detumes-cence in men and to the cessation of vaginal exudation in women. While ejaculation (orgasm) is almost always followed by the resolution phase and frequently the end

Stages Coital
FIGURE 2.3 The Masters and Johnson curve illustrating the phases of human coital interaction. (a) The male sexual response cycle; (b) the female sexual response cycle. (From Masters and Johnson, 1966. Reprinted with permission from Lippincott Williams & Wilkins.)

of copulatory activity in men, women may experience two or more orgasms in a row before entering the resolution phase. It is possible to interrupt this latter phase and initiate a new response cycle with arousal, plateau and so on, but this is rather uncommon. Most humans, at least in Western cultures, do not reinitiate copulatory activity until after a rather long and very variable interval, ranging from hours to weeks.

Logically, we begin our passage through the phases of copulatory activity with the arousal stage. According to the Master and Johnson model, the arousal or excitement phase is characterized by erection in men and vaginal lubrication in women. Clitoral tumescence is also common. All these responses are consequences of enhanced genital blood flow. There is no mention whatsoever of any motivational causes or consequences of arousal in the Masters and Johnson description. This is obviously a serious limitation. The intensity of the visceral response to an incentive stimulus must be determined by the intensity of motivation in exactly the same way as intensity of responses in skeletal muscles are determined by motivational factors. This fact was neglected by Masters and Johnson. The likely reason is that they were not particularly interested in the psychological processes behind the physiological responses that were their object of study. This is by no means a criticism of the very important contribution they made. It is not possible to cover all aspects of such a complex phenomenon as human copulatory behavior at the same time. Nevertheless, it was soon recognized that the arousal phase ignored a critically important portion of sexual interaction. Enhanced genital blood flow does only occur if the individual feels a desire to experience sexual stimulation, provided either by another individual or by himself or herself. In other words, the arousal phase follows an initial phase of sexual desire (Kaplan, 1979). Interestingly enough, the desire phase had been presented in van de Velde's (1965) model of human sexual behavior under the name of prelude, but it was suppressed by Masters and Johnson (1966). In another chapter (Chapter 4) we will analyze the sexual incentives that may give rise to this desire or initiate the prelude. The addition of a desire phase to the Masters and Johnson model has been almost universally accepted. Furthermore, the Masters and Johnson plateau phase has been collapsed with their arousal phase into something now known as the excitation phase (Robinson, 1976). Some scientists have further sophisticated the analysis of the sexual response cycle and divided the desire phase in two components, one endogenous, and one activated by sexual stimulation (Levin, 2000). The endogenous phase is supposed to be created spontaneously, meaning that there is no physical stimulation of the genitalia or other evident physical events that can explain the activation of desire. The second phase is associated with the presence of exogenous sexual stimuli such as mechanical stimulation of the genitals or other parts of the body. This second phase of desire is simultaneous with the excitation phase.

An interested reader may find it rewarding to compare the notion of endogenous desire phase with Freud's Ichlibido, and the second, exogenous desire phase, with Freud's Objektlibido (Freud, 1905). Some striking similarities are evident. It appears, then, that Helen Kaplan's (1979) addition of a desire phase to the Masters and Johnson (1966) model brought us some 40 years back to the van de Velde model, originally published in 1926. Levin, who published his division of the desire phase into endogenous and exogenous in 2000, brought us back to a conceptual framework from 1905. This nicely illustrates that scientific progress sometimes consists of the fruitful union of almost forgotten concepts with newly acquired data.

If the copulatory behavior consists of penile-vaginal intercourse, then the excitation phase sooner or later will be manifest as vaginal penetration and thrusting. The moment of the excitation phase at which penetration occurs is highly variable and probably determined by own experience or social learning or a combination of both. Manuals of copulatory behavior, in the Western hemisphere, hypocritically called marriage manuals, give usually extensive advice as to when penetration is convenient, desirable or permissible. Common sense suggests that penetration is problematic before the penis has attained an adequate erection and before vaginal lubrication is sufficient to make penetration possible. It might even be regarded as desirable to assure that vaginal lubrication has reached a level where penetration can be made without causing pain to the woman. Most other suggestions as to the ideal instant for penetration are most likely based on uninformed speculation. The moment of penetration does not seem to be of any particular interest and I will refrain from any further discussion of it and, above all, I will give no advice. Moreover, I will not discuss the many bodily orientations that may allow for vaginal penetration. Only the creativity of the human mind and the acrobatic capacities of the partners pose limits to these orientations. Again, manuals of copulatory behavior as well as women's and men's magazines offer abundant advice. Rather than dwelling on this vulgar subject I want to insist on the fact that penile-vaginal intercourse is only one of many possible copulatory behaviors in the human. Other bodily openings of sufficient size can be employed for penetrative intercourse, and these other openings have the advantage of being available in both sexes. Thus, men fond of penetrative copulatory behavior can have the choice between heterosexual and homosexual copulatory behavior as soon as they free themselves from the inevitable limitations inherent in the insistence on penile-vaginal intercourse. Behaviors executed during the excitation phase may also be non-penetrative. Mutual masturbation is one example, cunnilingus another. Just as is the case with non-vaginal penetration, non-penetrative copulatory behaviors are equally possible as homosexual or heterosexual activities. All copulatory behaviors other than vaginal intercourse and masturbation were ignored by Masters and Johnson (1966) as well as by most others who recently have studied human copulatory behavior. Consequently, we do not know much about the physiological reactions occurring during these alternative behaviors. It may be reasonable to assume that they are not fundamentally different from those occurring during penile-vaginal intercourse.

The excitation phase may or may not lead to the point of orgasm. In men, the sensation of orgasm is normally associated with seminal emission and ejaculation. The stimulus triggering ejaculation is normally mechanical stimulation of the glans penis. Sensory receptors in the glans penis were described in the mid-1800s and they have been termed genital end bulbs. Unlike glabrous skin, such as the finger tips, or the lips and palate, there are no Merkel endings in the glans and few Pacinian and Ruffino corpuscles. On the contrary, there is an abundance of free nerve endings. A peculiar type of corpuscular receptor has been described (Halata and Munger, 1986). It seems to consist of axon terminals resembling a tangled skein of free nerve endings. Somatosensory information is relayed from the glans penis to the spinal cord by the dorsal penile nerve (Yang and Bradley, 1999). Blockade of nerve impulse conduction in the dorsal penile nerve eliminates ejaculation (Wieder et al., 2000), making evident the crucial importance of sensory stimulation for ejaculation associated with copulatory behaviors. Parenthetically, it must be mentioned that ejaculation can occur in the absence of or with minimal sensory stimulation, as happens in 'wet dreams' in young men and at 'spontaneous ejaculation' in rats. Nevertheless, ejaculation in conjunction with copulatory behavior is always a consequence of mechanical stimulation. Returning to the dorsal penile nerve, it must be mentioned that this nerve innervates the penile shaft in addition to the glans (Yang and Bradley, 1998). This opens up the interesting possibility that mechanical stimulation of the shaft may trigger ejaculation. I know of no experimental study evaluating the possibility of triggering ejaculation through stimulation of the penile shaft without concomitant stimulation of the glans penis, but this possibility cannot be excluded. Anyway, this question can have no practical meaning since it is difficult, if not impossible, to imagine any copulatory behavior involving the penile body while leaving the glans penis completely without stimulation.

The male orgasm/ejaculation is associated with bodily reactions and psychic experiences. The bodily reactions were described by Masters and Johnson (1966) and I see no reason to repeat their description here. The psychic activities, frequently described in terms of emotion, have been the subject of myriads of comments from all kinds of experts, true and false, including cardinals, sexologists, psychoanalysts, charlatans of all sorts and innumerable scientists. There is no need to comment further on this issue. I refer the curious reader to the safe hands of any newsstand attendant.

Orgasm in women is sometimes considered to be less known than ejaculation/ orgasm in men. There may be many obscure reasons for this and at least one reason of some conviction: female orgasm is not as apparent as the male orgasm. This, together with the fact that female orgasm is not a requisite for fertilization, made it a rather uninteresting topic for centuries. We shall not forget, though, that in the times of Saint Augustine it was believed that the experience of orgasm was necessary for both the copulating man and woman for the act to be fertile. Since non-fertile copulation was a sin, we may conclude that sex without orgasm was sinful. This notion of Saint Augustine was lost sometime during the Dark Ages and for centuries European men did not pay much attention to orgasm in women. Whether women did or did not was of little or no concern. Nevertheless, female orgasm has been the subject of a respectable amount of research during most of the 20th century. In addition, and as is the case with orgasm in men, there is an enormous amount of popular descriptions of the orgasmic experience. I should not, in principle, waste words on commenting on this subject, but there are so many misconceptions about the female orgasm circulating in popular and some not so popular writings that I cannot resist the temptation to present my own parsimonious view as a counterpoise. At difference to most of the popular opinions, I try to base my point of view on empirical data. As with so many other aspects of human copulatory behavior, Masters and Johnson (1966) were the first to provide a rather detailed description of the physiological responses accompanying orgasm in women. These responses were quite similar to those found in men, i.e. a series of rhythmic genitopelvic and anal contractions. Other responses typically seen at orgasm in both men and women are body rigidity, muscle spasms, hyperventilation, sweating and vocalizations. Master and Johnson (1966) considered the orgasm experience to be more variable in women than in men, although its basic structure was the same in both sexes.

The similarity of the experience of orgasm in men and women was elegantly shown in a study where medical students, obstetricians-gynecologists and psychologists were asked to determine the sex of the author of 48 anonymous, written descriptions of orgasm. Both groups of professionals as well as the medical students failed on the task (Vance and Wagner, 1976). It appears, then, that women and men produce indistinguishable accounts of their experience of orgasm. A more complex study asked almost a thousand students to portray their orgasm experience by rating how well 60 adjectives described it. A Likert scale ranging from 'does not describe it at all' to 'describes it perfectly' was used for the rating of adjectives like incredible, euphoric, shuddering, relaxing, spurting or throbbing, just to give a few examples. Two orgasmic contexts were rated: solitary masturbation and sex with a partner. The partner could be of the same or of the opposite sex and orgasm could be obtained through coital (supposedly penetrative) or non-coital activities. Results were evaluated in the complicated ways that psychologists like, employing procedures such as principal component analysis, multivariate analysis of covariance, confirmatory factor analysis, and so on. The conclusion was, nevertheless, that there was no qualitative difference between sexes with regard to the description of orgasm while there were some marginal differences in intensity ratings of the adjectives. For example, men considered 'shooting sensations' more intense than women did (Mah and Binik, 2002). The authors point out that the minor sex differences perhaps can be explained by sex stereotypes of emotional responsiveness or simply by differential social desirability responding. Women have a tendency to report more intense emotions than men and they also show more intense physiological reactions to emotional stimuli than men (Grossman and Wood, 1993). This does not reflect any inherent sex difference. The larger emotional responses displayed by women are usually interpreted as socially constructed rather than biologically determined. Likewise, women have higher scores on scales evaluating social desirability and they are less comfortable than men answering questions about their sexuality (Plaud et al., 1998). These socially determined sex differences can and probably do account for the small discrepancies observed between women's and men's descriptions of the orgasm experience. It seems rather safe to posit that the experience of orgasm is similar, if not outright identical, in both sexes. The importance of this assertion will become evident in future chapters.

We saw a couple of paragraphs ago that ejaculation/orgasm in men is triggered by tactile stimulation of the penis. The stimulus triggering orgasm in women seems also to be tactile stimulation. The question of some importance here is tactile stimulation of what. Everyone familiar with classical psychoanalysis knows that Freud considered clitoral stimulation to be of importance for orgasm but, albeit important, clitoral orgasm represents a kind of primitive or immature sexual experience when compared to vaginal orgasm. This latter was supposed to be triggered by tactile stimulation of the vagina. Empirical studies, again performed by Masters and Johnson (1966), have failed to detect any difference between orgasm induced by the exclusive stimulation of the clitoris and orgasm induced by stimulation of the vagina. However, it is most likely that vaginal stimulation is associated with some degree of clitoral stimulation because of the morphology of the female genitalia. Vaginal stimulation caused by the thrusting penis during penetrative intercourse is always associated with indirect stimulation of the clitoris because of the back and forth movement of the labia minora. As expressed by Masters and Johnson (1966, p. 59):

A mechanical traction develops on both sides of the clitoral hood of the minor labia subsequent to penile distension of the vaginal outlet. With active penile thrusting, the clitoral body is pulled downward the pudendum by traction exerted on the wings of the clitoral hood. . . . The rhythmic movement of the clitoral body in conjunction with active penile stroking produces significant indirect or secondary clitoral stimulation.

The same series of events will obviously also occur if the penis is replaced by another thrusting object. An inevitable consequence of this is that it is technically difficult to determine the role of exclusive vaginal stimulation. I know of no study of 'vaginal orgasm' where clitoral stimulation has been convincingly excluded. Perhaps a purely vaginal orgasm does not exist. In any case, Masters and Johnson (1966) concluded that there is no difference between clitoral and vaginal orgasm. Moreover, the bulk of available data suggest that clitoral stimulation is crucial for orgasm in most women (see, for example Mah and Binik, 2001 for a discussion). This assertion does not exclude other means for women to obtain orgasm. Masters and Johnson (1966) mention examples of women who can obtain orgasm through the exclusive stimulation of the nipples, and we have already learned that some women may reach orgasm through mental representations alone, in complete absence of tactile stimulation of any kind. Despite these facts, it is no exaggeration to state that orgasm induced by copulatory behaviors normally requires tactile stimulation of the clitoris as a triggering stimulus.

Anatomical studies of the human clitoris have revealed that its general structure is rather similar to that of the penis. It consists of erectile tissue, corpora cavernosa and bulbs, as well as of a non-erectile glans, similar to the glans penis. The bulbs extend laterally around the urethral and vaginal openings. It has for a long time been discussed whether the bulbs are part of the clitoris or not, but recent anatomical studies seem to have settled the issue with the unequivocal conclusion that they are (O'Connell et al., 2005). A very important consequence of this is that the ventral side of the vaginal opening is bordered by clitoral tissue. In fact, there is such an intimate relationship between the ventral vaginal opening, the urethral opening and the clitoris that these three structures can be considered a functional unit. Indeed, branches of the cavernous nerve originate in the ventrolateral wall of the vaginal opening and join the dorsal nerve of the clitoris (Yucel et al., 2004). This nerve is equivalent to the main sensory nerve of the penis, the dorsal nerve. In addition to containing fibers from the vaginal opening, the dorsal nerve of the clitoris contains fibers from the urethral sphincter complex and an abundant supply from the glans clitoris. The glans contains a large number of sensory receptors, just as the glans penis does. The nature of these sensory receptors is not entirely clear. Pacini's corpuscles have been described, but the abundant free nerve endings and the particular receptor-like structure formed by them that has been observed in the glans penis have not been found in the glans clitoris. One reason is that the only detailed study of clitoral sensory receptors was performed more than 50 years ago (Yamada, 1951), with rather primitive histological techniques. In the absence of more data, I simply suggest that the sensory characteristics of the glans clitoris are not much different from those of the glans penis. In the same way as there is a structural and functional basis for the idea that tactile stimulation of the penis is crucial for ejaculation/orgasm in male copulatory behaviors, we have now shown that a similar structural and functional basis exists for the notion that tactile stimulation of the clitoris is essential for female orgasm produced by copulatory behaviors.

Before ending the discussion of female orgasm, I consider it useful to mention two very neat studies giving support to the conclusion presented in the preceding paragraph. In one, the vaginal wall was stimulated by a device delivering electric currents of variable intensity. The device was rather small and it was kept in a fixed position during the periods of stimulation, thereby avoiding most mechanical stimulation of the vaginal opening. Sensitivity to the electrical stimuli was generally low. In fact, it was lower than that of the back of the hand (Weijmar-Schultz et al., 1989). There was only one stimulus location that revealed high sensitivity to stimulation. Curiously enough, this was the ventral vaginal wall close to the introitus. In the other study, women were asked to stimulate the vaginal walls with their fingers and rate the 'erotic sensitivity' (Alzate and LondoƱo, 1984). Results showed that stimulation of a site described as 'the upper anterior wall' was the most efficient for attaining orgasm. If we assume that the upper anterior wall is equivalent to the ventral wall close to introitus, then we must conclude that both studies showed that the most sensitive part of the vagina is exactly where the clitoral bulbs are found, which is on the ventral part of the vaginal opening. Thus, without knowing it, these studies support the notion that clitoral stimulation is crucial for female orgasm. Just for the fun of it, I might add that none of the scientifically sound studies mentioned here (as well as many others that have not been mentioned) has provided any support for the existence of a site known as the G spot, or the Grafenberg spot, famous from popular magazines and some bestselling books.

After orgasm there are two possible ensuing events: continuation of copulatory activity, or the end of it. As already mentioned, the latter alternative is the most common in men, while many women may experience repeated orgasms before ending copulation. Regardless of the number of orgasms, the cessation of copula-tory activity leads to the resolution phase. During that phase, physiological parameters return to baseline. In later chapters, we will analyze some of the behavioral consequences of sexual stimulation, particularly of orgasm, but the resolution phase, as described by Masters and Johnson (1966), will not attract our attention.

This short description of copulatory behavior in men and women should satisfy the minimum requirements for an adequate appreciation of the following chapters. Before closing this section, though, we might venture into a short comparison of copulatory behavior displayed by non-human animals and those displayed by women and men. A salient difference is that copulatory behavior in non-humans is essentially limited to penile-vaginal intercourse, and occasionally masturbation, while it may be extremely variable in the human. The reasons for this dramatic difference are not completely clear. It is easy to suggest that non-human animals are not creative enough, or do not have the motor capacities needed for the enormous variation typical of the human. For example, it may be motorically impossible for a male rat to achieve anal or oral penetration of a female. Likewise, the rats' creativity may not be sufficient for getting the idea that prolonged licking of the genitals may be enough for triggering ejaculation and orgasm. However, the rat genital organs have structural and functional bases for activating orgasm rather similar to what I have described for the human (see Marson, 1995 for an example and some references). Considering that, it might be a mystery that rats do not masturbate. Likewise, a male chimpanzee would certainly be motorically able to stimulate a female's clitoris until orgasm, but for some reason he has never chosen to do so. Perhaps he does not find any pleasure in it and then he would have no reason to do it. More surprisingly is that the female chimp does not stimulate her own clitoris to orgasm. That would probably cause her pleasure, so it is difficult to see why she does not. Of course, one explanation can be that she has never discovered how pleasurable it is, perhaps because female chimps have an anatomy such that the clitoris is not readily stimulated except by a male's vaginal penetration. This hypothesis may be reinforced by the fact that male chimps do masturbate, and it is obvious even for a human observer that the chimp's penis is easily accessible for manual exercises. I will not penetrate the problem of stereotyped versus highly variable copulatory behaviors here, but it is important to keep this difference between men and mice in mind.

At the same time as there are fundamental differences between human and nonhuman animals with regard to copulatory behaviors, there are also basic similarities. The basic model of copulatory behavior posing the existence of an ordered sequence of desire - excitation - ejaculation/orgasm - return to baseline is applicable to males and females of any mammalian species. The basic behavioral and neural processes involved are probably also quite similar across species. Likewise, the fact that the model applies both to females and males is probably also true in all mammalian species. However, when we turn to a more detailed examination of each of the phases, we will find some differences in addition to similarities. Salient among the similarities is the fact that male ejaculation/orgasm in association with copulation is always triggered by mechanical stimulation of the penis. As far as we know, this is the case for all mammals and probably also for other classes of vertebrates. Most unfortunately, this clear-cut statement cannot be immediately applied to women. The origin of orgasm, and even the existence of it, in non-human females is unclear. We know that copulatory behaviors are associated with positive affect in females of all species where this question has been studied, but we do not know if this positive affect is associated with the physiological reactions typical of orgasm in women, notably the rhythmic contractions of pelvic muscles. Thus, while the mechanisms of ejaculation/orgasm in males seem to have been conserved during evolution, the mechanisms of orgasm in females may have been modified. The modifications may be limited to the physiological manifestations of orgasm but they may also involve the stimuli triggering it. It is only to be hoped that some day we will have enough data to answer this question. Nevertheless, the universal utility of the sequence desire - excitation - ejaculation/orgasm - return to baseline seems to be beyond doubt.

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