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CHAPTER Is Female Orgasm a Covert Mate 1 1 Choice Mechanism?

David A. Puts and Khytam Dawood

Abstract Debate surrounds whether women’s orgasm is an adaptation or by-product of orgasm in men. We clarify what evidence would decide this debate and review evidence that female orgasm is an adaptation for promoting fertilization by men of high genetic quality. Female orgasm does not appear vestigial as by-products do. Rather, cross-species data suggest that female orgasm depends on the quality of a female’s mate and evolves where females copulate polyandrously. Sex differences in human orgasm frequency mirror sex differences in choosiness over mates, and orgasm frequency tracks conception risk across the cycle, another indicator of a sire choice function. Manipulations of orgasm-related hormones and brain regions in humans and nonhuman mammals indicate that female orgasm promotes conception. We review evidence of men’s concern over female orgasm in order to gauge paternity certainty, and evidence that women feign orgasm in order to maintain male investment. Key Words: adaptation, by-product, female orgasm, mate choice, good genes

Introduction In this chapter, we review evidence for these In the 1989 fi lm When Harry Met Sally , the hypotheses and fi nd that the mate choice hypoth- title characters are eating lunch in a deli when esis receives the most support. Specifi cally, female Sally loudly fakes an orgasm, prompting a nearby orgasm appears to have been shaped by selection to woman to order “what she’s having.” Orgasms function in increasing the probability of fertilization are important to women, yet they are frequently from males whose genes would contribute to the fi t- absent during sex. When women do not achieve ness of off spring. If correct, this hypothesis predicts orgasm during sex, they often mimic it. A sat- that female orgasm will be absent at times that are isfactory evolutionary understanding of female advantageous to the female but disadvantageous to orgasm must explain these facts. Th ere are cur- the male sex partner. We conclude by considering rently two major competing explanations for how female orgasm might stimulate and mediate orgasm in women: the mate choice hypothesis, sexual confl ict. which states that female orgasm has been shaped by natural selection to function in selecting mates, By-Products and Adaptations and the by-product hypothesis, which states that In order to evaluate whether the human female female orgasm has no evolutionary function and orgasm is an adaptation—and if so, for what—it is is present only because women share some of their necessary to distinguish adaptation from nonadap- development with men, in whom orgasm is an tation, and to clarify how adaptive hypotheses are adaptation. tested.

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Adaptations demonstrated by showing that variation in the trait’s how to spot an adaptation expression corresponds with variation in its useful- An adaptation is a morphological, physiologi- ness for the hypothesized function. Such variation can cal, psychological, or behavioral trait that has been either occur naturally or be induced experimentally. shaped by natural selection to perform some func- tion that contributed to inclusive fi tness in its ances- Naturally Occurring Variation tral bearers. Th e main evidence for an adaptation is Traits vary both between and within species, and a demonstration that the trait performs a function both sources of variation off er tests of functional that is likely to have increased inclusive fi tness in hypotheses. In cross-species comparison, the expres- the past. According to Williams (1966), the more sion of the trait across species is related to the pres- effi ciently and precisely a trait performs its putative ence of some problem for which the trait putatively function, the more probable that it is an adaptation provides a solution. If a functional hypothesis is for this function. If a trait appears modifi ed to per- correct, then species possessing similar traits should form a function with effi ciency, economy, and pre- have experienced similar problems over their evolu- cision, then it is likely to be an adaptation for that tion. For example, if horns in male beetles function function. Th is is often called the argument for spe- in contests over females, then horns should occur cial design (Buss, Haselton, Shackelford, Bleske, & most frequently in beetle species in which males Wakefi eld, 1998; Williams, 1966). Th e converse fi ght for mates and be rare in beetle species with- is not necessarily true—that is, it is not necessary out male contests. However, species share traits not for an adaptation to perform a function precisely only because of similar selection pressures, but also and effi ciently. After all, selection modifi es traits because they inherited traits from a common ances- that evolved for some other function (our external tor. Consequently, the presence of horns (or similar acoustic meati, “ear holes,” were gill slits ancestrally) structures) will provide the strongest evidence for the and probably often encounters local optima in the above hypothesis if they occur in distantly related “adaptive landscape.” species with similar selection pressures. Antlers in Moreover, not all traits that appear useful are many deer species closely resemble beetle horns, for adaptations. It is possible for a preexisting trait to example. Th e fact that male deer use their antlers to acquire a new benefi cial eff ect without being modi- fi ght one another for mates is evidence that horns fi ed by selection for this fi tness-enhancing eff ect serve a similar function in beetles. (Andrews, Gangestad, & Matthews, 2002). In this Traits also vary naturally within species. Our beetle case, the trait is called an exaptation (Gould & Vrba, horn hypothesis, for example, predicts relationships 1982). Given that it was shaped by selection for a between horns and fi ghting ability among individu- previous function, an exaptation is unlikely to be als of a given species. We might predict that males exquisitely suited to its new role. Any heritable vari- with larger horns are more successful in contests with ation enhancing the fi t between an exaptation and other males. Sex diff erences off er another important its new utility will be selected. Th us, exaptations source of within-species variation. Because males should often become secondary adaptations (Gould & engage in more fi ghting over mates, we can predict Vrba, 1982) as they become modifi ed for their new that males will tend to have larger horns. functions. Finally, traits can vary within individuals. One need not become entangled in the arcane Behaviors change frequently, of course, but even nomenclature of evolutionary biology here. Th e anatomical traits can change within the lifetime of question regarding how female orgasm came to take the individual. For example, many species breed its present form is this: Did selection shape orgasm seasonally, and the color of their fur or feathers, and to adopt a special function in women, or do women even the presence of entire structures such as ant- experience orgasms merely as a fortuitous conse- lers, change according to the season. Organisms also quence of selection for orgasm in men? change over their development. If male beetles grow horns only (or more rapidly) at sexual maturity, this testing adaptive hypotheses would support the hypothesis that they function in One can infer ancestral selection pressures by competition over mates. studying the adaptations that they produced. Th e more effi ciently a trait performs its hypothesized func- Experimental Manipulation tion compared with alternative functions, the stronger When two variables are related in nature, we can- the support for the adaptive hypothesis. Th is can be not be sure which aff ects the other, or whether the

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two are associated only because they both depend and deep sea creatures). But strong selection for a on an unknown third variable. For example, if trait in one sex can maintain the vestiges of that trait beetles with larger horns are better fi ghters, how do in the opposite sex. Such reduction in size is apparent we know that horn size, rather than overall body in male nipples. It is also apparent in the copulatory size or some other variable, causes fi ghting success? system of leopard geckos. As in other lizards, male Experimental manipulations show that one variable leopard geckos possess paired, bilateral copulatory aff ects another, while holding all other variables organs called hemipenes (Holmes, Putz, Crews, & constant. Evolutionary hypotheses can be tested by Wade, 2005). Each hemipenis is controlled by its manipulating a relevant variable to see if the manip- own set of muscles and may be used during copula- ulation aff ects the hypothesized dependent variable. tion. Interestingly, adult female leopard geckos also To test the male contest hypothesis for beetle horns, possess hemipenes. Females’ hemipenes are less than we might artifi cially enlarge some males’ horns (for one twentieth the cross-sectional area of males’, and example, by gluing extra pieces on) to see if this the associated muscles are much smaller. Female increased fi ghting success. hemipenes do not have a known function. Apparently, selection tends to favor developmental How to Spot a By-Product patterns that reduce the expression of by-products. In evolutionary terms, a by-product, also called a In addition, whereas signs that a trait has been mod- spandrel , is a phenotypic feature that is not itself a ifi ed for a function suggest adaptation, the absence direct product of natural selection but rather arose of apparent design for effi cient function indicates as an indirect consequence of selection operat- that the trait may be a by-product. However, the ing on another aspect of the phenotype (Gould & apparent lack of design may refl ect only the present Lewontin, 1979). Examples of evolutionary by- state of knowledge, as future research may reveal a products include the red color of blood (a by-prod- convincing adaptive explanation. uct of the oxygen-carrying capacity of hemoglobin) In sum, by-products that arise via sexually antag- and the umbilicus (a by-product of the umbilical onistic pleiotropy are developmentally related to cord, which supplies the fetus with oxygenated, adaptations in the opposite sex. By-products also do nutrient-rich blood from the placenta). not appear to have been modifi ed over their evolu- Th ese examples illustrate by-products of adapta- tion to function effi ciently and often appear vesti- tions in the same individual, but a by-product could gial in relation to the corresponding adaptations in also develop due to an adaptation in the opposite the opposite sex. sex, a circumstance known as sexually antagonistic pleiotropy . Because the two sexes are nearly geneti- Orgasm cally identical and share much early development, Having established how to diff erentiate adapta- genes that produce an adaptation in one sex may tions from by-products, we now describe female produce a nonadaptive by-product in the other. orgasms before evaluating the evidence regarding Nipples are clearly adaptations in females but are orgasm in women. probably possessed by males only because males share genes and some of their development with What Is a Female Orgasm? females (Symons, 1979). Th e human female orgasm has been described as Such sexually antagonistic by-products may a subjective experience of both intense pleasure and be intuitive in principle, but diffi cult to identify release at sexual climax, accompanied by a distinct set conclusively in practice. Part of the reason is that of physiological processes. Th ese processes include shared development with an adaptation in the increases in respiration rate, heart rate, blood pres- opposite sex is insuffi cient to conclude that a trait sure, and involuntary rhythmic muscle contractions is a by-product. For example, the diff erent plumage in the vagina, uterus, anal sphincters, and even ovi- colorations of peahens and peacocks are develop- ducts (Komisurak et al., 2006; Masters & Johnson, mentally related, but both are probably adapta- 1966). In addition, behavioral responses at orgasm tions, the former for avoiding predators, the latter include rapid, regular vocalizations, which can be for attracting mates. verbal or nonverbal (Hamilton & Arrowood, 1978), Sexually antagonistic by-products may also appear involuntary contractions of facial muscles, arching reduced, rudimentary, or vestigial. In general, selec- of the back, and muscle tension (Komisaruk et al., tion tends to reduce or eliminate traits that have no 2006). Orgasm in women, as in men, is accompa- function (consider the many eyeless cave-dwelling nied by the release of oxytocin, which contributes

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to both muscle contractions and the pleasurable 70% of the variation among women in copulatory sensations of orgasm (Blaicher, Gruber, Bieglmayer, orgasm frequencies is due to environmental diff er- et al., 1999; Carmichael et al., 1987; Carmichael, ences (Dawood, Kirk, Bailey, Andrews, & Martin, Warburton, Dixen, & Davidson, 1994). Recent 2005; Dunn, Cherkas, & Spector, 2005), although studies have also demonstrated that the same this estimate subsumes all sources of nongenetic brain regions in men and women show increased infl uence, including developmental and even prena- (dopamine-related systems) and decreased (cerebral tal environment. Given that 90%–95% of Western cortex) activation during orgasm (Georgiadis et al., women report having experienced orgasm, and 2006; Holstege et al., 2003). some of the remainder are likely capable of experi- Some researchers distinguish between clitoral encing orgasm under more favorable circumstances orgasms, vaginal orgasms, G-spot orgasms, and (nearly 100%, if the Mangaian data are correct), it is uterine orgasms depending on the mode of induc- reasonable to conclude that orgasm is a species-wide tion (e.g., masturbation vs. vaginal intercourse) capacity in women (Symons, 1979). (Fisher, 1973; Singer, 1973). It is unclear whether women indeed experience entirely diff erent kinds We Are Not Alone: Female Orgasm in of orgasm, or whether stimulation of diff erent sites Nonhuman Primates actually leads to the same kind of orgasm, varying Th ere is no direct way to compare the subjective in intensity but not in physiological basis (Levin, experience of women’s orgasms with the experiences 2001; Masters & Johnson, 1966). Masters and of nonhuman primate females. However, several Johnson (1966) described a key physiological sign researchers have provided strong evidence that some of orgasm in women, spasmodic contractions of female nonhuman primates undergo other corre- the outer vaginal muscles, as the same regardless of lates of human female orgasm. Th is evidence can be whether orgasm was reached via stimulation of the broadly divided into two categories: physiological clitoris or within the vagina. signs and behavioral responses. Examples of physi- In Western surveys, between 90% and 95% of ological signs include strong uterine contractions women report having experienced orgasms in some and increased heart rate, and behavioral responses context, and between 88% and 89% report having include an open, round-mouthed facial expression, experienced orgasms specifi cally during copulation body tenseness and rigidity, vocalizations, and a (reviewed in Lloyd, 2005). It is important to note reaching back and clutching reaction. that these numbers do not represent the propor- Th e sexual behavior of female rhesus macaques tions of women who are capable of orgasm. Because has been studied extensively. Zumpe and Michael the quality of sexual experience can aff ect a woman’s (1968) describe rhythmic vaginal contractions sim- chances of experiencing orgasm, some women who ilar to those that occur during the orgasm phase have not experienced orgasm may be capable of in human females (Masters & Johnson, 1966). In doing so under diff erent circumstances. For exam- rhesus macaques, these vaginal contractions occur ple, on the Polynesian island of Mangaia, boys were during apparent sexual climax at the time of ejacu- taught techniques for sexually satisfying a female lation by the male and are associated with stereo- partner (Marshall, 1971). Boys then practiced these typed behavioral responses, such as a “clutching techniques on an experienced woman, who coached reaction” and reaching-back behavior. Physiological them on methods of delaying ejaculation in order responses have also been induced experimentally in to synchronize their orgasm with that of their part- female rhesus macaques (Burton, 1971). Th is study ner. All Mangaian women were reported to achieve was designed to produce female orgasms through orgasm during copulation. In many other societies, artifi cial stimulation using a silicone penis-sim- men are much less attentive to the woman’s sexual ulator. Burton described the animals as “clearly pleasure, and female orgasm rates are much lower exhibiting three of Masters and Johnson’s (1966) (Davenport, 1977). four copulatory phases: excitement, plateau, and Th ese cross-cultural data suggest that, in addi- resolution” and recorded physiological responses in tion to variability in women’s orgasmic potential, each phase. During the excitement phase, responses the quality of women’s sexual experiences infl uences included dilation of the vaginal opening, vaginal their probability of orgasm, and further, that the secretions, engorgement of the labia, and deepen- proportions of women who have ever had orgasm ing of the color of the perineal region. During the underestimate the proportion of women who are plateau phase, the vagina was observed to widen capable of experiencing orgasm. Indeed, about and deepen. In addition, Burton confi rmed the

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clutching reaction and reaching-back behaviors Female Orgasm Does Not Appear Vestigial observed by previous researchers. Finally, during Female orgasm has been described as psychologi- the resolution phase, the clitoris was observed to cally more complex and more elaborate than male undergo detumesence, coloration of the perineal orgasm, with women reporting signifi cantly more region receded, and the females exhibited a series intense experiences (Mah & Binik, 2001, 2002). of intense vaginal spasms. Multiple orgasms are far more frequently reported Female stumptail macaques have similarly been in women than in men (Masters & Johnson, 1966), observed in laboratory studies of coital behavior with one study reporting 43% of women in a sam- (Chevalier-Skolnikoff , 1974; Goldfoot, Westerborg- ple of college-educated U.S. nurses usually expe- van Loon, Groeneveld, & Slob, 1980; Slob, riencing multiple orgasms (Darling et al., 1991). Groeneveld, & van der Werff ten Bosch, 1986). Although orgasm is accompanied by the release of Chevalier-Skolnikoff (1974) noted the following oxytocin from the pituitary gland in both men and orgasmic behavioral response patterns as “essentially women (Blaicher, Gruber, Bieglmayer et al., 1999; identical to the behavior reported in the human Carmichael et al., 1987, 1994), in women, oxyto- female”: involuntary muscular tension through- cin has a direct eff ect on uterine transport mecha- out the body followed by muscular body spasms nisms and in generating peristaltic contractions of accompanied by characteristic facial expressions and the uterus (Wildt, Kissler, Licht, & Becker, 1998). vocalizations. According to Chevalier-Skolnikoff , According to Goldfoot (quoted in Wolf, 1980, there are two key indications of orgasm in female p. 8), while female orgasm is not a reproductive stumptail macaques: the reaching-back and clutch- necessity, “an event of such magnitude can’t just be ing behavior, and the postejaculatory phase, which fortuitous.” indicates that a genital lock occurs between males and females after ejaculation. She noted that the Evidence Th at Female Orgasm Promotes former behavioral responses are directly analogous Fertilization to the spasmodic hand grasp behavior in human Williams (1966) noted that the lateral line sys- females during orgasm described by Masters and tem of fi shes exhibits suffi cient complexity and Johnson (1966). Further, Chevalier-Skolnikoff constancy across species that it was recognized as a equated the vaginal muscle contractions during probable adaptation well before its sensory function orgasm in human females with the constriction was discovered. Female orgasms are likewise suffi - of the vaginal muscles likely occurring during the ciently complex and constant in their appearance genital lock in the postejaculatory phase in female across primates that one might suspect that they stumptail macaques. Similar behaviors have been were adaptations, even if there were no evidence of reported in female pigtail monkeys (Bernstein, their function. In fact, a variety of evidence suggests 1967; Kaufman and Rosenblum, 1966; Nadler and that female orgasm has been shaped by selection to Rosenblum, 1973), howler monkeys (Carpenter, increase the probability of fertilization. 1934), baboons (Bolwig, 1959; Hall, 1962) and First, orgasmic uterine contractions probably help chimpanzees (Goodall, 1965). transport sperm to the oviducts. In women, orgasm activates the cingulate cortex and medial amygdala Evidence Regarding the Function (Komisaruk et al., 2004). Electrical stimulation of of Female Orgasm these areas in experimental animals induces uter- Donald Symons (1979) fi rst suggested that ine contractions (Beyer, Anguiano, & Mena, 1961; human female orgasm is a nonfunctional by- Setekleiv, 1964), which transport sperm in rats, dogs, product of orgasm in men, a suggestion taken cows (Singer, 1973) and probably humans (Wildt up by Gould (1987) and Lloyd (2005). It is clear et al., 1998). Orgasm also causes the hormone oxy- that male and female orgasms are developmentally tocin to be released into the bloodstream (Blaicher et related and depend on many homologous anatomi- al., 1999; Carmichael et al., 1987, 1994), probably cal structures, for example, the glans penis and the through stimulation of the paraventricular nucleus glans clitoridis, respectively. Th e critical question in (PVN) (Cross & Wakerley, 1977; Komisaruk et al., determining whether female orgasm is a by-product 2004). Oxytocin also induces uterine contractions or an adaptation therefore involves whether female (Knaus, 1950; Wildt et al., 1998) and increases the orgasm appears vestigial relative to male orgasm, or transport of a semen-like fl uid into the uterus and whether it appears to have been shaped for some oviducts (Wildt et al. 1998). Wildt et al. (1998) dem- adaptive function. onstrated that oxytocin administration is followed by 188 is female orgasm a covert mate choice mechanism?

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an immediate increase in uptake from the vagina and Evidence that the Aff ective Aspects of directed transport into the oviducts. Signifi cantly, Orgasm Motivate Adaptive Behavior fl uid was transported to both oviducts, except dur- Physiological changes associated with female ing the preovulatory (fertile) phase of the cycle, when orgasm suggest a conception-promoting function, transport was directed to the oviduct of the domi- but it is also important to consider the aff ective nant follicle. Two earlier studies that failed to fi nd aspects. In this context, consider male orgasm, movement of semen-like substances through the cer- which has both physiological correlates (e.g., ejacu- vix following orgasm (Grafenberg, 1950; Masters & lation) and aff ective correlates (intense pleasure). Johnson, 1966) placed a cap over the cervix, which Why is the moment of ejaculation associated with apparently prevented any movement (Fox, Wolff , & such intense pleasure in men? Aff ect may evolve Baker, 1970; Puts & Dawood, 2006). to elicit adaptive behavior (Plutchik, 1980). For Transport of the sperm to the oviducts should pro- example, fear functions in danger avoidance, pain mote fertilization by increasing proximity between prevents the use of damaged body parts, and plea- sperm and ovum. In addition, interaction between sure likely evolved to motivate behaviors that con- sperm and oviductal epithelium may also prolong tributed to fi tness. It seems probable that orgasm sperm longevity, increase the number of capacitated would motivate whatever sexual behaviors elicited sperm (sperm capable of fertilizing an ovum), and it. For men, this would presumably mean copulat- lengthen the interval over which at least some sperm ing until ejaculation. Orgasm is salient to women as in an ejaculate are capacitated (Smith, 1998; Suarez, well. A survey of 202 Western women of reproduc- 1998; but see Levin, 2002). tive age found that 76% said that having an orgasm Second, oxytocin reverses uterine pressure from with a partner was somewhat important to very outward to inward (Fox et al., 1970; Knaus, 1950; important, compared with only 6% saying that it Wildt et al., 1998), which may prevent loss of would be somewhat unimportant to very unimport- sperm (“fl owback”) and aid sperm in reaching the ant (Eschler, 2004). Th e possibility of orgasm may oviducts. Baker and Bellis (1993) found that female motivate women not to terminate copulation until orgasm within one minute before and 45 minutes the conception-promoting physiological changes after ejaculation was associated with higher sperm associated with orgasm occur. Associated pleasur- retention than was no orgasm or orgasm at other able sensations may also induce women to copu- times, although these results have been questioned late again with males with whom they experienced (Lloyd, 2005; but see Puts & Dawood, 2006). Th ird, orgasm. female orgasm may allow the earlier entry of sperm Yet, it would not pay to promote copulation and into the cervix by resolving the “vaginal tenting” of conception with every mate. Because, on average, sexual arousal, which elevates the cervix from the women have invested more than men in off spring posterior vaginal wall, removing it from the semen over human evolution via gestation, lactation, pool (Levin, 2002). Earlier entry of sperm may fur- and child care, they have evolved to be relatively ther prevent loss of sperm and help sperm reach the choosy with regard to mates (Daly & Wilson, 1983; oviducts. Fourth, prolactin secretion during orgasm Symons, 1979; Trivers, 1972). If female orgasm is may capacitate sperm (Meston, Levin, Sipski, Hull, & a mate choice mechanism (Alcock, 1980; Smith, Heiman, 2004). Finally, orgasmic vaginal contrac- 1984; Th ornhill, Gangestad, & Comer, 1995), tions may excite male ejaculation (Meston et al., then one would predict it to have several properties. 2004), which could coordinate ejaculation with the First, because women have evolved to be choosier various conception-enhancing processes associated than men about mates, orgasm should be triggered with orgasm in women. less easily in women. Indeed, only about 60% of To sum up, at orgasm, vaginal contractions stim- Western women report experiencing orgasm most ulate ejaculation (if it has not yet occurred), the cer- (>50%) of the time during copulation (Lloyd, 2005; vix dips into the semen pool where sperm can be Puts, 2007), whereas men much more frequently drawn into the uterus by uterine pressure changes, experience orgasm during copulation. Second, prolactin secretion may help capacitate sperm, and the “choosiness” of female orgasm should be most peristaltic uterine contractions transport sperm to characteristic of copulation. Th at is, the lower fre- the oviducts where sperm can be capacitated and quency of induction should apply especially to conception can occur. Near ovulation, these peri- orgasms resulting from copulation, as opposed to staltic contractions transport sperm to the oviduct those resulting from self-masturbation or other sex- into which the ovum is released. ual behaviors more distally related to reproduction.

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Orgasm is, in fact, more easily achieved via mastur- and parasitic infection (Moller & Pomiankowski, bation than copulation, and orgasm via masturba- 1993; Parsons, 1990, 1992; van Valen, 1962) and tion is achieved with comparable ease in women is moderately heritable in several species (Moller & and men (Hite, 1976; Kinsey, Pomeroy, Martin, & Th ornhill, 1997). Gebhard, 1953). Th ird, if female orgasm functions As expected, women exhibit preferences for the in mate choice, then it should not occur with every odors (Gangestad & Th ornhill, 1998; Rikowski & copulation but should depend upon the quality of a Grammer, 1999; Th ornhill & Gangestad, 1999), woman’s mate. Evidence below suggests that it does. faces (Gangestad, Th ornhill, & Yeo, 1994; Scheib, Gangestad, & Th ornhill, 1999), and voices (Hughes, Good Genes Mate Choice Harrison, & Gallup, 2002) of men whose external Because female orgasm may promote fertilization, features are symmetrical. Women also prefer men a reasonable hypothesis is that it functions in sire who are somewhat more masculine than average in choice (Baker & Bellis, 1993; Smith, 1984; Th ornhill height (Pawlowski & Jasienska, 2005), body build et al., 1995). If so, then women should be more (Frederick & Haselton, 2007; Horvath, 1981), likely to experience orgasms with good-genes males. voice (Feinberg et al., 2005; Puts, 2005), and per- Good-genes mate choice is expected to target indi- haps face (e.g., Johnston, Hagel, Franklin, Fink, & cators of heritable parasite resistance (Hamilton & Grammer, 2001; but see Perrett et al., 1998). Some Zuk, 1982) and freedom from harmful mutations studies have found that men’s symmetry and mas- (Zahavi & Zahavi, 1997). Th is is because heritabil- culinity correlate (Gangestad & Th ornhill, 2003; ity in fi tness can be maintained by mutation and Scheib et al., 1999), as might be predicted if both fl uctuating selection, and parasites impose tempo- features index underlying genetic quality. rally fl uctuating selection on hosts. Women’s preferences for masculine and symmet- Th e primary genomic region mediating disease rical men are strongest for short-term, sexual (vs. resistance is the major histocompatibility com- long-term) relationships and during the fertile phase plex (MHC). Preferences for MHC-dissimilar of the ovulatory cycle (reviewed in Gangestad & mates should produce off spring that are more Th ornhill, 2008). Symmetrical, physically attrac- heterozygous at MHC loci, thus exhibiting stron- tive men are also more often the extra-pair sexual ger immune function. Olfactory preferences for partners of women (Gangestad & Th ornhill, 1997). MHC-dissimilar mates have been observed in Women’s extra-pair, but not intra-pair, sexual interest fi sh, reptiles, birds, rodents, and in most human increases near ovulation (Gangestad, Th ornhill, & studies (reviewed in Roberts & Little, 2008). Garver, 2002), and this shift occurs mainly in Preferences for MHC dissimilarity may produce women whose primary partners are putatively greater-than-chance MHC-specifi c genetic dis- of low genetic quality (Gangestad, Th ornhill, & similarity within human couples (Chaix, Cao, & Garver-Apgar, 2005; Haselton & Gangestad, 2006; Donnelly, 2008) and may produce attractive, Pillsworth & Haselton, 2006). Th ese fi ndings sug- healthy-looking off spring; MHC heterozygosity is gest that women’s sexual preferences are designed associated with facial attractiveness and healthy- partly to recruit genetic benefi ts from men who may looking skin (Lie, Rhodes, & Simmons, 2008; not be the women’s long-term partners. Evidence of Roberts et al., 2005). sperm competition and moderate rates of extra-pair Two possible good-genes indicators are andro- paternity across human societies (Simmons et al., gen-dependent traits and fl uctuating asymmetry. 2004) support this possibility. Androgen-dependent, masculine traits may indicate heritable disease resistance because androgens sup- evidence that orgasm is more likely press immune function (Grossman, 1985) and may with good-genes males be produced in proportion to inherited immuno- Women’s reported orgasm frequencies closely competence (Folstad & Karter, 1992). Males with parallel their preferences for putative good-genes few harmful mutations may also be able to produce males. For example, when several variables, includ- and maintain more elaborate androgen-dependent ing relationship satisfaction, relationship duration, traits (Zahavi & Zahavi, 1997). Fluctuating asym- and a woman’s rating of her partner’s attractiveness metry (FA) refers to asymmetry in anatomical traits were entered into a multiple regression, only part- that are normally bilaterally symmetric. FA may ner’s attractiveness predicted a woman’s probability negatively indicate genetic quality because it results of orgasm during their last copulation (Shackelford from developmental stresses such as mutation et al., 2000). Th is suggests that attractive men (who

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are presumably of high genetic quality, Gangestad & prior to the onset of the next menstrual bleeding, Buss, 1993) are more likely to give their partners the approximate day of ovulation. orgasms. Men’s attractiveness in this study was In a sample of lesbian couples, Matteo and assessed by their partners, so it is also possible that Rissman (1984) demonstrated this pattern of high orgasm rates cause women to fi nd their part- female sexual behavior without the confounds of ners more attractive. fear of pregnancy or male infl uence on female sexual However, Th ornhill, Gangestad and Comer interest and initiative. A signifi cant peak in reported (1995) examined women’s orgasm frequencies in orgasms was found near midcycle. As reported in relation to two objective proxy measures of genetic Hrdy (1997), Worthman (1978, unpublished dis- quality: (a) physical attractiveness assessed by raters sertation) studied a small sample of eight hunter- other than the men’s partners and (b) symmetry for gatherer women living in the Kalahari and reported anatomical traits that exhibit fl uctuating asymme- a nonsignifi cant increase in the probability of orgasm try, along with several control variables. Women’s at midcycle. Clayton et al. (1999) also reported reported orgasm frequencies were signifi cantly “a trend toward more frequent orgasms, better- higher if their mates were more symmetrical, and timed orgasms, and more pleasurable orgasms when marginally signifi cantly higher if their mates were women were assessed during the midfollicular phase more physically attractive. As noted above, women’s compared to the late luteal phase.” extra-pair sex partners have been found to be of Changes in women’s orgasmic response over the higher than average putative genetic quality (e.g., cycle are likely to be mediated by cyclically fl uc- more symmetrical). In 75 normally ovulating, poly- tuating hormones. Van Anders and Dunn (2009) androus women, Baker and Bellis (1993) found reported relationships between estradiol, testoster- signifi cantly elevated rates of self-reported copula- one, and women’s phenomenological experience of tory orgasm with extra-pair males relative to in-pair orgasm. Zumpe and Michael (1968) describe the males. eff ects of ovariectomy on the sexual behavior of More recently, Puts, Welling, Burriss, & Dawood female rhesus monkeys and note a signifi cant reduc- (2011) found that women in romantic couples tion in the incidence of the clutching reaction after reported more frequent and earlier-timed orgasms ovariectomy, and after administration of progester- when mated to masculine and dominant men— one in estrogen-treated females. Th ese fi ndings are those characterized by high objectively-measured particularly important in demonstrating that the facial masculinity, observer-rated facial masculinity, clutching reaction is mediated by estrogen, inhibited partner-rated masculinity, and partner-rated domi- by progesterone, and is not simply a conditioned nance. Women also reported experiencing orgasm motor response to male ejaculation (Zumpe & more frequently during or after male ejaculation Michael, 1968). when mated to men characterized by high observer- Th us, evidence indicates that women’s orgasms rated and self-rated attractiveness. Th ese putative are more frequent near ovulation and from copula- measures of men’s genetic quality did not predict tion with good-genes males—both of which follow their mates’ orgasms from self-masturbation or from the hypothesis that female orgasm functions from non-coital partnered sexual behavior. in sire choice. Perhaps most convincing is evidence of an interaction between menstrual cycle phase and evidence that orgasm is likelier near the genetic quality of a woman’s mate on her prob- ovulation ability of orgasm. In the only study to examine this A number of studies have shown that women have interaction, Garver-Apgar, Gangestad, Th ornhill, an increased likelihood of orgasm during the midcy- Miller, & Olp (2006) found that women reported cle portion of the menstrual cycle when fertilization more orgasms if their partner had a compatible is possible. Udry and Morris (1968) reported on two MHC genotype, but only during the fertile phase samples: 40 normally cycling, married women from of the menstrual cycle. Th is important fi nding thus North Carolina and 48 normally cycling women shows a direct link between the genetic quality of a contributing to a study at the Institute for Sex male and his ability to induce orgasm in his mate. It Research, Indiana University. Th e total number of would be diffi cult to explain why this was observed menstrual cycles evaluated across these two samples only near ovulation if female orgasm has not been was between 911 and 997. In both samples, women shaped to recruit high-quality genes for off spring. reported signifi cantly more orgasms near ovulation, Nonhuman primate research also supports the with the peak rate of orgasm occurring 14 days sire choice hypothesis. Troisi and Carosi (1998)

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examined copulatory orgasms in female Japanese been associated with women’s intercourse orgasm. macaques and explored the roles of both physical Women in long-term relationships such as marriage stimulation and social factors such as the age and and cohabitation reported greater emotional and dominance rank of the sexual partners in the inci- physical satisfaction with their sex lives than women dence of orgasms. Th e highest frequencies of female who had been relationships for shorter time periods orgasms were detected among pairs formed by high- (Laumann, Gagnon, Michael, & Michaels, 1994). ranking, dominant males and low-ranking females, Women who were most likely to report being orgas- and the lowest frequency among pairs formed by mic premaritally were having sex in a regular, stable low-ranking males and high-ranking females. Troisi relationship (Tavris & Sadd, 1977). and Carosi (1998) noted that mating with high- A link between orgasm and pair bonding could ranking males facilitated female orgasm only when be mediated by oxytocin. Oxytocin is released in combined with “an asymmetry of rank between the response to vaginal-cervical stimulation and orgasm partners.” Th ey conclude that the fi nding that male in both sexes (Carter, 1992b; Kruger et al., 2003; rank infl uences the probability of female orgasmic Murphy et al., 1987; Todd & Lightman, 1986) and response “places in question the alternative hypoth- has anxiety-reducing, prosocial eff ects (Kirsch et al., esis that female orgasm is an incidental by-product 2005). In nonhuman mammals, oxytocin plays a of male orgasm,” and that their fi ndings provide key role in the formation of pair bonds and part- “indirect evidence that primate female orgasm is an ner preferences (Carter, 1998; Sanchez et al., 2009; adaptation whose evolutionary function is selective Winslow et al., 1993), especially in females (Sanchez mate choice.” et al., 2009). Among 59 premenopausal women, those who reported more frequent hugs had higher Pair-Bonding Function oxytocin levels (Light, Grewen, & Amico, 2005). It has also been suggested that female coital orgasm Higher oxytocin levels were also associated with functions in selecting and securing a long-term, lower blood pressure and heart rate. While both men investing partner (Barash, 1977; Beach, 1974; Eibl- and women reporting greater partner support tend Eibesfeldt, 1975; Hamburg, 1978; Morris, 1967). to have higher oxytocin levels, only women who Th is could work if investing males were more likely reported greater partner support also demonstrated to induce orgasm, and orgasm bonded women to lower blood pressure and cortisol levels, suggesting these men. Perhaps men who are more attentive care- that oxytocin benefi ts may be more pronounced in givers are more likely to induce orgasms in women. women (Sanchez et al., 2009). Similarly, studies of To the degree that foreplay duration indexes a male’s women have found an increase in plasma oxytocin attentiveness, women appear to have more orgasms associated with being in a romantic relationship with attentive men (Singh, Meyer, Zambarano, & (Taylor et al., 2006; Turner et al., 1999). In sum, Hurlbert, 1998). Given that men are sensitive to the oxytocin released at orgasm could play a role cues of paternity in allocating investment (Bressan, in bond formation in both sexes, but the eff ects are Bertamini, Nalli, & Zanutto, 2009; Daly & Wilson, likely to be greater in women. 1982; DeBruine, 2004; Platek, Burch, Panyavin, Other evidence appears to contradict the pair- Wasserman, & Gallup, 2002; Platek et al., 2003; bonding hypothesis. Women’s reported orgasm rates Regalski & Gaulin, 1993), it may also have benefi t- were lowest in marriage and cohabitation and high- ted ancestral females to increase the odds of fertil- est in women who had been dating their partners for ization from males who demonstrated willingness less than three months (Laumann et al., 1994). In and ability to invest. addition, Th ornhill et al. (1995) found that women’s In support of the pair-bonding hypothesis, wom- orgasm frequency did not relate to their professed en’s copulatory orgasm frequency has been associ- love for a partner, relationship duration, or his pro- ated with favorable dimensions of relationship fessed love, nurturance, commitment, exclusivity, quality, including Satisfaction, Intimacy, Passion, socioeconomic status, or perceived future earnings. and Love (Costa & Brody, 2007). Noncoital Although Pollet and Nettle (2009) initially reported orgasm frequency with a partner was unrelated that Chinese women with wealthier partners reported to these dimensions, and masturbation frequency higher orgasm frequencies, this eff ect seems to have was associated with less Love. Similarly, “marital been due to these women being healthier, happier, happiness” (Gebhard, 1966; Singh et al., 1998) younger, and more educated (Herberich, Hothorn, and sexual compatibility (Singh et al., 1998) have Nettle, & Pollet, 2010; Pollet & Nettle, 2010). In

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75 normally ovulating polyandrous women, Baker Cross-species comparative data also chal- and Bellis (1993) found signifi cantly elevated rates lenge a pair-bonding function for female orgasm. of self-reported copulatory orgasm with extra-pair Compelling evidence of female orgasm has been males relative to in-pair males, which again contra- reported in several nonhuman primate species. Th ese dicts a pair-bonding function. species tend to be characterized by multimale groups Shackelford and colleagues (2000) found that and a promiscuous mating system (see Table 11.1). women who reported an orgasm during the last Th e physiological and behavioral similarities across copulation with their partner reported greater rela- species and the association between the presence of tionship satisfaction. However, when several other female orgasm and multimale groups would seem to variables, including relationship duration and a contradict a pair-bonding function. woman’s rating of her partner’s attractiveness were Of course, it is possible for the function of female entered into a regression, only partner’s attractive- orgasm to diff er between humans and nonhuman ness predicted a woman’s probability of orgasm dur- primates. Is it also possible that human female ing their last copulation. Th is suggests that measures orgasm serves both fertilization and pair-bonding of relationship satisfaction are related to orgasm functions. How fascinating it would be if orgasm frequency only through their association with the played such a dual role in women, promoting con- partner’s physical attractiveness. In other words, ception within the fertile window when orgasm is while it is plausible that a strong relationship would more easily induced and sex with good-genes males is increase orgasm frequency and/or that frequent more likely, and promoting pair-bonding outside of orgasm would cause relationship satisfaction, these the fertile window when greater partner attentiveness data suggest that the two are correlated due to their is required for its induction. It is also possible that relationships with a third variable: partner’s attrac- orgasm promotes pair-bonding to a greater extent in tiveness. A man’s attractiveness and genetic quality some women, for example those less likely to benefi t may increase both his mate’s orgasm frequency (as from extra-pair copulations. On the whole, however, data linking males’ FA and MHC genotype to their the present evidence would seem to support a good- partners’ orgasm frequencies also suggest) and her genes mate choice function of female orgasm more happiness in the relationship. strongly than it would a pair-bonding function.

Table 11.1 Female orgasm occurs in primate species where females copulate polyandrously. Species Mating system Reference for orgasm Reference for mating system

Chacma baboons Multimale, multifemale Blowig (1959), Cheney (1978), Seyfarth et al. promiscuous mating Hall (1962) (1978)

Chimpanzees Multimale, multifemale Goodall (1965) Nishida (1979), Goodall promiscuous mating (1983), Wrangham (1979)

Japanese macaques Multimale, multifemale Troisi & Carosi (1998) Kawai et al. (1967), promiscuous mating Norikoski & Koyama (1974)

Mantled howler Varies from one-male to Carpenter (1934) Glander (1980) monkeys multimale groups

Pigtail monkeys Multimale, multifemale Kaufman and Rosenblum Tokuda et al. (1968), promiscuous mating (1966), Bernstein (1967), Hadidian & Bernstein (1979) Nadler & Rosenblum (1973)

Rhesus macaques Multimale, multifemale Zumpe & Michael (1968), Lindburg (1971), Boelkins & promiscuous mating Burton (1971) Wilson (1972), Drickamer & Vessey (1973)

Stumptail Multimale, multifemale Chevalier-Skolikoff (1974), Bertrand (1969), Hadidian & macaques promiscuous mating Goldfoot et al. (1980), Bernstein (1979) Slob et al. (1986)

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Arguments Against Adaptation Wallen and Lloyd derived clitoral variability nor As noted above, several researchers have favored the study from which they derived penile variabil- the by-product hypothesis over an adaptive expla- ity reported intra- or intermeasurer reliability. Th is nation for female orgasm. Symons (1979) argued is a serious problem, as the authors acknowledge; that female orgasm is too diffi cult to induce, and its all of the reported diff erence in variability between expression too variable among women, for it to be clitorises and penises could be due to the greater dif- an adaptation. Lloyd (2005, p. 134) echoed these fi culty of precisely measuring smaller structures (cli- sentiments: “very wide variability of rates of orgasm toral length was 16% of penile length) or to other with intercourse suggests that there is no selection diff erences between the studies. Th ird, Wallen’s and on female orgasm with intercourse. If there was, Lloyd’s thesis relies upon clitoral and penile length then we would expect a high, consistent expression aff ecting orgasmic potential in women and men, of the trait of female orgasm with intercourse…” respectively. Th e authors provide no supporting Th e reasoning is that adaptations are generally found evidence for this questionable assumption. Finally, species wide, and if female orgasm is not reliably whereas both clitorises and penises are important in induced, then this challenges how effi ciently and orgasm, penises are intromittent organs necessary precisely it can perform any hypothetical function. for sperm transfer and also function in urination. However, given women’s (probably adaptive) Th ese additional roles mean that, whatever diff er- choosiness over mates, a certain intricacy in orgasm ences existed in selection on clitorises and penises induction would be predicted from a mate choice for sexual stimulation, selection on penis length hypothesis. Th is “choosiness” in orgasmic response certainly diff ered from that on clitoral length. Th e should be (and is) especially characteristic of sexual comparison appears fundamentally unsound. intercourse. Moreover, our reading of the data is that the variability among women in orgasmic potential Contexts Th at Could Favor a Copulatory does not warrant rejecting an adaptive hypothesis. Mate Choice Mechanism Women vary in rates of orgasm because they diff er Human female orgasm may be a functionless in the propitiousness of their mating circumstances, by-product of male orgasm, a pair-bonding adap- and because they diff er in the response pattern tation, or an adaptation for some other unknown relating those circumstances to their tendency to function, but evidence is accumulating that it has orgasm. Much of the variation among women in been shaped by selection to promote fertilization by orgasm frequencies likely results from the faculta- males of high genetic quality. Th is evidence includes tive nature of orgasm—not all sexual stimulation human and nonhuman data on physiological and is equal. Undoubtedly, much orgasmic variation behavioral consequences of orgasm that likely also results from diff erences in women’s response increase the probability of fertilization and evidence patterns, but how much is presently unknown. In of increased orgasm rates near ovulation and with any case, even traits that have probably experienced partners of high genetic quality. One could specu- strong selection, such as menstrual cycle length, late that each feature of female orgasm is a by-prod- stature, cognitive abilities, running speed, and facial uct of some other adaptation, but the number of attractiveness, are highly variable among individuals. these features, their deviations from the male pat- Facultative adaptations should exhibit even greater tern, and their consistent relationship to an elevated variation. We have considered these arguments and probability of fertilization from good-genes males others more fully elsewhere (Puts, 2006a, 2006b, suggest that female orgasm evolved for this func- 2007; Puts & Dawood, 2006). tion. What ancestral conditions might have favored Recently, Wallen and Lloyd (2008) reported such an adaptation? Several possibilities exist. that clitoral length was more variable than penile First, ancestral females may have been better able length, taking this as evidence of weaker selection to evaluate the genetic quality or other qualities of on orgasmic potential in women than in men. Th is their mates through the act of copulation. In some work suff ers from several fl aws. First, Wallen and insects, for example, females may terminate copu- Lloyd compared the external parts of the clitoris lation before insemination is complete, remate if a and penis, but clitorises diff er from penises in the male is of lower quality than other available mates, proportion that is external. To avoid this compari- or bias fertilization toward high-quality males son of “apples to oranges,” the authors might have (Dickinson, 1997). Across taxa, paternity has been compared the clitoral glans to the penile glans, for observed to be biased in relation to such variables example. Second, neither the study from which as the timing of copulation, copulation duration,

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copulatory courtship behavior, genital structure, for males who obtained copulations through invest- and male body size (reviewed in Jennions & Petrie, ment in a mate. Th ese conditions might then favor 2000). Evidence reviewed above suggests that female male vigilance to evidence of female orgasm, espe- orgasm may bias human paternity according to the cially among women’s long-term, investing mates, timing of copulation, male copulatory behavior, and for whom the costs of lost paternity were relatively male quality. Genital structure may also infl uence greatest. human paternity if penis morphology, for example, size (Lever, Frederick, & Peplau, 2006), infl uences Men’s Concern over Female Orgasm women’s probability of orgasm (Miller, 2000). A cursory examination of popular media and Second, ancestral females may have chosen men’s magazines (e.g., Men’s Health , GQ , Maxim ) some mates on the basis of sire quality and other shows an abundance of articles devoted to sexual mates based on other qualities, such as investment performance and satisfaction. Frequently, articles potential (Baker & Bellis, 1995; Buss & Schmitt, focus on women’s orgasms and instruct male readers 1993; Gangestad & Simpson, 2000; Gangestad & on how to improve their sexual technique and sat- Th ornhill, 2008). Among many bird species, females isfy their mates. A review of the scientifi c literature apparently pair with a male social partner for direct (Muehlenhard & Shippee, 2009; Wiederman, 1997) benefi ts, such as nesting sites and paternal care, and indicates that men are, indeed, often concerned obtain extra-pair copulations with males of superior with whether their partner experiences orgasm and genetic quality (Møller, 1992; Møller & Swaddle, feel a responsibility to ensure that she enjoys sexual 1997). As discussed above, evidence indicates that intercourse. Research also indicates that women women are more likely to obtain extra-pair copula- engage in “pretending” or “faking” orgasms partly tions from males of higher genetic quality than their because they believe that men care (Muehlenhard & long-term partner, especially near ovulation, and Shippee, 2009). that they are also more likely to achieve orgasm with Men’s concern with female sexual satisfaction good-genes males and extra-pair partners (Baker & can be viewed in a diff erent light by examining Bellis, 1993). the traditional practice of female “circumcision” or Th ird, ancestral females may not always have had “genital mutilation” in many African and Middle the opportunity to choose with whom they copu- Eastern societies. Th ese practices involve surgical lated. A variety of factors might have limited ances- removal of all or part of the external clitoris (cli- tral females’ ability to choose their mates, including toridectomy), this in combination with removal of familial infl uence of long-term partners (Apostolou, the labia minora, or stitching together of the labia 2007), sexual coercion (Smuts, 1996), and male majora (infi bulation). Studies of sexual function in exclusion of competitors through intense contest women who have undergone circumcision indicate competition (Puts, 2010). A covert copulatory mate that sexual arousal, lubrication, orgasm, and gen- choice mechanism would thus allow women to exert eral sexual satisfaction are all adversely impacted some control over paternity, even in the absence of by the practice (Alsibiani & Rouzi, 2010). Many control over sexual access. writers have documented the practice as originating Each of these possibilities likely applied to some in beliefs that circumcised women are more likely degree in ancestral human populations. Th e result to be chaste and socially and sexually acceptable to would have been selection favoring physiological future mates (Abusharaf, 1998). Th us, in societies and psychological variants that increased the prob- with female circumcision, a girl’s family and per- ability of fertilization from copulations with high- haps her future husband are concerned that she can- quality males. not obtain sexual satisfaction from other men.

Sexual Confl ict and Female Orgasm Faked Orgasm by Women Th e ability of human females to manipulate Given evidence of men’s attentiveness (at least in paternity via orgasm, however imperfectly, would some societies) to signs of orgasm in women, it is impose potential costs on ancestral males. Males reasonable to expect that women might sometimes who obtained costly copulations through contest feign orgasm so as to maintain male investment or competition, sexual display, or coercion might nev- avoid other potential costs. Most women (approxi- ertheless experience a decreased probability of con- mately 60%) report having faked an orgasm at some ception if they were unable to stimulate the female time (Darling & Davidson, 1986; Muehlenhard & to orgasm. Th e costs would be considerably higher Shippee, 2009; Wiederman, 1997). Th ornhill et al.

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(1995) found that the average woman faked orgasm adaptation.” While we may never be able to con- 13% of the time. Th is pretense was partly success- clude defi nitively that female orgasm is an adapta- ful; men reported that their partners faked orgasm tion, the past three decades have revealed important only 10% of the time. Other studies show that men observations that seriously question the by-product generally overestimate their partners’ enjoyment, hypothesis in favor of a sire choice hypothesis. especially with regard to female orgasm through Th ese observations include examples of each type of intercourse (von Sydow, 2002). evidence of adaptive design outlined above. Cross- In a recent study, Muehlenhard and Shippee species comparative data suggest that female orgasm (2009) found that the most frequently reported evolves where females copulate polyandrously, and reasons for college students to pretend orgasm were that female orgasm depends on the quality of a that (1) orgasm was unlikely, (2) they wanted sex female’s mate. Between-female variation in orgasm to end, (3) they wanted to avoid negative conse- frequency in humans is also linked to the quality of quences (e.g., hurting their partner’s feelings), and a female’s mate. Sex diff erences in human orgasm (4) to obtain positive consequences (e.g., pleasing frequency mirror sex diff erences in choosiness over their partner). Under the sire choice hypothesis for mates, and within-female variation in orgasm fre- female orgasm, women are expected to orgasm least quency tracks conception risk across the cycle, a often with males of low genetic quality. Women key indicator that orgasm functions in sire choice. might nevertheless pretend orgasms with these Experimental manipulations of orgasm-related hor- males if there were an advantage, such as garnering mones and brain regions in humans and nonhuman investment, to falsely signaling that these men were mammals also indicate that female orgasm promotes being chosen as sires. Th us, women might be like- conception. lier to fake orgasm if they play a mixed reproductive If female orgasm is indeed a copulatory sire choice strategy of obtaining investment from a long-term mechanism, then it has fi tness consequences for mate and recruiting better quality genes from one or both women and their mates. Men might attend to more extra-pair sex partners. evidence of orgasm in their mates in order to gauge Some evidence supports these predictions. For their probability of paternity and adjust investment example, more attractive women may be less sexually accordingly. Women might sometimes feign orgasm faithful (Singh, 2004), and women who rate them- in order to maintain investment from males who selves as more attractive and are higher in sexual self- did not induce orgasm. Some of these inferences esteem are more likely to fake orgasms (Wiederman, fi nd support in the literature. 1997). Women who report more sexual partners (Darling & Davidson, 1986; Wiederman, 1997) Future Directions and who tend to act in less exclusive ways with their Much work is still to be done, but the questions partners (e.g., by fl irting with other men, or neglect- surrounding the possible adaptedness of female ing their partners at social gatherings) (Th ornhill orgasm and its relationship to sexual confl ict are et al., 1995) are also more likely to fake orgasms tractable. Which variables predict the occurrence of than are other women. Th ornhill et al. conclude that female orgasm, and which variables predict when faked orgasms may correlate with lower reliability of it is faked? Is orgasm more likely when a woman’s paternity. In contrast, women who have engaged in mate is physically attractive and of high genetic extra-pair copulations report more frequent copula- quality, or when he is committed, investing, and tory orgasms with extra-pair males relative to in-pair attentive? Do the physiological sequelae of orgasm males (Baker & Bellis, 1993). However, no data of increase the probability of conception, as several which we are aware directly test the prediction that lines of evidence indicate? What are the behavioral women will more often fake orgasms with investing consequences of orgasm in women? Does having an mates of low genetic quality. We are also aware of orgasm with a partner make a woman more sexually no study testing the prediction that men decrease attracted to this partner, or cause her to fall deeply investment when they suspect their mates of faking in love? Do women fake orgasms a greater propor- orgasm. tion of the time when having sex with their long- term mates, or their extra-pair partners? Do men Conclusion decrease investment in partners when they suspect Symons (1979, p. 89) noted that “the avail- them of faking orgasm? Th ese questions should be able evidence is, by a wide margin, insuffi cient to explored across cultures, particularly in traditional warrant the conclusion that female orgasm is an societies. Key fi ndings require replication. We look

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