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Definition
A sexual climax typically experienced as a result of sexual stimulation, accompanied by involuntary muscle contractions and endorphin release.
Introduction
Orgasm is typically experienced as a sexual climax between the plateau and resolution phases of the human sexual response cycle (Masters and Johnson 1966). Whether the stimulation experienced during solitary or paired sexual behavior is induced manually, orally, or through the use of an object, orgasm in men is most reliably achieved through direct penile stimulation and in women through direct clitoral stimulation. Though there exists substantial overlap in the physiological and psychological sequelae of orgasm between the sexes, copulatory orgasm may serve different evolutionary functions in men and women. While men’s copulatory orgasm is necessary for reproductive success, it is unclear whether women’s copulatory orgasm is an adaptive mate-choice mechanism or a by-product of developmental processes shared with males.
Physiological and Psychological Effects of Orgasm
The physiological changes accompanying orgasm include accelerated heart rate, increased blood pressure, and pelvic muscle contractions (Masters and Johnson 1966). In men, muscle contractions in the penis and anal sphincter are typically accompanied by ejaculation, which function in sperm transfer during copulation. Women experience muscle contractions in the uterus, vagina, and anal sphincter during orgasm, which may by several mechanisms increase sperm retention and facilitate conception (Levin 2011; Lloyd 2005; Puts et al. 2012). These perceptible physiological changes are accompanied by differential patterns of neuronal activation in the orbitofrontal cortex, cerebellum, and paraventricular nucleus (e.g., Georgiadis et al. 2009), the latter of which modulates the release of oxytocin (OXT), a hormone implicated in human pair-bonding.
It may be because of these similar patterns of neuronal activation that the psychological experience of orgasm is similar for men and women. Written descriptions of orgasmic experiences do not differ significantly between men and women, though recent studies suggest that women may subjectively rate orgasms as more intense than do men. In both sexes, orgasm is described as satisfying and rewarding, which may encourage men and women to engage in behaviors that could result in orgasm and, in the case of copulation, to engage in copulation until orgasm is achieved.
As ejaculation is required for reproductive success in men, the ability to do so during intercourse has likely been under strong selective pressure throughout human evolution. More than 75 % of men report experiencing orgasm during every act of intercourse, and primary anorgasmia (never having experienced orgasm) in men is rare. By contrast, the prevalence of primary anorgasmia in women is much higher, with about 5–10 % of women never having experienced orgasm. Indeed, copulatory orgasm is experienced much less frequently in women than in men, with approximately 25 % of women always and 33 % never experiencing copulatory orgasm (Lloyd 2005).
Orgasm as an Adaptation
It has been suggested that orgasm serves a mate-choice function in women (Smith 1984). Orgasm may facilitate fertilization by high-quality mates by stimulating ejaculation, reinforcing copulation until ejaculation occurs, promoting repeated copulation with the same males, and inducing peristaltic contractions of the uterus and oviducts, which transport sperm toward the ovum (reviewed in Puts et al. 2012; Wheatley and Puts 2015).
Women may be more likely to experience orgasm during copulation with men possessing putative markers of genetic fitness (Smith 1984), such as facial attractiveness, facial masculinity, and low fluctuating asymmetry. Several studies have found a positive association between these traits and women’s frequency of copulatory orgasm, even when controlling for potential confounds such as relationship duration and relationship satisfaction (reviewed in Puts et al. 2012; Wheatley and Puts 2015). Women’s frequency of copulatory orgasm may also be positively associated with the major histocompatibility complex (MHC) genetic compatibility of their male sexual partners, in that that conception with MHC-discordant men should produce offspring with strong adaptive immunity. Interestingly, this association was only found when women were in the fertile phase of the menstrual cycle (Garver-Apgar et al. 2006). Thus, there may exist a largely unexplored interaction between women’s fertility and markers of men’s genetic quality and compatibility in predicting the frequency of women’s copulatory orgasm. More generally, women’s greater difficulty in achieving orgasm may reflect their overall greater choosiness over sexual partners resulting from their greater investment in offspring (Symons 1979).
Women’s copulatory orgasm may also serve a different adaptive purpose by facilitating long-term pair-bonding with men who are caring and would invest in offspring (Hamburg 1978). Some studies have found positive associations between women’s frequency of copulatory orgasm and numerous indices of relationship satisfaction. More frequent copulatory orgasm, and concomitantly, more frequent OXT release, may facilitate women’s desire to form pair-bonds with men who are perceived as caring, attentive, and investing. However, evidence for this hypothesis is mixed (for reviews, see Lloyd 2005; Puts et al. 2012; Wheatley and Puts 2015).
Orgasm as a By-Product
Alternatively, women’s orgasm may be a by-product of developmental processes shared with men and devoid of any adaptive function in women. Prior to the tenth week of gestation, male and female embryos are physically indistinguishable (Hamburg 1978). Sexual differentiation of external reproductive structures begins around the tenth week of gestation, when a previously undifferentiated structure called the genital tubercle develops into either the penis or the clitoris, both of which contain nerve and erectile tissues involved in sexual response. The presence of these tissues is patently adaptive in men, as they are required for ejaculation and sperm delivery. Such tissues implicated in orgasm in women, however, may not serve any adaptive function, but may remain present due to sexually antagonistic selection (Symons 1979).
Proponents of this by-product hypothesis have argued that if women’s copulatory orgasm was under selective pressure and conferred a reproductive advantage, the frequency distribution of women’s copulatory orgasm should look similar to that of men’s: a distribution with little variability and with the majority of women reporting experiencing orgasm reliably and frequently during copulation. On the one hand, the frequency distribution of women’s copulatory orgasm is almost flat and highly variable, suggesting a potential lack of selective pressure on a specific behavioral phenotype (Lloyd 2005). On the other hand, if female orgasm were selected to function in mate choice, then it should be less reliably induced on average compared to orgasm in men and exhibit greater variability due to its contingency on the quality of women’s mates (Puts et al. 2012). In addition, because ejaculation is essential for reproduction whereas female orgasm is not, selection should favor the tendency for men to more quickly and easily achieve orgasm compared to women.
Conclusion
Despite the fact that “no other human sexual topic has been the focus of so much speculation and hypothesizing” (Levin 2011), it remains equivocal whether women’s copulatory orgasm is an adaptation or simply a developmental by-product. Studies linking copulatory orgasm frequency to markers of relationship quality have necessarily been correlational. Consequently, causal relationships between these variables, as well as the directionality of any relationships, have not been established. In addition to investigating causality, perhaps through longitudinal studies and path analysis, future work should continue to explore associations between women’s copulatory orgasm and other markers of partner’s genetic fitness and investment. Additionally, research should examine potential interactions between such markers and women’s conception risk within the ovulatory cycle in modulating copulatory orgasm. Finally, future research should continue to investigate whether women’s orgasm promotes sperm transport and whether the occurrence of orgasm contributes to the probability that coitus will result in conception during fertile days of the ovulatory cycle.
Cross-References
References
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Georgiadis, J. R., Reinders, A. A., Paans, A. M., Renken, R., & Kortekaas, R. (2009). Men versus women on sexual brain function: Prominent differences during tactile genital stimulation, but not during orgasm. Human Brain Mapping, 30(10), 3089–3101.
Hamburg, B. A. (1978). The biosocial basis of sex differences. In S. L. W. E. R. McCown (Ed.), Human evolution: Biosocial perspectives (pp. 155–213). Menlo Park: Benjamin/Cummings.
Levin, R. J. (2011). Can the controversy about the putative role of the human female orgasm in sperm transport be settled with our current physiological knowledge of coitus? Journal of Sexual Medicine, 8(6), 1566–1578.
Lloyd, E. A. (2005). The case of the female orgasm: Bias in the science of evolution. Cambridge, MA: Harvard University Press.
Masters, W. H., & Johnson, V. E. (1966). Human sexual response. Boston: Little, Brown, and co.
Puts, D. A., Dawood, K., & Welling, L. L. M. (2012). Why women have orgasms: An evolutionary analysis. Archives of Sexual Behavior, 41(5), 1127–1143.
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Shirazi, T., Puts, D. (2016). Orgasm. In: Weekes-Shackelford, V., Shackelford, T., Weekes-Shackelford, V. (eds) Encyclopedia of Evolutionary Psychological Science. Springer, Cham. https://doi.org/10.1007/978-3-319-16999-6_3366-1
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DOI: https://doi.org/10.1007/978-3-319-16999-6_3366-1
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