The Evolution of Female Orgasm: Adaptation or Byproduct?

Masturbation in the Animal Kingdom

Masturbation is one of the most common sexual behaviors in humans. It is also a phylogenetically widespread trait of various other mammalian and some non-mammalian species. Several hypotheses have been proposed aiming to explain the function of masturbation in primates and other species. These were mainly based on observations of nonhuman primates such as rhesus macaques or bonobos and rodents such as African ground squirrels. Based on these observations various scholars suggested that masturbation improves ejaculate quality, decreases the risk of contracting sexually transmitted infections or is merely a by-product of sexual arousal and thus an alternate outlet to copulation. While these theories may explain some facets of masturbation in some species, they do not explain why masturbation is so widespread and has developed in various species as well as our hominid ancestors. Moreover, the research on which these theories are based is scarce and heavily focused on male masturbation, while female masturbation remains largely unexplored. This sex difference may be responsible for the one-sided theorizing that attributes a specific biological benefit to masturbation. We propose that the widespread prevalence of masturbation in the animal kingdom may be better explained by viewing masturbation as a primarily self-reinforcing behavior that promotes pleasure both in human and in nonhuman species.

Endocrinology of human female sexuality, mating, and reproductive behavior

Hormones orchestrate and coordinate human female sexual development, sexuality, and reproduction in relation to three types of phenotypic changes: life history transitions such as puberty and childbirth, responses to contextual factors such as caloric intake and stress, and cyclical patterns such as the ovulatory cycle. Here, we review the endocrinology underlying women's reproductive phenotypes, including sexual orientation and gender identity, mate preferences, competition for mates, sex drive, and maternal behavior. We highlight distinctive aspects of women's sexuality such as the possession of sexual ornaments, relatively cryptic fertile windows, extended sexual behavior across the ovulatory cycle, and a period of midlife reproductive senescence-and we focus on how hormonal mechanisms were shaped by selection to produce adaptive outcomes. We conclude with suggestions for future research to elucidate how hormonal mechanisms subserve women's reproductive phenotypes.

The whole versus the sum of some of the parts: toward resolving the apparent controversy of clitoral versus vaginal orgasms

BACKGROUND

The nature of a woman's orgasm has been a source of scientific, political, and cultural debate for over a century. Since the Victorian era, the pendulum has swung from the vagina to the clitoris, and to some extent back again, with the current debate stuck over whether internal sensory structures exist in the vagina that could account for orgasms based largely on their stimulation, or whether stimulation of the external glans clitoris is always necessary for orgasm.

METHOD

We review the history of the clitoral versus vaginal orgasm debate as it has evolved with conflicting ideas and data from psychiatry and psychoanalysis, epidemiology, evolutionary theory, feminist political theory, physiology, and finally neuroscience.

RESULTS

A new synthesis is presented that acknowledges the enormous potential women have to experience orgasms from one or more sources of sensory input, including the external clitoral glans, internal region around the "G-spot" that corresponds to the internal clitoral bulbs, the cervix, as well as sensory stimulation of non-genital areas such as the nipples.

CONCLUSIONS

With experience, stimulation of one or all of these triggering zones are integrated into a "whole" set of sensory inputs, movements, body positions, autonomic arousal, and partner- and contextual-related cues, that reliably induces pleasure and orgasm during masturbation and copulation. The process of integration is iterative and can change across the lifespan with new experiences of orgasm.

Human female orgasm as evolved signal: a test of two hypotheses

We present the results of a study designed to empirically test predictions derived from two hypotheses regarding human female orgasm behavior as an evolved communicative trait or signal. One hypothesis tested was the female fidelity hypothesis, which posits that human female orgasm signals a woman's sexual satisfaction and therefore her likelihood of future fidelity to a partner. The other was sire choice hypothesis, which posits that women's orgasm behavior signals increased chances of fertilization. To test the two hypotheses of human female orgasm, we administered a questionnaire to 138 females and 121 males who reported that they were currently in a romantic relationship. Key predictions of the female fidelity hypothesis were not supported. In particular, orgasm was not associated with female sexual fidelity nor was orgasm associated with male perceptions of partner sexual fidelity. However, faked orgasm was associated with female sexual infidelity and lower male relationship satisfaction. Overall, results were in greater support of the sire choice signaling hypothesis than the female fidelity hypothesis. Results also suggest that male satisfaction with, investment in, and sexual fidelity to a mate are benefits that favored the selection of orgasmic signaling in ancestral females.

A typological approach to testing the evolutionary functions of human female orgasm

Building on previous work that identified different types of orgasm in women (King, Belsky, Mah, & Binik, 2011), the goal of the present study was to extend such typological work and determine whether female orgasmic variability tracked potentially evolutionarily salient sexual partner characteristics (e.g., those displaying possible immune-system compatibility). A total of 265 females completed an Internet survey about their orgasmic experience-achieved either with partners or alone. For partnered orgasms, they also provided details of partner characteristics and sexual behaviors. Latent class analysis revealed two orgasm types which were meaningfully distinguishable in terms of sensations and location-either centered on the surface of genitalia or deep inside. Deep orgasms were associated with internal sensations consistent with proposed functions of female orgasm in terms of differential sperm insuck. Such orgasms were associated with partners who were perceived as considerate, dominant, with a noticeably attractive smell, and as providing firm penetration. However, some hypothesized reproductively significant partner characteristics were not differentially associated with deep orgasms (i.e., muscularity, aggression, masculinity). Results were discussed and future research directions outlined. In particular, it is suggested that sexual passion between partners is a non-accidental component of sexual functioning and that this has too frequently been missing in sex research involving humans. Direct physiological measures of the results of female orgasm need to be undertaken. Additionally, the intriguing phenomenon of female ejaculation deserves scientific attention.

Why women have orgasms: an evolutionary analysis

Whether women's orgasm is an adaptation is arguably the most contentious question in the study of the evolution of human sexuality. Indeed, this question is a veritable litmus test for adaptationism, separating those profoundly impressed with the pervasive and myriad correspondences between organisms' phenotypes and their conditions of life from those who apply the "onerous concept" of adaptation with more caution, skepticism or suspicion. Yet, the adaptedness of female orgasm is a question whose answer will elucidate mating dynamics in humans and nonhuman primates. There are two broad competing explanations for the evolution of orgasm in women: (1) the mate-choice hypothesis, which states that female orgasm has evolved to function in mate selection and (2) the byproduct hypothesis, which states that female orgasm has no evolutionary function, existing only because women share some early ontogeny with men, in whom orgasm is an adaptation. We review evidence for these hypotheses and identify areas where relevant evidence is lacking. Although additional research is needed before firm conclusions can be drawn, we find that the mate-choice hypothesis receives more support. Specifically, female orgasm appears to have evolved to increase the probability of fertilization from males whose genes would improve offspring fitness.

Sexual selection on human faces and voices

Humans are highly sexually dimorphic primates, and some of the most conspicuous human sex differences occur in the face and voice. Consequently, this article utilizes research findings on human faces and voices to illustrate how human sex differences may have arisen by sexual selection (i.e., the type of natural selection favoring traits that increase mating opportunities). Evidence suggesting that sexual selection shaped women's faces and voices is reviewed. However, sexual selection likely operated more strongly on men over human evolution. Thus, this research focuses on two types of sexual selection operating on men: female mate choice, which favors traits that attract females, and male contests, which favor traits for excluding competitors from mates by force or threat of force. This article demonstrates how masculine faces and voices advertize critical information about men's mate value and threat potential, and reviews evidence that women's preferences and men's deference to masculine faces and voices reflect this information content. Data suggesting that facial and vocal masculinity influences men's mating opportunities and reproduction are discussed, and the article concludes by highlighting directions for future research.

Human Sexual Conflict from Molecules to Culture

Coevolutionary arms races between males and females have equipped both sexes with mutually manipulative and defensive adaptations. These adaptations function to benefit individual reproductive interests at the cost of the reproductive interests of opposite-sex mates, and arise from evolutionary dynamics such as parental investment (unequal reproductive costs between the sexes) and sexual selection (unequal access to opposite-sex mates). Individuals use these adaptations to hijack others' reproductive systems, psychological states, and behaviors—essentially using other individuals as extended phenotypes of themselves. Such extended phenotypic manipulation of sexual rivals and opposite-sex mates is enacted by humans with the aid of hormones, pheromones, neurotransmitters, emotions, language, mind-altering substances, social institutions, technologies, and ideologies. Furthermore, sexual conflict may be experienced at an individual level when maternal genes and paternal genes are in conflict within an organism. Sexual conflict may be physically and emotionally destructive, but may also be exciting and constructive for relationships. By extending the biological concept of sexual conflict into social and cultural domains, scholars may successfully bridge many of the interdisciplinary gaps that separate the sciences from the humanities.

Evidence to suggest that copulatory vocalizations in women are not a reflexive consequence of orgasm

The current studies were conducted in order to investigate the phenomenon of copulatory vocalizations and their relationship to orgasm in women. Data were collected from 71 sexually active heterosexual women (M age = 21.68 years ± .52) recruited from the local community through opportunity sampling. The studies revealed that orgasm was most frequently reported by women following self-manipulation of the clitoris, manipulation by the partner, oral sex delivered to the woman by a man, and least frequently during vaginal penetration. More detailed examination of responses during intercourse revealed that, while female orgasms were most commonly experienced during foreplay, copulatory vocalizations were reported to be made most often before and simultaneously with male ejaculation. These data together clearly demonstrate a dissociation of the timing of women experiencing orgasm and making copulatory vocalizations and indicate that there is at least an element of these responses that are under conscious control, providing women with an opportunity to manipulate male behavior to their advantage.

Response to Puts and Dawood's ‘The Evolution of Female Orgasm: Adaptation or Byproduct?’ — Been There

David Puts and Khytam Dawood's recent critique of my book, The Case of the Female Orgasm: Bias in the Science of Evolution, attempts to make plausible an adaptive account of female orgasm based on a hypothesized mechanism of uterine upsuck and sperm competition. Yet the authors fail to respond to the criticisms of such accounts that I detailed previously in my book. They raise a further concern about my definition of adaptation — a red herring — and manufacture a conceptual error regarding heritability that they then attribute to me. Most seriously, they fail to address the glaring failure of sperm competition accounts to accord with evidence from sexology. Specifically, the distribution curve of orgasm-with-intercourse — according to Dawood et al.'s own data, as well as others' — is relatively flat across the various classes. This curve needs to be tested against a well-formed multistrategy adaptive hypothesis; it cannot be explained by the adaptive account defended by Puts and Dawood in their critique.