KeywordsSexual Satisfaction Relationship Satisfaction Sexual Contact Nonhuman Animal Sexual Infidelity
Both copulation and sexual intercourse have been used to refer to penetrative, heterosexual sexual activity, that is: penile-vaginal sex. Here, however, the term “copulation” will refer to both penetrative (i.e., vaginal or anal sex) as well as nonpenetrative (e.g., oral sex) forms of intercourse that occur between same-sex and opposite-sex partners.
Defining sex is – perhaps surprisingly – the subject of significant debate and disagreement in lay as well as scientific circles. Several works have investigated the ways in which individuals define “sex” or “having had sex” and have arrived at disparate findings, depending on when these attitudes are assessed (Rosenthal 2013), the age of the respondent (Sanders et al. 2010), and even how the question is asked (Peterson and Muehlenhard 2007). This ambiguity is mirrored in the scientific community, where “copulation” and “sex” have been used to refer to exclusively intromission (i.e., penile-vaginal sex) as well as intromission between opposite-sex conspecifics and penetrative sex, oral sex, and manual stimulation of genitals that occurs within both same and opposite-sex pairings (Bailey and Zuk 2009). Herein, human copulation, colloquially referred to as “sex” or “sexual intercourse,” will adopt an inclusive definition, including (1) oral sex, (2) vaginal sex, and (3) anal sex. The various expressions and function(s) of human copulation will be explored through the review of psychological, cross-cultural, and phylogenetic evidence.
This difference in lay definitions of “sex” is also represented as an age difference, wherein men aged 18–29 are less likely to define oral sex and manual genital contact as “sex,” compared to men over the age of 30 (Sanders et al. 2010). Interestingly, the same cross-sectional study of definitions of “sex” found that all men but those in their oldest age group (65+) were significantly more likely to indicate that anal sex was defined as “sex” (Sanders et al. 2010). All of these works have used a similar approach when assessing disagreement as to the definition of “sex,” that is, asking participants if, after engaging in these various behaviors with a partner, they would say that they have “had sex” with that partner. In other words, these studies have all relied upon a methodology wherein they ask participants if various sexual behaviors “count” as sex. However, this methodology involves the implicit assumption that participants have clear (that is, not contradictory) definitions of what contact constitutes “sex.” Conclusions about how laypersons define “sex” may change if these attitudes are assessed in a manner which allows for the representation of uncertainty.
Work does suggest that the way in which participants are asked to define sex can affect the definition of sex that they provide (Peterson and Muehlenhard 2007). Specifically, Peterson and Muehlenhard (2007) asked participants to share experiences which were “almost but not quite…” or “just barely qualified…” as sex and thereby employed a methodology that allowed for representations of uncertainty or ambiguity in definitions of sex. Activities that “almost but not quite…” fit participants’ definitions of sex included brief or partial intromission, genital to genital contact without penetration, oral sex, and manual genital stimulation. Interestingly, these same activities were represented (albeit, to a lesser degree) as behaviors that “just barely…” fit participants’ definitions of sex. Participants explained that these nonintromission types of sexual contact “fit” within their definitions of sex because they contained what participants perceived to be other, critical defining features, such as genital contact, orgasm, “an exchange of fluids,” sexual arousal, and/or nudity. Taken together, it seems as though individuals have an overwhelming tendency to define sex/copulation as intromission, while expressing some uncertainty as to whether or not other types of genital contact such as oral sex, anal sex, and manual genital contact fit within this definition – among other factors (such as age, the specific ways in which definitions of sex are elicited from participants, etc.) it seems as though these definitions depend upon the anticipated consequences of labeling a specific type of sexual contact as “sex.” Specifically, when the consequences of labeling a specific type of sexual contact as “sex” are negative, individuals’ are more likely to label these experiences as “not quite sex” (Peterson and Muehlenhard 2007). For example, some individuals reported a motivation to define sexual contact with a same-sex partner as “not quite sex” in order to preserve their identity as heterosexual.
Disagreements as to the definition of copulation/sex are also present in the scientific community, where “copulation” and “sex” have been used to refer exclusively to intromission (i.e., penile-vaginal sex) as well as penetrative sex, oral sex, and manual stimulation of genitals that occurs within both same and opposite-sex pairings (Bailey and Zuk 2009). Studies of copulation in nonhuman primates often define copulation as sexual contact between opposite-sex conspecifics that must include intromission. Alternatively, comparative studies of sexual activity and contact between same-sex pairs have defined a variety of activities between same-sex conspecifics (e.g., oral sex, manual stimulation of genitals) as “copulation” (Bailey and Zuk 2009). The following evolutionary analysis of human copulation will adopt the latter, more inclusive definition, including (1) oral sex, (2) vaginal sex, and (3) anal sex. These various expressions and function(s) of human copulation will be explored through the review of psychological, cross-cultural, and phylogenetic evidence.
Evolutionary analysis of a given behavior, cognition, or affect typically follows one of the two paths of inquiry: (1) from form to function or (2) from function to form (Buss 1995). The former refers to the process of identifying a particular behavior of interest and searching for the adaptive problem for which that particular behavior may have been preferentially selected and perpetuated; in other words, what is the function of this behavior? The latter refers to the process of identifying a particular adaptive problem of interest and searching for the behavior, cognition, or affect that might have solved that adaptive problem in ancestral environments; in other words, what psychological mechanism might have evolved to address this problem? Therefore, an analysis of a given behavior of interest – oral sex – will take the following form; what is the function of oral sex?
Oral sex has long been colloquially understood as a form of sexual contact that often serves as a substitute for other forms of sexual contact during one’s sexual debut (Duberstein Lindberg et al. 2008). As Duberstein Lindberg puts it, “There is a widespread belief that teens engage in nonvaginal forms of sex, especially oral sex, as a way to be sexually active while still claiming that technically, they are virgins” (Wind 2008). Indeed, oral sex is more common among teens than vaginal sex, and teens perceive oral sex to be less likely to precipitate many negative emotional and social outcomes, compared to vaginal sex (e.g., less likely to get them into trouble, cause them to feel guilty or bad about themselves, less likely to conflict with their ethical, religious, or moral values; Halpern-Felsher et al. 2005). However, recent work suggests that oral sex often closely corresponds with the initiation of other types of sexual contact (including vaginal sex), rather than serving as an alternative for these other types of sexual contact (Duberstein Lindberg et al. 2008). Findings from a national survey indicate that the proportion of teens and young adults that engage in oral sex before other forms of sexual contact (26 % of females and 24 % of males) is strikingly similar to the proportions of teens and young adults that engage in oral sex after other forms of sexual contact (27 % of females and 24 % of males; Copen et al. 2012). It seems as though oral sex is common in both teens and adults but does not serve the function of initiating sexual activity or delaying other forms of sexual contact.
Comparative analysis of a particular behavior – such as oral sex – is often illuminating when determining the evolutionary function and adaptive importance of that behavior. Phylogenetic evidence, comparing the prevalence and function of a behavior in other species, supports the proliferation of an adapted behavior through generations of selection pressures. Oral sex is quite common in nonhuman animals, appearing in bonobos, stumptail macaques (Chevalier-Skolnikoff 1974), lemurs, and orangutans. In nonhuman primates the function of oral sex is often affiliative between same-sex conspecifics. For example, observations of oral-genital contact in male stumptail macaques suggests that such contact is most likely to occur between an adult male and a juvenile that has not yet reached sexual maturity, in the context of other positive, affiliative behaviors (such as grooming, carrying, and other parental care; Chevalier-Skolnikoff 1974). Also, the frequency of same-sex oral-genital contact between primates of disparate age suggests that this contact likely serves an educational function, critical to subadult primates’ sexual development (Gray and Garcia 2013). Alternatively, it is possible that oral sex in nonhuman animals is simply reflexive, a response to attractant pheromones secreted near the genitals. In nonhuman animals, there is little consensus as to the function of oral sex, but the behavior may serve the complex function of assisting in the sexual development of subadults or fostering social ties between same-sex conspecifics.
Recent work has illuminated three potential functions of oral sex in humans (Pham and Shakelford 2013) – which, unlike other types of sexual contact (i.e., kissing), is a practice that is quite common cross-culturally – (1) the detection of rivals’ semen, (2) the increase of partner’s relationship satisfaction (and thereby, fidelity), and (3) the facilitation of sperm retention. The first proposed function of oral sex in humans, the detection of rivals’ semen, is detailed in the infidelity-detection hypothesis that suggests cunnilingus may allow heterosexual males to test for the presence of rival semen in the vulva and therefore determine (at least in part) the sexual fidelity of their current mate. If cunnilingus serves this infidelity-detection function, one would expect that these concerns would be particularly great during female peak fertility (when the likelihood of conception is greatest); indeed, nonhuman animals display more frequent oral-genital stimulation during female’s peak fertility and humans indicate that the odor of vaginal secretions during peak fertility is more pleasant, compared to secretions during low fertility (Doty et al. 1975). Furthermore, men that are at a greater risk of experiencing sexual infidelity (operationalized as having a partner high in sexual and physical attractiveness) are more interested in performing oral sex and perform oral sex for longer durations than those with lower risks of experiencing sexual infidelity on part of their heterosexual partner (Pham and Shakelford 2013).
The second proposed function of oral sex – that it increases partner’s relationship satisfaction – is partially supported throughout literature exploring the relationship between sexual satisfaction and relationship satisfaction. Frequency of oral sex has been repeatedly linked to sexual satisfaction, in same-sex and opposite-sex couples. Oral sex seems particularly important for the sexual satisfaction of individuals in same-sex couples, as lesbians report oral sex as their most-preferred technique for achieving orgasm and oral sex has been found to be the most-frequently engaged in sexual contact for gay men (Ekstrand and Coates 1990). What seems clear from this work is the relationship between oral sex and sexual satisfaction – the relationship that is more contentious in the literature is that between sexual satisfaction and relationship satisfaction.
Evident in the literature are both strong correlations between sexual satisfaction and various indicators of relationship satisfaction, such as overall satisfaction with one’s marriage, satisfaction with one’s dating relationship, and feelings of love and commitment, as well as longitudinal works indicating that relationship satisfaction and sexual satisfaction seem to change concurrently (Sprecher 2002). However, these findings are qualified by works which have failed to demonstrate strong evidence that changes in relationship satisfaction can account for or explain changes in sexual satisfaction or that changes in sexual satisfaction can explain observed changes in relationship satisfaction (Byers 2005). Instead, it seems as though the relationship between sexual satisfaction and relationship satisfaction is mediated by the quality of communication in an intimate relationship (Byers 2005) and the similarity of sexual desire between partners in an intimate relationship (Davies et al. 1999). Overall, it seems as though oral sex can facilitate greater sexual satisfaction in both same-sex and opposite-sex mateships, but evidence providing a direct causal link between sexual satisfaction and relationship satisfaction is lacking.
The third proposed adaptive function of oral sex in humans – the facilitation of sperm retention – has received support in literature exploring the relationship between cunnilingus and the experience of female orgasm. Originally termed the Upsuck Theory by Masters and Johnson, recent work indicates that the experience of female orgasm (and the associated release of oxytocin) is associated with uterine contractions that act as a peristaltic pump, promoting the travel of sperm and other fluids in the reproductive tract towards the oviduct (Zervomanolakis et al. 2009). Therefore, if a female experiences orgasm during a heterosexual encounter, the likelihood of conception is promoted as the experience of female orgasm decreases the distance that must be traveled by spermatozoa to reach the ovum. Using both objective and subjective estimates of sperm retention (that is, sperm retained in the reproductive tract following penile-vaginal copulation), Baker and Bellis (1993) find that female orgasms occurring within a specific time window relative to male ejaculation (between 1 min before male ejaculation to 45 min after male ejaculation) increased the amount of sperm retained in the reproductive tract. This sperm retention theory appears to be relevant only to penile-vaginal copulation, rather than the current behavior of interest – oral sex. However, the likelihood of experiencing female orgasm through penile-vaginal copulation alone (that is no clitoral stimulation with the hands or the mouth, etc.) is quite low. Data from a national survey suggests that the likelihood of experiencing female orgasm during penile-vaginal copulation is dramatically increased (from around 12 % to around 32 %) if oral stimulation of the clitoris (e.g., oral sex) occurs (Richters et al. 2006). Therefore, the likelihood of retaining sperm in the reproductive tract and the travel of spermatozoa and seminal fluid to the oviduct are both facilitated by the female orgasm, which is more likely to occur when the clitoris is stimulated, particularly via cunnilingus.
Vaginal sex can take many forms in primates, including (1) male on top, female supine (ventro-ventral position), (2) female on top, male supine (ventro-ventral position), (3) rear entry (dorso-ventral position), (4) inverted rear entry (inverted dorso-ventral position), and (5) side by side (latero-lateral position). The ventro-ventral position of vaginal sex, with the male on top, female supine (i.e., “missionary position”) is by far the most common among humans (Gray and Garcia 2013) and has even been demonstrated to be common across cultures, appearing in the Alorese, Hopi, Marshallese, Tikopia, Balinese, among others (Dixson 2012). While occurring less frequently in other primates, orangutans, bonobos, and gorillas also engage in ventro-ventral vaginal sex, both with the male and with the female on top (Dixson 2012); some researchers hypothesize that this face-to-face copulatory posture evolved as primate intelligence increased, which gave way to variations in copulatory postures (specifically, variations from the dorso-ventral “norm”). Also, this face-to-face copulatory posture facilitates communication between opposite-sex conspecifics during vaginal sex, and such eye contact can be used to assess interest in sexual contact (Dixson 2012); indeed, eye contact has been demonstrated to be used as a method of communicating sexual intent. However, ventro-ventral vaginal sex with the male on top, female on bottom has been associated with low female sexual satisfaction and female difficulty achieving orgasm (Rosenthal 2013), as this position does not afford the female control over the depth, speed, or angle of intromission and often does not stimulate the clitoris. Variations on the ventro-ventral position that involve manual stimulation of the clitoris are associated with a greater likelihood of females achieving orgasm during vaginal sex. It is important to note that it is uncommon for females to achieve orgasm when engaging in penile-vaginal copulation alone, that is, without additional clitoral stimulation orally or manually (“vaginal orgasm”; Gray and Garcia 2013). There is some evidence to suggest that the vaginal orgasm might play a role in mate choice, wherein longer penises are associated with a greater likelihood of achieving female vaginal orgasm and females that report a preference for longer penises are more likely to experience vaginal orgasms (Costa et al. 2012). These differences in orgasm outcomes might create a selection pressure favoring longer penises, particularly for females that experience vaginal orgasm.
Dorso-ventral copulatory positions (i.e., “doggie style”), less common but still preferred among male and female humans, are much more common among nonhuman primates. These positions allow the, typically larger, males to remain standing, while the female crouches on all fours. In primates the dorso-ventral copulatory position is often accompanied by the use of the feet (termed the “leg lock” position) or the hands to restrain the female, particularly common during long duration vaginal sex (between 15 and 35 min; Dixson 2012). Importantly, the dorso-ventral copulatory position allows the males to retain a grip on tree branches with their prehensile tails and/or their feet. A fascinating variation on the dorso-ventral copulatory position is the inverted dorso-ventral position, wherein both the male and female primate are suspended, upside-down, by a tree branch. This position, common in the slow loris, slender loris, and the aye-aye, is thought to provide protective benefits, allowing the partners to hide during copulation and to quickly drop from their tree limb if disturbed or pursued by a predator (Dixson 2012).
As above, evolutionary analysis of a given behavior, cognition, or affect typically follows one of the two paths of inquiry: (1) from form to function or (2) from function to form (Buss 1995). Again, an analysis of the given behavior of interest – vaginal sex – will take the following form; what is the function of vaginal sex? There are two prevailing explanations as to the function of human vaginal sex, (1) (the somewhat obvious adaptive function) the production of offspring and therefore the proliferation of one’s genes in future generations and (2) the increase of partner’s relationship satisfaction (and thereby, fidelity). The former function refers to the fact that in the Environment of Evolutionary Adaptedness (EEA) for humans (roughly 10,000 years ago) – a time characterized by significant selection pressures in the environment which provided significant advantages in survival and reproduction for certain members of a species, with specific traits – modern medical technologies which enable individuals to conceive in the absence of penile-vaginal copulation were not available. Therefore, the production of offspring depended entirely upon fertilization as a result of penile-vaginal copulation between opposite-sex conspecifics. The necessity of sexual behavior (such as vaginal sex) for the survival and perpetuation of one’s genes has likely produced natural rewards for engaging in such behavior, such as the production and release of neurotransmitters that produce feelings of desire and pleasure (e.g., dopamine and serotonin) in the nucleus accumbens and the ventral tegmental area in the midbrain. Sexual behaviors are a “natural reinforcer” along with other behaviors that have been historically critical for survival and reproduction, such as eating, drinking, and seeking social connectedness. Activity in specific pathways of the brain (e.g., the nucleus accumbens and the ventral tegmental area) rewards individuals for and motivates individuals to engage in behaviors – like vaginal sex – which have been critical for the perpetuation of genes into subsequent generations. Put simply, were it not for the existence of vaginal sex and a desire to engage in vaginal sex, the human species would not continue to exist.
The second proposed function of vaginal sex – that it increases partner’s relationship satisfaction and fidelity – is partially supported throughout literature exploring the relationship between sexual satisfaction and relationship satisfaction. As mentioned above, the existence of a direct causal relationship between sexual satisfaction and relationship satisfaction is somewhat contentious; however, the use of vaginal sex to maintain a long-term bond between mates is supported in both the human and nonhuman animal literature. Among nonmonogamous species of primates, such as lemurs and chimpanzees, almost all of their vaginal sex takes place during periods of female peak fertility, ovulation (Gray and Garcia 2013). Therefore, one might infer that in these nonmonogamous species, vaginal sex serves the primary function of producing offspring and that vaginal sex is motivated by cues indicative of female receptivity. These cues can be detected by scent or sight; male tamarins display increased erections and mounting behaviors when exposed to the scent marks of an ovulating female tamarin and vaginal sex increases in frequency while female bonobos display a swollen perineum, which is indicative of female ovulation. It is important to note that while these cues have evolved to signal times of peak fertility to sexually mature males and therefore motivate vaginal sex behaviors, the accuracy of these cues is a matter of some debate. According to the graded-signal hypothesis, in nonmonogamous species that live in multi-male, multi-female groups, signals of ovulation vary in their strength to indicate variations in the probability of conception (Nunn 1999). For example, sexual swellings would vary in terms of their size with their largest expression indicative of the greatest probability of conception. This type of variable signaling provides benefits to both males and females of nonmonogamous species; it allows the males to preferentially engage in vaginal sex with fertile females, and it provides a sufficiently wide window of possible ovulation (i.e., “graded”) to produce paternity uncertainty – if males in these multi-male, multi-female social groups are unsure as to which offspring are their genetic relatives or that of their rivals, they will be less likely to engage in infanticide.
Alternatively, in monogamous species, such as gibbons and siamangs, vaginal sex (while taking place relatively infrequently) does take place during periods of female infertility (e.g., when a female is currently pregnant; Gray and Garcia 2013). There are several physical markers that indicate a species evolved to engage in monogamous mating in the context of long-term pair bonds – such as small testicles (indicative of infrequent sperm competition) and low sexual dimorphism (indicative of similar parental investment provided by males and females, and relatively infrequent male physical competition for access to mates). In fact, some scholars argue that monogamy may have evolved, in part, to solve the adaptive problem of producing and caring for offspring that require significant investment; for instance, due to a long juvenile period or ecological constraints that make acquiring enough resources to produce healthy offspring difficult without male–female cooperation.
It is possible that the vaginal sex that takes place during periods of female infertility, observed in gibbons, siamangs, and many monogamous bird species (Eens and Pinxten 1995), may serve a function of increasing relationship satisfaction and protecting against threats to the fidelity of the mateship. For example, female sterlings have been observed courting and initiating copulation with males whom were seeking the attention of other females even after they have laid fertilized eggs (Eens and Pinxten 1995). Humans also engage in several mate retention tactics, including enhancing one’s appearance to appear more attractive to one’s partner and engaging in more frequent sexual contact, in response to perceived threats to their relationship. Interestingly, both men and women have reported faking orgasm during vaginal sex in order to increase their partner’s satisfaction (Muehlenhard and Shippee 2010) and women are more likely to fake orgasm during vaginal sex when they perceive that their relationship may be at a greater risk for experiencing partner infidelity and defection.
Anal sex refers to penetrative copulation, which can be classified as the penetration of the anus with the penis (penile-anal copulation). While often colloquially assumed to be prevalent in gay sexual relationships, 20 % of gay men report never experiencing anal sex (Laumann 1994). Furthermore, the incidence of anal sex has been steadily increasing among heterosexual couples in the United States, with estimates (of having “ever” engaged in anal sex with an opposite-sex partner) increasing from around 20 % in 1995 to approximately 36 % in 2013 (Chandra et al. 2013). While anal sex is overall a relatively uncommon form of copulation, individuals that are more likely to engage in anal sex are typically those that report higher levels of education and lesser religiosity (Baumeister 2001). Interestingly, women report anal sex as being their least preferred and the least satisfying type of sexual behavior (Hurlbert et al. 1993). While oral sex is reported as being the most preferred and most frequently engaged-in type of sexual behavior by gay men, anal sex is still a relatively frequent type of sexual contact in male same-sex couples; 75 % of men reported engaging in the “insertive role” while 81 % reported engaging in the “receptive role” (Laumann 1994).
Anal sex in the primate world is rare, and often occurs during isosexual encounters (male-male mounting and intromission; Dixson 2012). The male same-sex mounting and anal sex which take place in nonhuman primates is proposed to serve a developmental function – occurring frequently in juveniles and decreasing in frequency as the primate reaches sexual maturity – and is proposed to be maintained and motivated by the pleasurable sensation associated with engaging in anal sex from the “insertive role” (Dixson 2012). This type of male same-sex anal intromission has been observed in orangutans, stumptail macaques (Chevalier-Skolnikoff 1974), and rhesus monkeys, among others. As discussed relevant to oral sex above, same-sex sexual contact via anal sex in nonhuman primates likely serves the adaptive function of facilitating sexual development (Chevalier-Skolnikoff 1974). Interestingly, some documented cases of anal sex between same-sex conspecifics do not seem to be developmental and transitory in nature; in the case of two male rhesus monkeys raised in captivity, the two monkeys were capable of successfully mounting and achieving ejaculation during mating with a female, but demonstrated a preference for social and sexual contact with their male partner when given the option to interact with a female.
In the current discussion of human copulation, it has been demonstrated that the definition of copulation or “sex” is the topic of significant debate and disagreement, both within lay and scientific circles. Overall, one’s individual definition of “sex” or “having had sex” seems to depend on when these attitudes are assessed (Rosenthal 2013), the age of the respondent (Sanders et al. 2010), and how the question is asked (Peterson and Muehlenhard 2007). This ambiguity in the scientific community may be indicative of a transition away from heteronormativity, where “copulation” and “sex” have historically been used to refer exclusively to penile-vaginal sex but are now adopting more inclusive definitions which can capture partnered sexual contact between same-sex conspecifics (Bailey and Zuk 2009).
Taken together, human copulation in the form of oral sex is becoming increasingly common and likely serves the adaptive function of promoting social connectedness (in nonhuman primates) or is simply a reflexive response to appetitive vaginal secretions. Furthermore, oral sex in humans may serve the adaptive function of allowing for the detection of infidelity, increasing relationship satisfaction, and facilitating sperm retention (Pham and Shakelford 2013). Vaginal sex is the most common and most preferred form of sexual contact between heterosexual couples. While dorso-ventral forms of vaginal sex are most common in the primate world, the appearance of ventro-ventral (face-to-face) vaginal sex in some primates (including humans) may have evolved to facilitate communication or is simply indicative of increasing species intelligence. Vaginal sex likely serves many functions, including the propagation of one’s genes into subsequent generations, increasing relationship satisfaction, and increasing the likelihood of partner fidelity. Anal sex, while relatively uncommon in the primate world, is becoming increasingly common in heterosexual human mateships (Chandra et al. 2013). It likely serves affiliative and relationship maintenance functions in humans, as well as functions relevant to sexual development in nonhuman primates (Chevalier-Skolnikoff 1974). Overall, it is clear that human copulation, in its many forms, serves important functions for developing and maintaining social connections, sexual development, and satisfaction in intimate mateships. Many of these behaviors have been present throughout evolutionary history, as evidenced by the many copulatory behaviors shared with nonhuman primates. Future work will continue to illuminate the nature of the changing social and sexual world and the implications these changes might have given the ways in which evolution has shaped humans into sexual creatures.
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