Part 11. Humans Are Blank-ogamous. Sexaptation: The Many Functions of Sex

This is part 11 of a series on the evolution of human mating behavior. Please see the introduction here.


“When the gods gave people sex, say the !Kung, they gave us a wonderful thing. Sex is often referred to as food: just as people cannot survive without eating… hunger for sex can cause people to die.” (Shostak 2000: 237) 

“sex can be many things to many people, including but not limited to a blend of personalities, social rules, desire, intimacy and performance, moral order and national image that speak to processes of sexual embodiment, varieties of sexual practice and the dynamics of culture.” (Donnan and Magowan, 2010: 175)

E unum, pluribus. (Out of one, many).


Penis Festival

Genital-themed ashtrays from the Komaki Penis Festival in Japan. For humans, sex is more complex than just getting genitals together. (

Last month, representatives in Montana debated whether to repeal an old law that made homosexual sex illegal in that state (the law was in fact overturned). Apart from the fact that private, consensual sexual behavior is still considered a matter to be legislated, there were other interesting developments from the discussion. A representative named Dave Hagstrom raised a deep question when he asked: “What is the purpose of sex?” I appreciate Hagstrom’s line of inquiry, as we could probably use more reflection on human sexuality. Unfortunately his own answer did not live up to the profundity of the question: 

“To me, sex is primarily purposed to produce people. That’s why we’re all here. Sex that doesn’t produce people is deviate. That doesn’t mean it’s a problem, it just means it’s not doing its primary purpose.”

From there, he made an analogy comparing sex to a pen (an odd comparison), and referred to homosexual sex as ‘deviate’ because it was ‘not normal’ and not doing its primary purpose. In the video of his comments, he actually seemed quite considerate and sincere. Yet, there a couple of problematic areas with his reasoning, including defining normalcy and deviancy, as well as overlooking the fact that biological variation is the norm within species.

However, the other problem that I wanted to focus on here is trying to delimit human sexuality to any single function, even to something as fundamental as procreation. While sex certainly has its origins in reproduction, likely as a means to create genetic diversity in offspring, the story doesn’t end there. Origins do not tell us everything. Stephen Jay Gould once wrote that there is often a “disconnection between historical (evolutionary) origin and current utility” (2002: 85).  For example, a number of evolutionary scenarios have been proposed for the origins of grasping hands – and forward facing eyes – in early primates tens of millions of years ago, including for holding onto branches for climbing, for hunting insects, or for better hand-eye coordination to obtain fruit under low-light conditions (Conroy 1990).

Today, we apply our modified primate hands for activities far removed from our ancestral life in the trees such as typing, using chopsticks, or for sign language. The point is that evolution is a tinkerer, often tweaking and redeploying pre-existing structures for new functions, a concept known as exaptation (Gould and Vrba, 1982). This may also apply to sex. Sexaptation?

It doesn’t take much mental effort to come up with a list of the many possible forms of sex that do not result in pregnancy, and this applies to all sexualities. Heterosexual individuals often go well out of their way to avoid procreation by using contraception, but they also engage in sex after menopause or during pregnancy, as well as through a variety of other creative acts that cannot result in conception (with the rarest of exceptions). Even kissing is a part of human sexuality, and it doesn’t produce people (Kirshenbaum 2011). What it comes down to is this: human sexuality is far more complex than merely trying to arrange a play date between genitals and gametes. As Agustin Fuentes noted: 

“For humans sex is not confined to genitals or even to purely physical contact. Human sex can be had in conversation, over the phone, while dancing, over a well cooked meal, in one’s mind, as well as physically between two (or more) people. For humans sex is a seriously complicated and totally biocultural act. It is never just about biological processes, ever.”

It almost seems self-evident that an enormous part of the human sexual repertoire lies outside of the intention to procreate, yet for some reason this often slips the mind of those who argue that sex is solely for reproduction. Some have argued that sex primarily became separated from marriage and childbearing after the sexual revolution of the 1960s, which made birth control more accessible. However, that narrative oversimplifies things by a wide margin. Greg Downey has referred to the past few million years of human evolution as one long, slow sexual revolution, in continuous flux as humans have migrated and adapted to various ecological and social circumstances.

Humans have probably been aware of the link between sex and reproduction since we first evolved, and Holly Dunsworth has hinted that this ‘reproductive consciousness’ is likely unique to our species. With this consciousness in hand, women have known about and used contraceptives or abortifacents for millennia (ex. see Emily Willingham’s post). Children are wonderful, and often sources of joy and meaning – not to mention genetic continuity – but given the health risks and personal sacrifices involved in pregnancy, birth, and raising children, it makes perfect sense that women throughout history would have wanted to take more control over their own reproductive histories. Yet they presumably had sex during that time, even if they wished to become pregnant on their own terms.


The Sex Connection

Why do it then? The common response is that sometimes sex is for procreation; other times it’s for recreation. Yet even though those two sound well together, that dichotomy doesn’t quite go far enough. To continue with the alliteration, human sexuality is also pro-relational. As Chris Ryan put it:

“For Homo sapiens, sex is primarily about establishing and maintaining relationships—relationships often characterized by love, or at least affection. Reproduction is a by-product of human sexual behavior, not its primary purpose.”

Feathers as display and for flight. Lesser bird of paradise, New Guinea. (Tim Laman, National Geographic)

Feathers as display and for flight. Lesser bird of paradise, New Guinea. (Tim Laman, National Geographic)

This directly contradicts the premise that the ‘primary purpose’ of sex in humans is for reproduction. But perhaps more important than trying to single out any single primary purpose is the recognition that sex can have many functions, which depend on context. One thing can be many things. E unum, pluribus. If the pro-relational attributes of human sexuality are an exaptation, this need not mean that when new functions evolve that older ones have to wither away. For example, feathers likely first evolved in some dinosaurs for warmth and thermoregulation, after which they became useful for attracting mates in sexual display, and were only later coopted for flight in birds (see Ed Yong’s posts on these). In the present, they can still retain all three functions: warmth, display, flight. The same goes for human sexuality: it can certainly be for reproduction, but it can also be strictly for pleasure, or for connection. Or some combination of the three. Or more.

When Meston and Buss (2007) asked a sample of college students (an admittedly WEIRD sample) why they had sex, they came up with 237 reasons, an amusingly high number. These roughly fell into four categories, including: Physical (stress reduction, pleasure, fun), Goal Attainment (resources, social status, revenge), Emotional (love, commitment), and Insecurity (self-esteem boost, duty/pressure, mate guarding). [1]  We are not always proficient at explaining why we behave the way we do, but conscious explanations of why we have sex should be taken seriously. However, the only ones that would have a real evolutionary impact are those that would curtail sex or make celibacy sacrosanct (this didn’t work out too well for the Shakers). All other roads lead to Rome. Sexy Rome. Still, it seems that sex is akin to a Swiss Army knife with a variety of functions, with some being more socially acceptable than others.

But I want to return to Chris Ryan’s point that sex is largely about maintaining and establishing relationships, because I think it cuts through a lot of the noise surrounding questions of human sexuality. I think his other writings (Ryan and Jethá 2010) would clarify that he did not mean these relationships needed to be long-term or exclusive, only that sex was an important form of interpersonal connection – if even a fleeting one – for an intrinsically social species like ours. While sex can be multi-functional, the one consistent feature of it is rather obvious: we usually prefer it when another person is present (for whichever reason). This need not occur in a ‘relationship’ in the traditional sense of the term, but sex can occur between any two people, ranging from lifelong ‘soul-mates’ to virtual strangers. But even strangers are capable of having what psychologist Barbara Fredrickson (2013) calls “micro-moments of connection” (hopefully this lasts more than a micro-moment in the case of sex). Along these lines, in his essay “Why Do We Have Sex?” Noam Shpancer (2012) wrote that:

human sexuality can be fully understood only in a social context…. Sexual desire, thus, is not chiefly aimed at physical pleasure or the production of children, but at connectedness with others. Sexual pleasure is fundamentally a social construct, an emergent property of social exchange.”

Shpancer used the example of prostitution to bolster his case, arguing that if physical pleasure were truly the primary goal of sex, that this could be achieved alone through self-stimulation, at no financial cost, and without risk of pregnancy or sexually-transmitted disease (not to mention possible social sanctions). The ethics of prostitution are complex and nuanced, and there are a variety of opinions on the topic, but that’s not the point here. Rather, I find Shpancer’s argument worth considering because it emphasizes the importance of another person’s presence. He goes on to say that the experience is enhanced when pleasure is shared: “What excites him about the thought that she is enjoying herself? Fundamental social, interpersonal dynamics are apparently present even here, inside the most alienated transaction.” For the same reasons even when sex is a solo activity, it is not a strictly mechanical event, but is pseudo-interpersonal since it usually involves fantasies or even pornographic images of another person (or more).

But what is meant by connectedness, and what exactly is being exchanged? Like the 237 reasons given in the Meston and Buss study, this is hard to pin down to any one thing. Sex has many forms of currency that can span across a continuum of commitment, or ‘sociosexuality’ (Simpson and Gangestad, 1991). These include physical pleasure, feeling desirable, appreciating beauty, finding commonality, or some elusive concept of ‘chemistry’ that occurs when individuals cross some threshold together. This list is not complete, but the important thing is that these are shared or exchanged between individuals, when sex is consensual (!) and mutual. There is also some evidence that our neurobiology contains mechanisms which reinforce cooperation and reciprocal altruism, at least in lab settings (Rilling et al 2002). To find someone willing to cooperate sexually with us could only be that much more satisfying. In fact, sex seems to be one of the most satisfying things that we do.

Happiness by activity. Dashed line indicates average happiness, and size of the circle indicates the frequency of occurrence (from Killingsworth and Gilbert, 2010).

Happiness by activity. The line  at 65 indicates average happiness, with making love as the clear standout. Size of the circle indicates the frequency of occurrence (from Killingsworth and Gilbert, 2010).

When researchers text-messaged over 2,000 people (mostly Americans) at random times about what they were currently doing, “making love” stood out as the activity with the greatest levels of focus and happiness (Killingsworth and Gilbert, 2010). An unpublished 2012 New Zealand study found similar results. Volunteers were texted at random several times a day about their activities and emotions.  Sex/making love ranked first not only for pleasure and happiness, but also for meaning and engagement (housework, Facebook, and being sick ranked near the bottom). Furthermore, there is also evidence that sexual stimulation has secondary benefits in other species. For example, it may restore cognitive function in middle-aged rats (Glasper ER, Gould E. 2013). And sex is probably a pleasurable experience for most animals (at least some of the time). But do any of them, other than humans, find it a source of meaning and engagement? I have my doubts. Hmm, maybe bonobos.

Tyrannosaurus sex. In search of meaning and engagement? Also, Rawr. (Jurassic Museum of Asturias. Wikipedia).

Tyrannosaurus sex. Probably not a quest for meaning and engagement. Also, rawr. (Jurassic Museum of Asturias. Wikipedia).

Attitudes from other cultures vary, but appear comparable. Aka foragers and Ngandu farmers in the Central African Republic refer to sex as “the work of the night” to create a pregnancy, but they also view it as fun, as an expression of love, and much more pleasurable than the “work of the day” acquiring food (Hewlett and Hewlett, 2008). If it is work, they don’t mind doing it too much. Married Ngandu and Aka couples had sex about 228 and 439 times a year, respectively, compared to 86 times per year for U.S. couples aged thirty- to thirty-nine. Hewlett and Hewlett attributed these high numbers at least partly to cultural beliefs that repeated intercourse and sperm were required to sustain a pregnancy. Both males and females expressed equal and high amounts of desire, however, reinforcing that this was not merely seen as drudgery to procreate. 

There are other hints that indicate sex is often a means of connection.


1. Copulation Positions

Much has been made about the ability of humans and bonobos to mate face-to-face.  Most primate species mate front-to-back, or ‘dorso-ventrally’ in anatomy-speak.  But the ‘missionary’ position (‘ventro-ventral’) is very common in people and also occurs in about a quarter to a third of copulations in bonobos (Dixson 2009: 88). It has also been seen, albeit rarely, in orangutans and gorillas, though not at all in chimpanzees. Dixson noted that:

“face-to-face patterns of copulation facilitate eye contact and continued facial communication between partners during mating. Eye contact plays an important role during pre-copulatory behavior in many anthropoids (monkeys and apes). Female monkeys of various species frequently attempt to look back at males during dorso-ventral copulations. Such communication is greatly facilitated during ventro-ventral copulation, however…” (p. 90)

Comedian Louis CK, demonstrating why giraffes can't have sex face-to-face.

Comedian Louis CK, demonstrating why giraffes can’t have sex face-to-face (click to animate).

Dixson suggested that the missionary position may not have evolved ‘for’ communication, but is likely a side effect of a more flexible ape/human anatomy as well as the creativity that comes with the increased intelligence in greater apes. Maybe it’s boredom (“Oh, not dorso-ventral again?!”). But some level of mental/emotional connection is certainly increased in highly intelligent animals staring into the face of another inches away, including for us (though you may know of exceptions). Of course, in humans this is further enhanced by language and being able to speak with one’s partner, though language barriers are not impermeable either. We can only speculate how affectionate were the intimate encounters between earlier modern humans and Neanderthals, but they almost certainly spoke different languages.

Ventro-ventral copulation is not limited to humans. Clockwise from top left: rhesus macaques, bonobos, orangutans, gorillas. The first is a monkey; the last three are apes.

Ventro-ventral copulation is not limited to humans. Clockwise from top left: rhesus macaques, bonobos, orangutans, gorillas. The first is a monkey; the last three are apes.

However, Neanderthals would have had a highly sophisticated intelligence and a well-developed theory of mind, so even across phenotypically distinct subspecies with different languages there was still the possibility of affection. Admittedly, that is just speculation. Greg Laden has written that in too many pop science accounts of sex, human male sexuality is programmed for reproduction, making them sexually indiscriminate and able to “make love to mud.” Though there is evidence that men are more apt to separate sex from love than are women (Schmitt 2005), a fuller account of human sexuality reminds us that this is far from universal and there is a lot of overlap between males and females (see Fuentes 2012b). Men are more complex than they are often portrayed, with communication/connection remaining an important part of the equation (again, you may know exceptions). In his essay, “What Do Men Really Want?,” Eric Jaffe wrote:

“While some guys do view sex and desire as one and the same, many others—even those in the early stages of a casual engagement—want someone they know and trust on a deeper level.”


Complexities of ‘Hookup Culture’

In her book “The End of Sex,” Donna Freitas (2013) discussed the ‘hookup culture’ common among young adults, focusing primarily on college students in the United States. As mentioned above, college students are a limited sample, but we can still learn from them. Definitions of ‘hookups’ are slippery, but Garcia et al 2012 (p. 162) referred to them as casual, uncommitted sexual contact between strangers or brief acquaintances, outside of a ‘formal’ relationship (dating, marriage, etc.), without a ‘traditional’ reason (such as love, procreation, or commitment).  Freitas argues that hookups, or at least repeated ones without room for more traditional intimate relationships, leave many young adults dissatisfied. In her assessment, this stems from the fact that emotional entanglement is verboten in hookups, and casual partners enter into any intimate encounter with the tacit agreement there will be no strings attached. However, some people have an easier time with this than others.

“Men and women learn to shut down emotionally in order to ‘safely’ turn on physically…. In a way, one can liken a hookup to masturbating with another person present. The problem is, most students fail at this goal, walking away with feelings for their partner.”  (Freitas, 2013: 30-1)

In one study of 500 college students, 64.9% of women and 45.2% of men expressed hope that their hookup experience would become a committed relationship, indicating that casual sex is often not so casual after all (Owen & Fincham, 2011). There is also evidence that young adults who have more uncommitted sexual encounters experience more psychological distress for both women and men (contrary to stereotypes), but it is unclear if this relationship is correlational or causal (Whitbourne, 2013). However, this was not true across the board, and there is a robust amount of variation in people’s attitudes, behaviors, and psychological responses to uncommitted sex.

Silly string overload.

Some strings attached.

Reiber and Garcia (2010) found that in a few studies college students often overestimated how comfortable their peers were with uncommitted sex, implying that perceptions do not always match reality, a concept known as “pluralistic ignorance.”  In another paper, they concluded that:  

 “the findings that a majority of both men and women are motivated to engage in hookups, but often desire a more romantic relationship, is also consistent with a more nuanced evolutionary biopsychosocial perspective that takes into account social context and the cross-cultural and biological centrality of the pair-bondHookups, although increasingly socially acceptable, may leave more “strings” than public discourse would suggest.” (Garcia et al 2012: 172).


Female Orgasm in One-Night Stands

Finally, Armstrong et al (2012) assessed women’s sexual satisfaction in uncommitted and relationship scenarios, looking at 6,500+ undergraduate heterosexual women at 21 colleges and universities. They found that:

“Women had orgasms more in repeat hookups compared to hookups with a new partner, and relationship sex led to orgasm most often. Women reported orgasms in 11 percent of first hookups, 16 percent of second or third hookups, 34 percent of higher-order hookups, and 67 percent of relationship sexual events.”

The authors attributed this to two things: a double-standard, at least in heterosexual encounters, in prioritizing men’s pleasure over women’s, and men being less concerned with women’s pleasure in uncommitted sex compared to sex within relationships. At least in this sample, sexual satisfaction was higher in more committed relationships. 

Percent of college women  achieving orgasm in different relationships (from Armstrong et al).

Percent of college women achieving orgasm in different types of committed and uncommitted relationships (from Armstrong et al).


Take-home message

 Robert Sapolsky once wrote that the same area of our brain, the insula, becomes active when we experience disgust, whether it applies to thinking about rotten meat or someone’s reprehensible behavior. In his words: “When we evolved the capacity to be disgusted by moral failures, we didn’t evolve a new brain region to handle it. Instead, the insula expanded its portfolio.” Similarly, we may have expanded our sexual portfolio over our evolutionary history. 

As the !Kung believe, sex may be a gift from the gods, but it seems that the type of gift that it is depends on circumstances. Collectively, the above points suggest that human sexuality is highly flexible with a number of functions used in a variety of contexts, though usually with a strong emotional component to it that goes beyond ‘creating people.’ Evolutionarily speaking, it is likely that this came about through exaptation and co-opting sex for more than reproductive purposes or pleasure, and into establishing and maintaining relationships. 


[1] In the Meston and Buss study, students were given the open-ended question “Please list all the reasons you can think of why you, or someone you have known, has engaged in sexual intercourse in the past.” One response was “I wanted to get closer to God.” I’m skeptical.



Armstrong EA, England P, Fogarty ACK. 2012. Accounting for women’s orgasm and sexual enjoyment in college hookups and relationships. American Sociological Review 77(3): 435-62. Link
Barriger M, Vélez-Blasini CJ. 2013.  Descriptive and injunctive social norm overestimation in hooking up and their role as predictors of hook-up activity in a college student sample. J Sex Res. 50(1):84-94. Link
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Dixson A. 2009. Sexual Selection and the Origins of Human Mating Systems. Oxford. Link
Donnan H, Magowan F. 2010. The Anthropology of Sex. Bloomsbury Academic. Link
Downey G. 2012. The long, slow sexual revolution (Part 1). Plos Blogs: Neuroanthropology. Link
Fredrickson BL. 2013.  Love 2.0: How Our Supreme Emotion Affects Everything We Feel, Think, Do, and Become. Penguin. Link
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Fuentes A. 2012b. Why is sex so complicated? Psychology Today, Dec 3. Link
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  1. Part 1. Introduction Link
  2. Part 2. Promiscuity Link
  3. Part 3. Promiscuity (Genetics) Link
  4. Part 4. Promiscuity (Anatomy/Physiology) Link
  5. Part 5. Pair-Bonding and Romantic Love Link
  6. Part 6. Many Intimate Relationships Link
  7. Part 7. Is It Possible to Love Two People? Link
  8. Part 8. Love and Suffering Link
  9. Part 9. Love Is an Evolutionary Compromise  Link
  10. Part 10. Wondrously Complex Paleo-Sex Link
  11. Part 11. Sexaptation: The Many Functions of Sex Link
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4 thoughts on “Part 11. Humans Are Blank-ogamous. Sexaptation: The Many Functions of Sex

  1. Pingback: Links: From The Golden Bough to Digital Public Anthropology | Anthropology Report

  2. Alain de Botton on what makes something sexy. It sounds a lot like connection. He says it is about curing loneliness and “an echo for our deepest needs,” “a miniature utopia,” when “another person accepts the whole of us.” We often see it as depraved, but de Botton say that through sex, “the dirty becomes clean.”

    He goes on to critique evolutionary psychology, not necessarily because it’s wrong, but because it falls short: “we are turned on not just by signs of health, but by the whole person.”

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