Sex Basics

Hunter Gatherer Notes

Some of the population-level differences that have long been attributed to men and women turn out to be mutable—women should not be confined to healing or teaching professions, nor men to ones requiring brute strength or raw ambition. Recognizing these things does not mean, however, that we are the same at the population level. For instance, “Men are taller than women” is a true statement about averages.

An average difference does not imply that all members of population Y (men) are taller than all members of population Z (women). True statements about populations do not manifest in all individuals in those populations; believing otherwise is falling prey to the “fallacy of division” (first described by Aristotle). In populations where the overlap of a trait is significant, it can be difficult to parse population-level patterns from individual experience.

If you, as an individual, do not fit a particular pattern, the discrepancy can feel like evidence that the pattern is false, but that feeling does not make it so.

Female doctors are more likely to go into pediatrics; men are more likely to become surgeons. In retail, men are more likely to sell cars, women are more likely to sell flowers. And while retail jobs in the US were split nearly evenly between men and women in 2019, wholesale jobs skewed strongly male. In tasks that require physical strength, men, on average, are simply stronger. An all-female force engaging in hand-to-hand combat would not beat an all-male one, and it would be beyond foolish to pretend otherwise.

We should also not expect that men and women will make identical choices, or be driven to excel at identical things, or even, perhaps, be motivated by the same goals. To ignore our differences and demand uniformity is a different kind of sexism. Differences between the sexes are a reality, and while they can be cause for concern, they are also very often a strength, and we ignore them at our peril.

Sex: The Deep History

Sexual reproduction has always been a messy, costly operation. You have to find an appropriate mate. You have to convince the mate that you’re a good bet. It may need to be the right time of year—the mating season—or else it’s possible that your gonads have been reabsorbed to save weight and to use those resources for something else (many migratory birds do this). If you do manage to find and convince another individual of the right species and breeding status to mate with you, you may have to tend to the developing egg or fetus. You may have responsibilities that extend for years—decades even—after you have a child that is the result of sexual reproduction.

In terms of genetic fitness, when you reproduce sexually, you are taking a 50 percent hit. If you cloned yourself, you would be 100 percent related to all of your offspring, spreading your genes with perfect accuracy. With sex, only half of your genes are in each of your kids. Asexual reproduction is only a win for you and your offspring if the future looks exactly like the past. So long as conditions stay the same, if life worked out for you, it should work out for your clones.

Our chromosomes start us down a path toward femaleness and maleness. One of the genes on the Y chromosome, for instance, is SRY, which, when initiated, controls the regulation of a whole series of masculinizing actions, including the formation of what will become the testis, in which sperm is made. Hormonal cascades further masculinize a body (with testosterone and other androgens) or feminize it (with estrogens and progesterone). Even when the quantity of gonadal hormones is controlled for, though, sex chromosomes themselves affect such varied differences between males and females as pain perception and response, the anatomy of individual neurons, and the size of various brain regions, including parts of the cerebral cortex and corpus callosum.

In order to reproduce sexually, you need two things: DNA from multiple individuals, and a cell. The machinery of the cell—for example, mitochondria and ribosomes—is big and bulky, at least compared to DNA, but it’s necessary for life. So, if you are going to reproduce sexually, at least one partner has to contribute that cellular machinery, known as the cytoplasm. That cell—which we call the egg—is therefore big (for a cell). Trade-offs being what they are, that big cell is also largely sessile—it doesn’t move.

• The next problem in sexual reproduction is how the gametes will find each other. Since some gametes are sessile, others need to be zippy, which is facilitated by being stripped of most of the cellular machinery that would be required to make a zygote. Those gametes are called sperm in animals, pollen in plants. They move around their environment, “looking for” eggs.
• Intermediate gametes—ones with some cytoplasm and some ability to move—would be worse at both things: they would have insufficient cytoplasm to create a zygote on their own (and upon meeting a gamete that also had cytoplasm, there would be disagreement about whose to use to make a whole new life), and they would be slow to find other gametes to hook up with.
• Anisogamy, the evolution of two different (aniso) gamete sizes (gamy), therefore occurs because of the poor performance of intermediate gametes.

Humans are sexually dimorphic across many domains, extending far beyond reproduction. Men and women have different disease risks, etiology, and progression, for conditions from Alzheimer’s to migraine, from drug addiction to Parkinson’s disease. Our brains are structured differently. We tend to have different personality traits by sex, and they are mediated by our environment: personality differences are greater in countries that have abundant food and a low prevalence of pathogens. In general, women are more altruistic, trusting, and compliant as well as more prone to depression than men. Men are more likely to be diagnosed with ADHD, women are more likely to have anxiety disorders. Finally, on average, men prefer working with things, and women prefer working with people.

Sex Changes and Sex Roles

Sometimes, conditions are so dire that normally sexually reproducing individuals become asexual in order to reproduce.

Similarly, sometimes conditions are such that it is evolutionarily appropriate to change sex. In a few species of plants, many species of insects, and several clades of reef fish, sequential hermaphroditism is common, in which an individual begins as one sex and switches to the other at some point in their life.

Among sequential hermaphrodites like flame wrasses, after a female changes into a male, he has changed not only his sex—which gamete he produces, once eggs, now sperm—but also his “sex role,” which is the behavioral expression of his (new) sex. In humans, we call this gender, or sometimes, gender expression.

The usual rules of sex roles, then, are ones of male display and female choosiness. This stems from that long-ago difference in investment between the sexes—the large resource-rich egg and the small streamlined sperm. Furthermore, in those species in which parental care is necessary for the offspring to survive—which means all of mammals and birds, and a good proportion of reptiles, amphibians, fish, and insects as well—males tend to put more effort into what happens before sex, and females put more effort into what happens afterward.

In the vast majority of species, females are the limiting sex. Because females invest more in offspring— from eggs being larger than sperm through parental care typically falling to females more than males—males must compete for access to females, and females get to choose among their suitors. Males thus tend to be larger or more aggressive; or gaudier, louder, or more melodious than the females of the species.

Sex-role reversed species have also flipped which sex invests the most when; in these cases, the male is the limiting sex. Several polyandrous waterbirds do this, including Northern jacanas. Among them, we find dominant females defending large territories, inside of which a female’s many male mates build nests, incubate eggs, and tend to the young.

Sex-role reversal—what we might call gender switching in humans—is not the same as changing sex. In mammals and birds, with our genetic sex determination, there is no sex change possible. Behavior, though—call it sex role, call it gender—that is highly labile (open to change). We humans are the most labile, behaviorally, of all the animals. So, it shouldn’t surprise us too much that many of us are abandoning some old gender norms—behaviors that have, in the past, been tightly coupled with our sex—and configuring new ones.

What it would be foolish to do—and what many WEIRD people in the 21st century are doing—is to pretend that sex equals gender, or that gender has no relationship to sex, or that either sex or gender is not wholly evolutionary. Remember the Omega principle, which tells us that adaptive elements of our software (e.g., gender) are no more independent of our hardware (e.g., sex) than the diameter of a circle is independent of that circle’s circumference. Gender is more fluid than sex, and has many more manifestations, but “acting feminine” (gender) is not the same as “being female” (sex).

Women are, on average, more likely to nest and nurture, and men are more likely, on average, to defend and explore. Observing this does not mean that men don’t nurture, or that women don’t explore, but these population-level differences have evolved because of the underlying differences between the sexes. Pretending otherwise puts us all at risk—ask people to believe things that are patently untrue and they will be ever less likely to form a coherent worldview, one based in observation and reality, rather than fantasy. Men will never ovulate, gestate, lactate, menstruate, or go through menopause. Women who identify as men might, but that is different.

Sexual Selection in Humans

Human girls develop breasts at puberty, and they persist throughout the woman’s life. Breasts are, of course, functional as mammary glands with which to feed babies. In no other species of primate do breasts persist when there are no babies around to benefit. Human breasts are sexually selected, and they are doing more than feeding babies. They are also advertisements to men.

The concealment of ovulation in humans is also sexually selected. While nearly all mammals advertise fertility by physiological means, humans do not—or at least, we do so far less than other species. We have also become sexually receptive throughout the year, rather than just seasonally. Concealment of ovulation serves some reproductive ends, but it also encourages something that humans do a lot of: we engage in nonreproductive sex—sex for pleasure, sex for bonding.

The physical ornamentation of women—including not just makeup and heels and jewelry, but also breasts that stay enlarged throughout our reproductive cycles—is an indicator of partial sex-role reversal in humans. What does that mean? Whereas most animal species exhibit male-male competition and female choice of mates, a species with partial sex-role reversal, such as we have, will also show competition between females and male choice of mates. This can manifest as anything from women advertising to attract the attention of men to outright brawls between women. Men, not coincidentally, will also be more likely to be in a position to choose their partners.

Division of Labor

The division of labor can and has created rigid roles, however, many of which are outdated in the 21st century. It is useful to understand some of where those roles came from in order to determine which ones are unlikely to change, and which ones might.

• Among hunter-gatherers, men have been far more likely to be hunters of large game, women more likely to be gatherers of plant foods and smaller animals. Hunter-gatherer women likely spent most of their adult, premenopausal lives pregnant or breastfeeding infants and toddlers. When breast milk is all or most of a child’s diet, the mother is effectively on birth control, as she experiences physiologically induced amenorrhea—she cannot get pregnant when breastfeeding at frequent intervals. This keeps birth intervals relatively long, and the birth rate fairly low.
• Jump forward to the human transformation of landscapes with agriculture, and gender roles became even more constrained. Being tied to a particular piece of land, we were now more sedentary and had ample grain stores with which to supplement our and our children’s diets at any time. Agriculturalist women thus experienced a decrease in the birth interval—babies came at a faster rate—and so the birth rate climbed. This increase in fertility tied women to hearth and home, and we saw a concomitant decrease in women’s roles in economic, religious, and other culturally important realms.
• Some of the average differences between the sexes include that men have more “investigative” interests, while women have more “artistic” and “social” interests. Men are also, on average, more interested in math, science, and engineering. On tests, girls score higher in literacy, while boys score higher in math. And although average intelligence is the same between boys and girls, the variability in intelligence is not: there are more boy geniuses, and more boy complete dullards, than there are girls in either category.
• Across several domains—including both emotional memory and spatial ability—women are better at details; men are better at “gist.” This finding manifests in the average man’s superior ability to remember a route, and the average woman’s superior ability to remember the location of the keys, the cup of coffee, the document in need of being signed.
• Given a choice, neonate girls spend more time looking at faces, while neonate boys spend more time looking at things.
• In an analysis of 185 cultures, in every culture studied, some tasks are always gendered in the same direction: iron smelting, the hunting of large marine mammals, metalworking—all of these are done by men only (in those cultures that do these things at all). More interesting is the tasks that are highly gendered across cultures, but for which some cultures curtail female involvement, while other cultures curtail male involvement. These include weaving, the preparation of skins, and the gathering of fuel, among others. This suggests that there is a value in the division of labor, even when neither sex is inherently better at the task.

Historically, women and men have had division of labor, both in family units and in societies, but other than those tasks mandated by anatomy and physiology (gestation, lactation), there is little in the modern world that some women might not choose to do. Similarly, men are ever more welcome in traditionally female fields such as nursing and teaching, although we shouldn’t expect parity there either. Different preferences lead to different choices. Pretending that we are identical, rather than ensuring that we are equal under the law, is a fool’s game.

Sexual Strategies

While most readers of this book are likely living in a culture that assumes monogamy and biparental care, absent those constraints, men don’t contribute much to the production of babies. It is also true that baby production does not end at birth, as once a baby has been successfully gestated for nine months, mother’s milk may nourish that baby for six months, two years, or more, depending on cultural norms. Maternal investment in offspring is mandated, and high; paternal investment can be high as well, of course, but is negotiable. There are many humans alive now, and throughout history, who never met their fathers.

In every culture studied, women were more interested in mates with high earning potential than were men. Furthermore, men were more interested in potential mates who are young, and physically attractive, than were women.

Women who might get pregnant will have an easier time if that child has a father who contributes to the well-being of his child and mate. So, women will be selected to prefer men with the capacity to earn. Because female fertility peaks early, and falls off far more steeply than male fertility does, men who might father children are more likely to be interested in youth and beauty in their mates—both of which can be understood to be proxies for fertility.

Because fathers have never had certainty of paternity until recent technological advances have made it possible, the evolution of jealousy and mate guarding is far more prevalent in men than in women. Across cultures, men have tried to control the reproductive activities of women in such a way that they could increase their own certainty of paternity. Among the most patently divisive and destructive are menstrual huts that isolate women during menses (and so allow men to know when in their cycle women are) and female genital mutilation (which reduces or eradicates the possibility of sexual pleasure).

Among the Nayar of southwest India, wives and husbands do not live together, they share little beyond sexual activity, and women may have multiple husbands. With such uncertainty of paternity, fathers do no paternal care, but the mothers’ brothers have both rights and responsibilities toward their nieces and nephews that are, to our WEIRD eyes, quite paternal. In general, though, a man who is tricked into raising someone else’s children is ridiculed.

Broadly speaking, there are three possible reproductive strategies:

1. Partner up and invest long term, reproductively, socially, and emotionally.
2. Force reproduction on an unwilling partner.
3. Force nobody, but also invest little beyond short-term sexual activity.

Until very recently women preferred long-term partners to one-night stands, and they were far more likely than men to be sexually reticent. Women have thus tended to engage in a long game, looking for a man with whom to partner, to co-parent, and to grow old together. Gestation and lactation are anatomically, physiologically mandated features of being a female mammal. Given that women are forced by nature to invest in their children, they have a better chance of successfully parenting if they have a partner who is invested as well.

All three of these strategies are open to men, but the first one is the male strategy that is best for society, best for children, best for women, and best for all but a handful of men.

Rape has allowed men to successfully reproduce, especially in times of war. Nobody will defend rape as honorable or desirable for individuals, or for society. Rape is reproductive strategy two.

The final alternative male strategy, however, is also not honorable or desirable for society, and yet has been encouraged for women by “sex-positive” activists as a sign of freedom and escape from puritanism. This third reproductive strategy is that of one-night stands. Of sex with strangers and without commitment or expectation. This strategy does not involve force—many women will often willingly sleep with a guy they just met. Such sexual liaisons do tend to come with some deceit, though, often by both parties. If women adopting some of the worst traits of men is our evidence of equality and freedom, we need to reinvestigate our values. This is reproductive strategy three: sex with neither force nor investment, the short game.

Contra the messaging of sex-positive feminists, engaging in this short game diminishes the sexual power of women. When people of both sexes are routinely seeking frivolous, no-emotional-connection sex, they are creating conditions in which everyone is behaving like men at their (second) worst. It’s not as bad as rape, obviously. But it’s not as good as strategy one, either. Society sliding toward this third reproductive strategy for both men and women is a variation on the Sucker’s Folly—the tendency of concentrated short-term benefit (sexual pleasure) not only to obscure risk and long-term cost, but also to drive acceptance even when the net analysis is negative.

Women often don’t intuit that there are two distinct male strategies (aside from rape), and so are often advertising toward men engaged in strategy three, the one that looks for one-night stands, while actually angling for men interested in strategy one. Hotness is a manifestation of sexual strategy three gaining primacy. Beauty, by comparison, is a manifestation of the first sexual strategy, the one that’s in it for the long term. Hotness fades fast with reproductive potential. Beauty fades far more slowly.

Failures of Reductionism, Redux: Pornography

There is no such thing as “having sex.” It’s not like “watching Netflix,” or “playing the guitar.” Sex is interactional and emergent, such that “having sex” with person A is not the same as “having sex” with person B. This is, once again, the mistake of reductionism, imagining that a proxy for a thing is the (whole) thing.

Of course, people are fascinated by human sexuality. There is information, in watching others, about hazards and opportunities, both evolutionary and personal. Doing so triggers that third sexual strategy, though, the one that wants to get to the act of sex right away, the one in which it doesn’t matter who it is that one is having sex with.

Men are more likely to be jealous of physical infidelity, women are more likely to be jealous of emotional infidelity. In general, women prefer erotica, which brings with it a backstory, as it were. Porn, targeting that third sexual strategy, reduces human bodies to our constituent parts, and puts a premium on extreme sexual acts as a result of economic competition for attention. Among populations that have come of age on a steady diet of porn, women are far more likely to report being asked to engage in anal sex, strangling, and other violent “games” that are represented on-screen, even though few real women want these things in life.

Porn, we posit, produces what we will call sexual autism. Here, we take the diagnostic criteria for autism and suggest that porn produces, in its adherents, something similar with regard to sexuality: incoming sensory data are of primary importance, and emotional and social communication is backgrounded, if considered at all.

Those who learn sexual behavior via porn tend to display repetitive behavior, and atypical sensitivity to sensory inputs. Sexual communication is difficult for them—probably because communication is a two-way street, and the other person cannot be fully predicted in advance, or controlled. People who have learned about sex through porn have difficulty developing, maintaining, and understanding sexual relationships; insist on inflexible adherence to routine; and show intense fixation on narrow interests. It is, in short, difficult for them to contend with novelty and surprise, with discovery, and with emergence.

If the most complete human sexuality is, as we argue, an emergent property between whole individuals—bodies and brains, hearts and psyches—porn reduces sex to commodity, to acts, to mere bodies. Selecting from a narrow menu of options, sex learned from porn will be repetitive and inflexible, with a narrow focus on orgasm. Those who learn about sex from porn are likely to be insensitive to feedback from anything but their own body. Communication and feedback will not be priorities, nor perhaps understood as values at all. Relationships will be difficult to form, harder still to understand. This is safer, in a way—while you risk not discovering the true highs of human relationship and connection, you are also protected from some of the true lows. What of the world of emotional, deeply human discoveries that are possible with a richly connected sexuality? 

The Corrective Lens

• Avoid sex without commitment. Cheapening sex by seeking it everywhere and at all times renders it more difficult to form a stable bond with one individual, which is the best predictor of a relationship of equals, in which neither partner feels chronically subservient or undervalued. Instead, seek ecstasy and passion, which are more consistently discovered with someone whom you know well, and who knows you well.
• For straight women: Do not succumb to social pressure to embrace easy sex. If you aren’t interested in sleeping with a guy within hours or days of meeting him, and he passes you by for someone who is—what have you lost? You have lost access to a guy dedicated to strategy three. You are better off looking for a good guy who is capable of, and interested in, refusing to act on his basest impulses.
• Keep children far away from pornography. Try to keep yourself away from it as well. The market should not be allowed to intrude on several things, including but not limited to love and sex, music, and humor.
• Do not interfere with children’s development by trying to block, pause, or radically alter their development. Gender is the behavioral expression of sex, and so is both a product of evolution and more fluid than is sex.
• Childhood is a time of identity exploration and formation. Children’s claims to be the sex that they are not should therefore not be indulged as anything beyond normal play and searching for boundaries. While actually intersex individuals are real and incredibly rare, and actually transgendered people are also real and very rare, much of modern “gender ideology” is dangerous and contagious, and many of the interventions (hormonal, surgical) are not reversible.
• Keep contaminants away from fetuses and children. In several species of frogs, there is an established relationship between exposure to common environmental contaminants, like atrazine (an herbicide), and an increase in hermaphroditic individuals. While sex determination in frogs is different than in humans, we will not be surprised if it turns out that some of the modern confusion around sex and gender ends up attributable to widespread endocrine disrupters in our environment.
• Recognize that our differences contribute to our collective strength. If we more highly valued work that women are more likely to be drawn to (e.g., teaching, social work, nursing), perhaps we could stop demanding equal representation of men and women in fields that women are simply not as likely to be interested in. Recognizing that we are, on average, different is the critical first step to building a society in which all opportunities are truly open to everyone.
• Equal opportunity is an honorable goal in step with reality, whereas aiming for equal outcome—in which every occupation, from day-care workers to garbage collectors, has equal representation between the sexes—will disappoint everyone involved.

Love is a state of the emotional mind that causes one to prioritize someone or something external as an extension of self. Love, the genuine article, is a matter of intimate inclusion. When it is real, there are few forces more powerful.

Love evolved first between mother and child, but then expanded its scope. Soon enough, adults reliably experienced love between partners, and then other forms of love began to blossom—between fathers and children, grandparents and grandchildren, among siblings. Love then found a place between friends and between soldiers, between those who shared intense experiences, good or bad.

Eventually, love evolves to include abstractions—love of country and love of God, love of honor and service, truth and justice.

Love as we experience it first evolved nearly two hundred million years ago, when mammals diverged from reptiles. As with the evolution of sex, the egg is foundational to our understanding of the evolution of love.

The first mammals were egg layers, and eggs do not require love, although in many species they benefit from parental vigilance. But the five extant species of egg-laying mammals—four species of echidnas, and the duck-billed platypus—are substantively different from all other egg-laying species. Mammals, even the ones that lay eggs, make milk. Early on, it was a crude operation: modified sweat glands secreted nutritious fluid that was lapped up off the surface of the mother’s skin. Later, a more elegant solution for delivery evolved: the nipple. In all mammals, nippled or not, milk solves a problem.

A mammal mother can leave her babies in a safe place while she forages, freeing her from having to provide all their food in advance or having to shuttle morsels back to her burrow. Milk also allows the baby’s food to be chemically and nutritionally adjusted in various ways that facilitate development.

Parental Care: Moms, Dads, and Other

Helplessness of hatchlings and newborns—altriciality—is not itself an asset, but this helplessness opens the door to extraordinary things. Major programming of the brain can take place through cultural transmission when offspring are in close contact with their parents. This is far faster than genetic change, and allows not only for rapid behavioral evolution, but also for tailoring of behavioral patterns to the local environment—physical, chemical, biological, and social.

In birds and mammals, altriciality is the downside of behavioral flexibility, which is an asset. Behavioral flexibility, plasticity, emerges in organisms that are not fully programmed by the genome.

In part, cooperative breeding, which is on full display in many human societies, is most likely to evolve when rates of promiscuity are low and resources are distributed across the landscape such that any given individual cannot monopolize them. The monopolization of resources opens the door to the monopolization of mates—indeed, the distribution of resources in space and time has far-reaching effects on mating systems.

Mating Systems

Sexual size dimorphism is a strong predictor of polygyny across vertebrate species. Men are, on average, about 15 percent larger than females, and significantly stronger. This tells us that our ancestors were at least somewhat polygynous or promiscuous.

And while the majority of human cultures have been polygynous at some point in time, the majority of people alive today belong to cultures in which monogamy is the norm. Monogamy is fragile, and easily and often breaks down into polygyny in mammals. Despite this, monogamy is the superior system.

Types of Mating Systems:

• Monogamy: individuals of both sexes have just one partner at a time
• Polygamy: individuals of one sex have just one reproductive partner, but individuals of the other sex have multiple partners. Subtypes include:
  • Polygyny: (poly—many, gyn—female): One male and multiple females
  • Polyandry: (poly—many, andr—male): One female and multiple males.
• Promiscuity: members of both sexes have multiple partners (in humans this is sometimes called polyamory)

Monogamy is the mating system with the greatest potential for cooperation and fairness, beginning with child-rearing. In primates, monogamy is also correlated with the largest relative brain size. Across the biota, females are the limiting sex, which allows them to be choosy about their partners. In a polygynous system, sexual partners abound for females, but they are in short supply for males and—absent an intent to invest beyond the act of sex—males tend to have incredibly low standards for sex partners. If a female does not show obvious signs of a communicable disease, a male is likely to accept just about any willing female. Absent monogamy, this is what sexuality reduces to: females burdened with the entire chore of reproduction, and undiscerning males always on the make.

Because monogamous males select a female and forgo sexual opportunities with others, they have as much reason as females to be choosy about sexual partners. Males being choosy in this way reduces their tendency toward violence. They may yet fight over access to the best females, but no longer need aspire to the acquisition and defense of “harems,” which are closely associated with aggression and physical weaponry like antlers and piercing teeth.

Monogamy also creates a system in which nearly everyone has a mate, as sex ratios tend to be one-to-one within populations, regardless of mating system. This prevents the accumulation of sexually frustrated males for whom violence may be the only path to reproduction, either through the overthrow of harem owners—as in lions and elephant seals—or through rape—as in ducks and dolphins.

Males without “certainty of paternity” are unlikely to stick around to pair-bond with a female and help raise the kids. Male birds tend to have high certainty of paternity (not long between fertilization, egg laying, and hatching), but male mammals are rarely certain at all.

Once pair-bonded, a male is faced with a choice. He can merely guard his chosen female against rivals, or he can contribute in some way to the provisioning and care of their offspring. Paternal care is not universal in monogamous species, but it is common. Paternal care increases the chances that offspring will live to be reproductively viable, and also increases the number of viable offspring that can be produced—both of which contribute to the fitness of male and female alike.

In humans, there seems to have been a positive feedback loop: as babies got more and more helpless, with longer and longer childhoods, the bond between co-parents got tighter and tighter. Love is a manifestation of the tightness of that bond.

Given that monogamy is the route to having full siblings, monogamy thus enhances cooperation between offspring and reduces the tendency of intra-brood conflict.

Illnesses faced by mothers during pregnancy are often the result of a conflict of interest between mother and fetus—an often gentle but real tug-of-war for resources. A mother has a strong interest in dividing resources between her various offspring over her entire reproductive life span, whereas a developing fetus—which has hormonal access to its mother’s bloodstream and is only 50 percent related to its mother—has an interest in taking more than its share, so long as its take doesn’t put its mother’s life at risk. That conflict of interest is mitigated in monogamous populations, in which the fetus has double the interest in the survival of future full siblings compared to populations in which future siblings will be sired by different fathers. From the perspective of a father in a non-monogamous species, another way to put this is that males take advantage of the maternal behavior of females, and their genes continue this parasitism through pregnancy.

Implications of Monogamy in Humans

Sex, gender, relationship, and mating systems: four topics intimately intertwined and shot through with complexity and significance. 

When resources are in surplus, monogamy is likely favored. It provides a clear advantage at the lineage/population level by bringing all able adults into child-rearing, allowing the population to capture resources at the fastest rate. When a population reaches carrying capacity, though, and zero-sum dynamics are once again in play, the incentives of high-ranking men tend to shift in the direction of polygyny. Male-male competition becomes a driving force, as men with wealth and power seek to dominate the reproductive output of multiple women.

When polygyny is on the rise in a society, we see an increase in sexually frustrated young men who are willing to take big risks for the possibility of gaining a mate. The lineages of the relatively few powerful males who benefit from polygyny also benefit by arming frustrated young men and sending them abroad with dreams of coming back with a bride or as a marriageable war hero.

• The possibility of gaining territory and treasure through military adventurism has clear evolutionary implications—it expands the resources available to the conquering population and thereby increases its size. Such military adventurism is also a “transfer of resource” frontier, which is a form of theft.

All else being equal, people in monogamous cultures experience lower birth rates, and higher socioeconomic status, than those in polygynous ones. Furthermore, the age difference between spouses is smaller. In part, this likely reflects a move away from viewing women and girls as commodities, which strongly polygynous cultures often do.

Polygyny is often conflated with a fantasy of promiscuity in which there is an increase in sexual and reproductive freedom without negative consequence. The sexual revolution would seem to bear this out, but it is an illusion for two reasons.

• First, absent birth control, promiscuity is good for men, who procreate almost without cost, and dangerous to women, to whom the entire burden of child-rearing gets shifted.
• Second, promiscuity tends to break down into polygyny as powerful men discover that they are in a position to demand exclusivity from multiple reproductive partners, either in sequence or in tandem.

Prior to birth control, women (and their close kin) guarded their reproductive capacity. Because human babies are so hard to raise, a woman who could insist on help from a man would be foolish to forgo it. In such a world, men had great incentive to impress women to whom they would commit. Our modern arrangement gives women a lot more freedom to enjoy sex without the risk of becoming single parents, but it also radically undercuts their bargaining position with respect to long-term commitment—especially if they engage in reproductive strategy three, the short game.

Men may physically feel as though they have been judged sexually worthy by large numbers of women, but subconsciously they know that the bar for such acceptance has been lowered so far as to be meaningless. Yes, it is sex, but it is junk sex, formulaic and without depth.

Women may consciously want to enjoy sex without commitment, but they are wired to fall in love with the men they bed because sex, babies, and commitment are evolutionarily inextricably linked for women. Sex and orgasm trigger the release of oxytocin in females, which promotes bonding. The situation is similar for males, except that it is not oxytocin but vasopressin that does the job.

The pair-bond-facilitating vasopressin, they argue, will be most present in men when they have sex with women they have known for a longer time. If true, what this would mean for women is that sleeping with a guy you’re really into on the first or second date will actually decrease the likelihood that he will fall in love with you.

Two categories of men benefit from type 3: the wealthy and powerful who are in a position to have multiple partners, and those who delight in pretending to be interested in commitment in order to bed women in whom they then invest nothing. Some men in both of those categories also propose sexual quid pro quo with women trying to advance their careers, a position that women can find impossible to recover from without damage. Somehow, we have replaced a deeply flawed system of mating and dating with one perfectly positioned to transfer all the spoils to kings and cads.

Committed relationships are a good thing, so valuable to the rearing of healthy children. Yet if women in the modern mating and dating scene don’t accept casual sex as normal, they’re often ignored. If they do embrace casual sex, they often unwittingly trigger fear of commitment. Men could be seen as profiting at the expense of women in this situation, which is partly true, but their windfall is mostly an illusion. Yes, men are built to find commitment-less sex rewarding, but they are also built to value loving partnerships. 

Committed relationships are a good thing, valuable to the rearing of healthy children. Yet if women in the modern mating and dating scene don’t accept casual sex as normal, they’re often ignored. If they do embrace casual sex, they often unwittingly trigger fear of commitment. Men could be seen as profiting at the expense of women in this situation, which is partly true, but their windfall is mostly an illusion. Yes, men are built to find commitment-less sex rewarding, but they are also built to value loving partnerships. Casual sex is disrupting that.